U.S. patent application number 09/737851 was filed with the patent office on 2002-08-22 for drilling system and method.
Invention is credited to Leuchtenberg, Christian.
Application Number | 20020112888 09/737851 |
Document ID | / |
Family ID | 24965564 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020112888 |
Kind Code |
A1 |
Leuchtenberg, Christian |
August 22, 2002 |
Drilling system and method
Abstract
A closed-loop drilling system and method of drilling oil, gas,
or geothermal wells is described, whereby through the control of
the flow rates in and out of the wellbore, and by adjusting the
pressure inside the wellbore by a pressure/flow control device
installed on the return line, surface pressure being increased or
decreased as required, this in turn decreasing or increasing
downhole pressure, occurrence of kicks and fluid losses may be
greatly minimized and quickly controlled. Through the method of the
invention the elimination of the kick tolerance and tripping margin
on the design of the well is made possible, since the pore and
fracture pressure will be determined in real-time while drilling
the well, and, therefore, nearly no safety margin is necessary when
designing the well, reducing significantly the number of casing
strings necessary. The inventive method can be called intelligent
safe drilling since the response to influx or fluid loss is nearly
immediate and so smoothly done that the drilling can go on without
any break in the normal course of action. The new method is
applicable to the whole wellbore from the first casing string with
a BOP connection, and it can be implemented and adopted to any rig
or drilling installation that uses the conventional method with
very few exceptions and limitations. The new method is applicable
to all types of wells, onshore, offshore, deepwater and
ultra-deepwater, with huge safety improvement in difficult drilling
scenarios.
Inventors: |
Leuchtenberg, Christian;
(Swansea, GB) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Family ID: |
24965564 |
Appl. No.: |
09/737851 |
Filed: |
December 18, 2000 |
Current U.S.
Class: |
175/48 ;
175/38 |
Current CPC
Class: |
E21B 44/00 20130101;
E21B 21/08 20130101; E21B 21/085 20200501 |
Class at
Publication: |
175/48 ;
175/38 |
International
Class: |
E21B 044/00; E21B
021/08 |
Claims
I claim:
1. A system for drilling a well while injecting a drilling fluid
through an injection line of said well and recovering through a
return line of said well where the well being drilled is closed at
all times comprises a pressure containment device and pressure/flow
control device to a wellbore to establish/maintain a back pressure
on the well, means to monitor the fluid flow in and out, means to
monitor flow of any other material in and out, means to monitor
parameters affecting the monitored flow value and means to predict
a calculated value of flow out at any given time and to obtain real
time information on discrepancy between predicted and monitored
flow out and converting to a value for adjusting the pressure/flow
control device and restoring the predicted flow value.
2. A system of drilling a well while being drilled with a drill
string having a drilling fluid circulated therethrough, while the
well is kept closed at all times, wherein the system comprises: a)
a pressure containment device; b) a pressure/flow control device on
the outlet stream; c) means for measuring mass or volumetric flow
and flow rate on the inlet and outlet streams to obtain real time
mass or volumetric flow signals; d) means for measuring mass or
volumetric flow and flow rate of any other materials in and out; e)
means for directing all the flow, pressure and temperature signals
so obtained to a central data acquisition and control system; and
f) a central data acquisition and control system programmed with a
software that can determine a real time predicted out flow and
compare it to the actual out flow estimated from the mass and
volumetric flow rate values and other relevant parameters.
3. A system of drilling a well while being drilled with a drill
string having a drilling fluid circulated therethrough, while the
well is kept closed at all times, wherein said system comprises: a)
a pressure containment device; b) a pressure/flow control device on
the outlet stream; c) means for measuring mass flow rate on the
inlet and outlet streams; d) means for measuring volumetric flow
rate on the inlet and outlet streams; e) at least one pressure
sensor to obtain pressure data; f) at least one temperature sensor
to obtain temperature data; g) a central data acquisition and
control system that sets a value for an expected out flow and
compares it to the actual out flow estimated from data gathered by
the mass and volumetric flow rate meters as well as from pressure
and temperature data, and in case of a discrepancy between the
expected and actual flow values, adjusting the said pressure/flow
control device to restore the outflow to the expected value.
4. A system according to claim 1 wherein the well is a gas, oil or
geothermal well.
5. A system according to claim 3 wherein the at least one pressure
sensor is located at the wellhead.
6. A system according to claim 3 wherein the system comprises two
pressure sensors.
7. A system according to claim 6, wherein one pressure sensor is at
the wellhead and the other one at the bottomhole.
8. A system according to claim 1 wherein the said central data
acquisition and control system is provided with a time-based
software to allow for lag time between in and out flux.
9. A system according to claims 1 or 8 wherein said software is
provided with detection filters and/or processing filters to
eliminate/reduce false indications on the received mass and fluid
flow data, and any other measured or detected parameters.
10. A system according to claim 1 which comprises three safety
barriers, the drilling fluid, the blow-out preventer equipment and
the pressure containment device.
11. Method comprising, in relation to the system according to claim
1, the following steps of injecting drilling fluid through said
injection line through which said fluid is made to contact said
means for monitoring flow and recovering drilling fluid through
said return line; collecting any other material at the surface;
measuring the flow in and out of the well and collecting flow and
flow rate signals; measuring parameters affecting the monitored
flow value and means; directing all the collected flow, correction
and flow rate signals to the said central data acquisition and
control system; monitoring parameters affecting the monitored flow
value and means to predict a calculated value of flow out at any
given time and to obtain real time information on discrepancy
between predicted and monitored flow out and converting to a value
for adjusting the pressure/flow control device and restoring the
predicted flow value.
12. A method of drilling a well while being drilled with a drill
string having a drilling fluid circulated therethrough, while the
well is kept closed at all times, said method comprising the steps
of: a) providing a pressure containment device to the wellbore; b)
providing a pressure/flow control device to control the flow out of
the well and to keep a back pressure on the well; c) providing a
central data acquisition and control system and related software;
d) providing mass flow meters in both injection and return lines;
e) providing flow rate meters in both injection and return lines;
f) providing at least one pressure sensor; g) providing at least
one temperature sensor; h) injecting drilling fluid through an
injection line through which said fluid is made to contact said
mass flow meters, said fluid flow meters, and said pressure and
temperature sensors, and recovering drilling fluid through a return
line; i) collecting drill cuttings at the surface; j) measuring the
mass flow in and out of the well and collecting mass flow signals;
k) measuring the fluid flow rates in and out of the well and
collecting fluid flow signals; l) measuring pressure and
temperature of fluid and collecting pressure and temperature
signals; m) directing all the collected flow, pressure and
temperature signals to the said central data acquisition and
control system; n) the software of the central data acquisition and
control system considering, at each time, the predicted flow out of
the well; o) having the actual and predicted out flows compared and
checked for any discrepancy; p) in case of a discrepancy, having a
signal sent by the central data acquisition and control system to
adjust the pressure/flow control device and restore the predicted
out flow rate, without interruption of the drilling operation.
13. A method of continuous, safe drilling of a well being drilled
with a drill string having a drilling fluid circulated
therethrough, while the well is kept closed at all times, said
method comprising the steps of: a) providing a pressure containment
device to the wellbore; b) providing a pressure/flow control device
to control the flow out of the well and to keep a back pressure on
the well; c) providing a central data acquisition and control
system and related software; d) providing mass flow meters in both
injection and return lines; e) providing flow rate meters in both
injection and return lines; f) providing at least one pressure
sensor to measure pressure; g) providing at least one temperature
sensor to measure temperature; h) injecting drilling fluid through
an injection line through which said fluid is made to contact said
mass flow meters, said fluid flow meters and said pressure and
temperature sensors, and recovering drilling fluid through return
line; i) collecting drill cuttings at the surface; j)measuring the
mass flow in and out of the well and collecting mass flow signals;
k) measuring the fluid flow rates in and out of the well and
collecting fluid flow signals; l) measuring pressure and
temperature of fluid and collecting pressure and temperature
signals; m) directing all the collected flow, pressure and
temperature signals to the said central data acquisition and
control system; n) the software of the central data acquisition and
control system considering, at each time, the predicted flow out of
the well; o) having the actual and predicted out flows compared and
checked for any discrepancy; p) in case of a discrepancy, having a
signal sent by the central data acquisition and control system to
adjust the pressure/flow control device and restore the predicted
out flow rate without interruption of the drilling operation.
14. A method according to claims 12 or 13 wherein the well is a
gas, oil or geothermal well.
15. A method according to claims 12 or 13 wherein the at least one
pressure sensor is located at the wellhead.
16. A method according to claims 12 or 13 wherein the method
comprises two pressure sensors.
17. A method according to claim 16, wherein one pressure sensor is
at the wellhead and the other one is at the bottomhole.
18. A method according to claims 12 or 13 wherein the Equivalent
Circulating Density of the well being drilled is adjusted by
closing or opening the pressure/flow control device.
19. A method according to claims 12 or 13 wherein the discrepancy
between actual and predicted out flows is a fluid loss and the
adjustment of the pressure/flow control device comprises opening
said device to the extent required to counteract fluid loss and
reduce backpressure.
20. A method according to claims 12 or 13 wherein the discrepancy
between actual and predicted out flows is a fluid gain and the
adjustment of the pressure/flow control device comprises closing
said device to the extent required to counteract fluid gain and
increase backpressure.
21. A method according to claims 12 or 13 wherein the predicted
ideal value for the outflow is based on calculations taking into
account among others rate of penetration, rock and drilling fluid
density, well diameter, in and out flow rates, cuttings return
rate, bottomhole and wellhead pressures and temperatures.
22. A method according to claims 12 or 13 wherein the software
provided to the central data acquisition and control system
receives as input any early detection parameters to ascertain that
an influx/loss has occurred.
23. A method according to claims 12 or 13 wherein the mass flow
metering comprises any subcomponents designed to improve accuracy
of the measurement.
24. A method according to claim 23, wherein the subcomponents
comprise measuring the mass flux of cuttings being produced at the
shakers and mass outflow of gas from the said degasser.
25. A method according to claims 12 or 13 wherein means are
provided to pressurize the well bore through the annulus,
independently of the current fluid injection path.
26. A method according to claim 23, wherein the subcomponents
comprise measuring the mass flow and fluid flow into the well bore
through the annulus, independently of the current fluid injection
path.
27. A method for the real time determination of the fracture
pressure of a well being drilled with a drill string and drilling
fluid circulated therethrough, while the well is kept closed at all
times, said method comprising the steps of: a) providing a pressure
sensor at the bottom of the drill string; b) having fluid and mass
flow data generated collected and directed to a central data
acquisition and control device that sets an expected value for
fluid and mass flow; c) the said central data acquisition and
control device continuously comparing the said expected fluid and
mass flow to the actual fluid and mass flow; d) in case of a
discrepancy between the expected and actual value, the said central
data acquisition and control device activating a pressure/flow
control device; e) the detected discrepancy being a fluid loss, the
value of the fracture pressure being obtained from a direct reading
of the bottomhole pressure.
28. A method for the real-time determination of the pore pressure
of a well being drilled with a drill string and drilling fluid
circulated therethrough, while the well is kept closed at all
times, said method comprising the steps of: a) providing a pressure
sensor at the bottom of the drill string; b) having fluid and mass
flow data generated collected and directed to a central data
acquisition and control device that sets an expected value for
fluid and mass flow; c) the said central data acquisition and
control device continuously comparing the said expected fluid and
mass flow to the actual fluid and mass flow; d) in case of a
discrepancy between the expected and actual value, the said central
data acquisition and control device activating a pressure/flow
control device; e) the detected discrepancy being an influx, the
value of the pore pressure being obtained from a direct reading of
the bottomhole pressure provided by the said pressure sensor.
Description
FIELD OF THE INVENTION
[0001] The present invention deals with a closed-loop system for
drilling wells where a series of equipment, for the monitoring of
the flow rates in and out of the well, as well as for adjusting the
back pressure, allows the regulation of the out flow so that the in
and out flows are constantly balanced at all times. A pressure
containment device keeps the well closed at all times. Since this
provides a much safer operation, its application for exploratory
wells will greatly reduce the risk of blow-outs. In environments
with narrow margin between the pore and fracture pressure, it will
create a step change compared to conventional drilling practice. In
this context, applications in deep and ultra-deep water are
included. A method for drilling, using said system, is also
disclosed. The drilling system and method are suited for all types
of wells, onshore and offshore.
BACKGROUND INFORMATION
[0002] Drilling oil/gas/geothermal wells has been done in a similar
way for decades. Basically, a drilling fluid with a density high
enough to counter balance the pressure of the fluids in the
reservoir rock, is used inside the wellbore to avoid uncontrolled
production of such fluids. However, in many situations, it can
happen that the bottomhole pressure is reduced below the reservoir
fluid pressure. At this moment, an influx of gas, oil, or water
occurs, named a kick. If the kick is detected in the early stages,
it is relatively simple and safe to circulate the invaded fluid out
of the well. After the original situation is restored, the drilling
activity can proceed. However, if, by any means, the detection of
such a kick takes a long time, the situation can become out of
control leading to a blowout. According to Skalle, P. and Podio, A.
L. in "Trends extracted from 800 Gulf Coast blow-outs during
1960-1996" IADC/SPE 39354, Dallas, Tex., March 1998, nearly 0.16%
of the kicks lead to a blowout, due to several causes, including
equipment failures and human errors.
[0003] On the other hand, if the wellbore pressure is excessively
high, it overcomes the fracture strength of the rock. In this case
loss of drilling fluid to the formation is observed, causing
potential danger due to the reduction in hydrostatic head inside
the wellbore. This reduction can lead to a subsequent kick.
[0004] In the traditional drilling practice, the well is open to
the atmosphere, and the drilling fluid pressure (static pressure
plus dynamic pressure when the fluid is circulating) at the bottom
of the hole is the sole factor for preventing the formation fluids
from entering the well. This induced well pressure, which by
default, is greater than the reservoir pressure causes a lot of
damage, i.e., reduction of near wellbore permeability, through
fluid loss to the formation, reducing the productivity of the
reservoir in the majority of cases.
[0005] In the last 10 years, a new drilling technique,
underbalanced drilling (UBD) is becoming more and more popular.
This technique implies a concomitant production of the reservoir
fluids while drilling the well. Special equipment has been
developed to keep the well closed at all times, as the wellhead
pressure in this case is not atmospheric, as in the traditional
drilling method. Also, special separation equipment must be
provided to properly separate the drilling fluid from the gas,
and/or oil, and/or water and drilled cuttings.
[0006] The UBD technique has been developed initially to overcome
severe problems faced while drilling, such as massive loss of
circulation, stuck pipe due to differential pressure when drilling
depleted reservoirs, as well as to increase the rate of
penetration. In many situations, however, it will not be possible
to drill a well in the underbalanced mode, e.g., in regions where
to keep the wellbore walls stable a high pressure inside the
wellbore is needed. In this case, if the wellbore pressure is
reduced to low levels to allow production of fluids the wall
collapses and drilling cannot proceed.
[0007] Since among the most dangerous events while drilling
conventionally is to take a kick, there have been several methods,
equipment, procedures, and techniques documented to detect a kick
as early as possible. The easiest and most popular method is to
compare the injection flow rate to the return flow rate.
Disregarding the drilled cuttings and any loss of fluid to the
formation, the return flow rate should be the same as the injected
one. If there are any significant discrepancies, drilling is
stopped to check if the well is flowing with the mud pumps off. If
the well is flowing, the next action to take is to close the
blow-out preventer equipment (BOP), check the pressures developed
without circulation, and then circulate the kick out, adjusting the
mud weight accordingly to prevent further influx.
[0008] This procedure takes time and increases the risk of
blow-out, if the rig crew does not quickly suspect and react to the
occurrence of a kick. Procedure to shut-in the well can fail at
some point, and the kick can be suddenly out of control. In
addition to the time spent to control the kicks and to adjust
drilling parameters, the risk of a blow-out is significant when
drilling conventionally, with the well open to the atmosphere at
all times.
[0009] Among the methods available to quickly detect a kick the
most recent ones are presented by Hutchinson, M and Rezmer-Cooper,
I. in "Using Downhole Annular Pressure Measurements to Anticipate
Drilling Problems", SPE 49114, SPE Annual Technical Conference and
Exhibition, New Orleans, La., 27-30 September, 1998. Measurement of
different parameters, such as downhole annular pressure in
conjunction with special control systems, adds more safety to the
whole procedure. The paper discusses such important parameters as
the influence of ECD (Equivalent Circulating Density, which is the
hydrostatic pressure plus the friction losses while circulating the
fluid, converted to equivalent mud density at the bottom of the
well) on the annular pressure. It is also pointed out that if there
is a tight margin between the pore pressure and fracture gradients,
then annular pressure data can be used to make adjustments to mud
weight. But, essentially, the drilling method is the conventional
one, with some more parameters being recorded and controlled.
Sometimes, calculations with these parameters are necessary to
define the mud weight required to kill the well. However, annular
pressure data recorded during kill operations have also revealed
that conventional killing procedures do not always succeed in
keeping the bottomhole pressure constant.
[0010] Other publications deal with methods to circulate the kick
out of the well. For example, U.S. Pat. No. 4,867,254 teaches a
method of real time control of fluid influxes into an oil well from
an underground formation during drilling. The injection pressure
p.sub.i and return pressure p.sub.r and the flow rate Q of the
drilling mud circulating in the well are measured. From the
pressure and flow rate values, the value of the mass of gas M.sub.g
in the annulus is determined, and the changes in this value
monitored in order to determine either a fresh gas entry into the
annulus or a drilling mud loss into the formation being
drilled.
[0011] U.S. Pat. No. 5,080,182 teaches a method of real time
analysis and control of a fluid influx from an underground
formation into a wellbore being drilled with a drill string while
drilling and circulating from the surface down to the bottom of the
hole into the drill string and flowing back to the surface in the
annulus defined between the wall of the wellbore and the drill
string, the method comprising the steps of shutting-in the well,
when the influx is detected; measuring the inlet pressure P.sub.i
or outlet pressure P.sub.o of the drilling mud as a function of
time at the surface; determining from the increase of the mud
pressure measurement, the time t.sub.c corresponding to the minimum
gradient in the increase of the mud pressure and controlling the
well from the time t.sub.c.
[0012] It is observed that in all the cited literature where the
drilling method is the conventional one, the shut-in procedure is
carried out in the same way. That is, literature methods are
directed to the detection and correction of a problem (the kick),
while there are no known methods directed to eliminating said
problem, by changing or improving the conventional method of
drilling wells.
[0013] Thus, according to drilling methods cited in the literature,
the kicks are merely controlled. On the contrary, the present
application relates to a new concept of drilling whereby a method
and corresponding instrumentation allows that kicks may be detected
early and controlled much quicker and safer or even
eliminated/mitigated than in state-of-the-art methods.
[0014] Further, it should be noted that the present method operates
with the well closed at all times. That is why it can be said that
the method, herein disclosed and claimed, is much safer than
conventional ones.
[0015] In wells with severe loss of circulation, there is no
possibility to detect an influx by observing the return flow rate.
Schubert, I. J. and Wright, J. C. in "Early kick detection through
liquid level monitoring in the wellbore", IADC/SPE 39400, Dallas,
Tex., March 1998 propose a method of early detection of a kick
through liquid level monitoring in the wellbore. Having the
wellbore open to atmosphere, here again the immediate step after
detecting a kick is to close the BOP and contain the well.
[0016] The excellent review of 800 blow-outs occurred in Alabama,
Texas, Louisiana, Mississipi, and offshore in the Gulf of Mexico
cited hereinbefore by Skalle, P. and Podio, A. L. in "Trends
extracted from 800 Gulf Coast blow-outs during 1960-1996" IADC/SPE
39354, Dallas, Tex., March 1998 shows that the main cause of
blow-outs is human error and equipment failure.
[0017] Nowadays, more and more oil exploration and production is
moving towards challenging environments, such as deep and
ultra-deepwater. Also, wells are now drilled in areas with
increasing environmental and technical risks. In this context, one
of the big problems today, in many locations, is the narrow margin
between the pore pressure (pressure of the fluids--water, gas, or
oil--inside the pores of the rock) and the fracture pressure of the
formation (pressure that causes the rock to fracture). The well is
designed based on these two curves, used to define the extent of
the wellbore that can be left exposed, i.e., not cased off with
pipe or other form of isolation, which prevents the direct
transmission of fluid pressure to the formation. The period or
interval between isolation implementation is known as a phase.
[0018] In some situations a collapse pressure (pressure that causes
the wellbore wall to fall into the well) curve is the lower limit,
rather than the pore pressure curve. But, for the sake of
simplicity, just the two curves should be considered, the pore
pressure and fracture pressure one. A phase of the well is defined
by the maximum and minimum possible mud weight, considering the
curves mentioned previously and some design criteria that varies
among the operators, such as kick tolerance and tripping margin. In
case of a kick of gas, the movement of the gas upward the well
causes changes in the bottomhole pressure. The bottomhole pressure
increases when the gas goes up with the well closed. Kick tolerance
is the change in this bottomhole pressure for a certain volume of
gas kick taken.
[0019] Tripping margin, on the other hand, is the value that the
operators use to allow for pressure swab when tripping out of the
hole, to change a bit, for example. In this situation, a reduction
in bottomhole pressure, caused by the upward movement of the drill
string can lead to an influx.
[0020] According to FIG. 1 attached, based on state-of-the-art
designing of wells for drilling, typically a margin of 0.3 pound
per gallon (ppg) is added to the pore pressure to allow a safety
factor when stopping circulation of the fluid and subtracted from
the fracture pressure, reducing even more the narrow margin, as
shown by the dotted lines. Since the plot shown in FIG. 1 is always
referenced to the static mud pressure, the compensation of 0.3 ppg
allows for the dynamic effect while drilling also. The compensation
varies from scenario to scenario but typically lies between 0.2 and
0.5 ppg.
[0021] From FIG. 1, it can be seen that the last phase of the well
can only have a maximum length of 3,000 ft, since the mud weight at
this point starts to fracture the rock, causing mud losses. If a
lower mud weight is used, a kick will happen at the lower portion
of the well. It is not difficult to imagine the problems created by
drilling in a narrow margin, with the requirement of several casing
strings, increasing tremendously the cost of the well. In some
critical cases, a difference as small as 0.2 ppg is found between
the pore and fracture pressures. Moreover, the current well design
shown in FIG. 1 does not allow to reach the total depth required,
since the bit size is continuously reduced to install the several
casing strings needed. In most of these wells, drilling is
interrupted to check if the well is flowing, and frequent mud
losses are also encountered. In many cases wells need to be
abandoned, leaving the operators with huge losses.
[0022] These problems are further compounded and complicated by the
density variations caused by temperature changes along the
wellbore, especially in deepwater wells. This can lead to
significant problems, relative to the narrow margin, when wells are
shut in to detect kicks/fluid losses. The cooling effect and
subsequent density changes can modify the ECD due to the
temperature effect on mud viscosity, and due to the density
increase leading to further complications on resuming circulation.
Thus using the conventional method for wells in ultra deep water is
rapidly reaching technical limits.
[0023] On the contrary, in the present application the 0.3 ppg
margins referred to in FIG. 1 are dispensed with during the
planning of the well since the actual required values of pore and
fracture pressures will be determined during drilling. Thus, the
phase of the well can be further extended and consequently the
number of casing strings required is greatly reduced, with
significant savings. If the case of FIG. 1 is considered, the
illustrated number of casings is 10, while by graphically applying
the method of the invention this number is reduced to 6, according
to FIG. 2 attached. This may be readily seen by considering only
the solid lines of pore and fracture gradient to define the extent
of each phase, rather than the dotted lines denoting the limits
that are in conventional use..
[0024] In order to overcome these problems, the industry has
devoted a lot of time and resources to develop alternatives. Most
of these alternatives deal with the dual-density concept, which
implies a variable pressure profile along the well, making it
possible to reduce the number of casing strings required.
[0025] The idea is to have a curved pressure profile, following the
pore pressure curve. There are two basic options:
[0026] injection of a lower density fluid (oil, gas, liquid with
hollow glass spheres) at some point;
[0027] placement of a pump at the bottom of the sea to lift the
fluid up to the surface installation.
[0028] There are advantages and disadvantages of each system
proposed above. The industry has mainly taken the direction of the
second alternative, due to arguments that well control and
understanding of two-phase flow complicates the whole drilling
operation with gas injection.
[0029] Thus, according to the IADC/SPE 59160 paper "Reeled Pipe
Technology for Deepwater Drilling Utilizing a Dual Gradient Mud
System", by P. Fontana and G. Sjoberg, it is possible to reduce
casing strings required to achieve the final depth of the well by
returning the drilling fluid to the vessel with the use of a subsea
pumping system. The combination of seawater gradient at the mud
line and drilling fluid in the wellbore results in a bottomhole
equivalent density that can be increased as illustrated in FIG. 2
of the paper. The result is a greater depth for each casing string
and reduction in total number of casing strings. It is alleged that
larger casing can then be set in the producing formation and deeper
overall well depths can be achieved. The mechanism used to create a
dual gradient system is based on a pump located at the sea
bottom.
[0030] However, there are several technical issues to be overcome
with this option, which will delay field application for some
years. The cost of such systems is also another negative aspect.
Potential problems with subsea equipment will make any repair or
problem turn into a long down-time for the rig, increasing even
further the cost of exploration.
[0031] There are three other main methods of closed system
drilling: a) underbalanced flow drilling, which involves flowing
fluids from the reservoir continuously into the wellbore is
described and documented in the literature; b) mud-cap drilling,
which involves continuous loss of drilling fluid to the formation,
in which fluid can be overbalanced, balanced or underbalanced is
also documented; c) air drilling, where air or other gas phase is
used as the drilling fluid. These methods have limited application,
i.e., underbalanced and air drilling are limited to formations with
stable wellbores, and there are significant equipment and
procedural limitations in handling produced effluent from the
wellbore. The underbalanced method is used for limited sections of
the wellbore, typically the reservoir section. This limited
application makes it a specialist alternative to conventional
drilling under the right conditions and design criteria. Air
drilling is limited to dry formations due to its limited capability
to handle fluid influxes. Similarly Mud-Cap drilling is limited to
specific reservoir sections (typically highly fractured vugular
carbonates).
[0032] Thus, the open literature is extremely rich in pointing out
methods for detecting kicks, and then methods for circulating kicks
out of the wellbore. Generally all references teach methods that
operate under conventional drilling conditions, that is, with the
well being open to the atmosphere. However, there is no suggestion
nor description of a modified drilling method and system, which, by
operating with the well closed, controlling the flow rates in and
out of the wellbore, and adjusting the pressure inside the wellbore
as required, causing that influxes (kicks) and fluid losses do not
occur or are extremely minimized, such method and system being
described and claimed in the present application.
SUMMARY OF THE INVENTION
[0033] In its broadest aspect the present invention is directed to
a system and method of drilling a well by monitoring the flow in
and out of the well, as well as monitoring of the flow rates in and
out, together with other parameters that produce an early detection
of influx or loss independent of the mass flow in and out at that
point in time, the well drilled being closed with a pressure
containment device at all times. Monitoring of flow may be by
measurement of mass and/or volume flow. In a particularly preferred
embodiment the system and method of the invention comprises
monitoring the mass flow in and out of the well. Preferably
monitoring is constant throughout a given drilling operation.
[0034] The back pressure in the well is automatically adjusted by
pressure/flow control device, controlled by a central control
device. This central control device regulates the out flow to keep
the flows in and out balanced at all times, or to preemptively
adjust the backpressure to change the ECD (Equivalent Circulating
Density) instantaneously in response to an early detection of
influx or fluid loss.
[0035] Accordingly the system of the present invention for drilling
a well while injecting a drilling fluid through an injection line
of said well and recovering through a return line of said well
where the well being drilled is closed at all times comprises a
pressure containment device and pressure/flow control device to a
wellbore to establish/maintain a back pressure on the well, means
to monitor the fluid flow in and out, means to monitor flow of any
other material in and out, means to monitor parameters affecting
the monitored flow value and means to predict a calculated value of
flow out at any given time and to obtain real time information on
discrepancy between predicted and monitored flow out and converting
to a value for adjusting the pressure/flow control device and
restoring the predicted flow value.
[0036] In a further aspect the corresponding method of the present
invention comprises, in relation to the system of the invention as
hereinbefore defined, the following steps of injecting drilling
fluid through said injection line through which said fluid is made
to contact said means for monitoring flow and recovering drilling
fluid through said return line; collecting any other material at
the surface; measuring the flow in and out of the well and
collecting flow and flow rate signals; measuring parameters
affecting the monitored flow value and means; directing all the
collected flow, correction and flow rate signals to the said
central data acquisition and control system; monitoring parameters
affecting the monitored flow value and means to predict a
calculated value of flow out at any given time and to obtain real
time information on discrepancy between predicted and monitored
flow out and converting to a value for adjusting the pressure/flow
control device and restoring the predicted flow value.
[0037] We have found by means of the system and method of the
invention that the generation of real time metering using a full
mass balance and time compensation as a dynamic predictive tool,
which can be compensated also for any operational pause in drilling
or fluid injection enables for the first time an adjustment of
fluid injection rate while continuing normal operations. This is in
contrast to known open well methods which require pausing fluid
injection and drilling to unload excess fluid, and add additional
fluid, by trial and error until pressure is restored, which can
take a matter of hours of fluid circulation to restore levels.
Moreover the system and method provide for the first time a means
for immediate restoration of pressure, by virtue of the use of a
closed system whereby addition or unloading of fluid immediately
affects the well backpressure.
[0038] We have also found that the system and method of the
invention provide additional advantages in terms of allowing
operation with a reduced reservoir volume of fluid, by virtue of
closed operation under back pressure. Moreover the system and
method can be operated efficiently, without the need for repeated
balancing of the system after any operational pause in drilling, by
virtue of the ability to continuously circulate fluid even during
pauses in drilling, avoiding any undue changes in fluid density and
temperature.
[0039] Preferably the system for drilling a well while injecting a
drilling fluid through an injection line of said well and
recovering through a return line of said well where the well being
drilled is closed at all times comprises:
[0040] a) a pressure containment device;
[0041] b) a pressure/flow control device on the outlet stream;
[0042] c) means for measuring mass and/or volumetric flow and flow
rate on the inlet and outlet streams to obtain real time mass or
volumetric flow signals;
[0043] d) means for measuring mass and/or volumetric flow and flow
rate of any other materials in and out;
[0044] e) means for directing all the flow, pressure and
temperature signals so obtained to a central data acquisition and
control system; and
[0045] g) a central data acquisition and control system programmed
with a software that can determine a real time predicted out flow
and compare it to the actual out flow estimated from the mass and
volumetric flow rate values and other relevant parameters.
[0046] Preferably the means c) for measuring mass flow comprises a
volume flow meter and at least one pressure sensor to obtain
pressure signals and at least one temperature sensor to obtain
temperature signals; and may be a mass flow meter comprising
integral pressure and temperature sensors to compensate for changes
in density and temperature; and the means c) for measuring flow
rate comprises means for assessing the volume of the hole at any
given time, as a dynamic value having regard to the continuous
drilling of the hole. At least one additional pressure and
temperature sensor may be provided to monitor other parameters that
produce an early detection of influx or loss independent of the
mass flow in and out at that point in time.
[0047] Preferably the means d) comprise means for measuring
cuttings volume/mass out.
[0048] Most preferably the system comprises:
[0049] a) a pressure containment device;
[0050] b) a pressure/flow control device on the outlet stream;
[0051] c) means for measuring mass flow rate on the inlet and
outlet streams;
[0052] d) means for measuring volumetric flow rate on the inlet and
outlet streams;
[0053] e) at least one pressure sensor to obtain pressure data;
[0054] f) at least one temperature sensor to obtain temperature
data;
[0055] g) a central data acquisition and control system that sets a
value for an expected out flow and compares it to the actual out
flow estimated from data gathered by the mass and volumetric flow
rate meters as well as from pressure and temperature data, and in
case of a discrepancy between the expected and actual flow values,
adjusting the said pressure/flow control device to restore the
outflow to the expected value.
[0056] In a further aspect of the invention there is provided a
method for drilling a well while injecting a drilling fluid through
an injection line of said well and recovering through a return line
of said well where the well being drilled is closed at all times
comprising the following steps:
[0057] a) providing a pressure containment device, suitably of a
type that allows passage of pipe under pressure, to a wellbore;
[0058] b) providing a pressure/flow control device to control the
flow out of the well and to keep a back pressure on the well;
[0059] c) providing a central data acquisition and control system
and related software;
[0060] d) providing mass flow meters in both injection and return
lines;
[0061] e) providing flow rate meters in both injection and return
lines;
[0062] f) providing at least one pressure sensor;
[0063] g) providing at least one temperature sensor;
[0064] h) injecting drilling fluid through said injection line
through which said fluid is made to contact said mass flow meters,
said fluid flow meters and said pressure and temperature sensors,
and recovering drilling fluid through said return line;
[0065] i) collecting drill cuttings at the surface;
[0066] j) measuring the mass flow in and out of the well and
collecting mass flow signals;
[0067] k) measuring the fluid flow rates in and out of the well and
collecting fluid flow signals;
[0068] l) measuring pressure and temperature of fluid and
collecting pressure and temperature signals;
[0069] m) directing all the collected flow, pressure and
temperature signals to the said central data acquisition and
control system;
[0070] n) the software of the central data acquisition and control
system considering, at each time, the predicted flow out of the
well taking into account several parameters;
[0071] o) having the actual and predicted out flows compared and
checked for any discrepancy, compensated for time lags in between
input and output;
[0072] p) in case of a discrepancy, having a signal sent by the
central data acquisition and control system to adjust the
pressure/flow control device and restore the predicted out flow
rate, without interruption of the drilling operation.
[0073] Optionally the method comprises additionally providing a
means of measurement of drill cuttings rate, mass or volume, when
required, to measure the rate of cuttings being produced from the
well.
[0074] Preferably the system and method comprise additionally means
to pressurize the wellbore through the annulus, and a step of
pressurising the wellbore through the annulus, independently of the
current fluid injection path.
[0075] Therefore, the present invention provides a safe method for
drilling wells, since not only is the well being drilled closed at
all times, but also any fluid loss or influx that occurs is more
accurately and faster determined and subsequently controlled than
in state-of-the-art methods.
[0076] One advantage of the present method over state-of-the-art
methods is that it is able to instantly change the ECD (Equivalent
Circulating Density) by adjusting the backpressure on the wellbore
by closing or opening the pressure/flow control device. In this
manner the method herein described and claimed incorporates early
detection methods of influx/loss that are existing or yet to be
developed as part of the method herein described and claimed, e.g.,
tools under development or that may be developed that can detect
trace hydrocarbon influx, small temperature variations, pressure
pulses etc. The output of these tools or technology that indicates
a kick or fluid loss can be used as a feedback parameter to yield
an instant reaction to the detected kick or fluid loss, thus
controlling the drilling operation at all times.
[0077] As a consequence, in a patentably distinguishing manner, the
method of the invention allows that drilling operations be carried
out in a continuous manner, while in state-of-the-art methods
drilling is stopped and mud weight is corrected in a lengthy,
time-consuming step, before drilling can be resumed, after a kick
or fluid loss is detected.
[0078] This leads to significant time savings as the traditional
approach to dealing with influxes is very time-consuming: stopping
drilling, shutting in the well, observing, measuring pressures,
circulating out the influx by the accepted methods, and adjusting
the mud weight. Similarly a loss of drilling fluid to the formation
leads to analogous series of time-consuming events.
[0079] The present invention provides also a method of drilling
where the bottomhole pressure can be very close to the pore
pressure, thus reducing the overbalanced pressure usually applied
on the reservoir, and consequently reducing the risk of fluid
losses and subsequent contamination of the wellbore causing damage,
the overall effect being that the well productivity is
increased.
[0080] The present invention provides further a method to drill
with the exact bottomhole pressure needed, with a direct
determination of the pore pressure.
[0081] The present invention provides also a method for the direct
determination of the fracture pressure if needed.
[0082] Since both the fracture and pore pressure curves are
estimated and usually are not accurate, the present invention
allows a significant reduction of risk by determining either the
pore pressure or the fracture pressure, or, in more critical
situations, both the pore and fracture pressure curves in a very
accurate mode while drilling the well. Therefore by eliminating
uncertainties from pore and fracture pressures and being able to
quickly react to correct any undesired event, the present method is
consequently much safer than state-of-the-art drilling methods.
[0083] The present invention provides further a drilling method
where the elimination of the kick tolerance and tripping margin on
the design of the well is made possible, since the pore and
fracture pressure will be determined in real time while drilling
the well, and, therefore, no safety margin or only a small one is
necessary when designing the well. The kick tolerance is not needed
since there will be no interruption in the drilling operation to
circulate out any gas that might have entered into the well. Also,
the tripping margin is not necessary because it will be replaced by
the back pressure on the well, adjusted automatically when stopping
circulation.
[0084] By the fast detection of any influx and by having the well
closed and under pressure at all times while drilling, the present
invention allows the well control procedure to be much simpler,
faster, and safer, since no time is wasted in checking the flow,
closing the well, measuring the pressure, changing the mud weight
if needed, and circulating the kick out of the well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0085] FIG. 1 attached is a state-of-the-art log of pore and
fracture pressure curves indicated hereinbefore. Included in this
figure are the kick tolerance and tripping margin, used for
designing the casing setting points, in this case taken as 0.3 ppg
below the fracture pressure and above the pore pressure,
respectively. This value is commonly used in the industry. On the
right hand side the number and diameter of the casing strings
required to safely drill this well using the current conventional
drilling method is shown. As pointed out before, the two curves
shown are estimated before drilling. Actual values might never be
determined by the current conventional drilling method.
[0086] FIG. 2 attached is a log of the same curves according to the
invention, without the kick tolerance and tripping margin of 0.3
ppg included. On the right hand side the number of casing strings
required can be seen. With the drilling method described in the
present application the elimination of the kick tolerance and
tripping margin on the design of the well is made possible, since
the pore and fracture pressure will be determined in real time
while drilling the well, with the well being drilled closed at all
times, and, therefore, no safety margin is necessary when designing
the well.
[0087] FIG. 3 attached is a state-of-the-art schematics of the
circulating system of a standard rig, with the return flow open to
the atmosphere.
[0088] FIG. 4 attached is a schematic of the circulating system of
a rig with the drilling method described in the application. A
pressure containment device located at the wellhead, mass flow and
fluid flow rate meters on the inlet and outlet streams, pressure
and temperature sensors, cuttings mass/volume measurement device,
and other pieces of equipment have been added to the standard
drilling rig configuration. The control system receives all the
data gathered and actuates the pressure/flow control device on the
outlet stream.
[0089] FIG. 5 attached is a general block diagram of the method
described in the present invention.
[0090] FIG. 6 attached is a flowsheet that schematically
illustrates the method of the invention.
PREFERRED MODE--DETAILED DESCRIPTION
[0091] As pointed out hereinbefore, the present system and method
of drilling wells is based on a closed-loop system. The inventive
method and system is applied to oil and gas wells, as well as to
geothermal wells.
[0092] As regards the mud circuit, the circulation of the drilling
fluid down the wellbore may be through the drill string and the
return through the annulus, as in state-of-the-art methods, but not
limited to it. As a matter of fact, any way of circulation of the
drilling fluid may be successfully employed in the practice of the
present system and method, no matter where the fluids are injected
or returned.
[0093] In a very broad way, the system and method of the invention
comprises adjusting the wellbore pressure with the aid of a
pressure/flow control device to correct the bottomhole pressure to
prevent fluid influx or losses in a pro-active as opposed to the
state-of-the-art reactive manner.
[0094] While several of the devices being described have been used
in some configuration or combination, and several of the parameter
measurements have been included in descriptive methods on patents
or literature, none have ever:
[0095] 1. Simultaneously combined the measurement of all critical
parameters to ensure the necessary accuracy required allowing such
a system to effectively function as a whole method.
[0096] 2. Utilized mass flow meters simultaneously on inlet and
outlet flows.
[0097] 3. Utilized mass measurement of cuttings in conjunction with
mass flow measurement on inlet and outlet.
[0098] 4. Utilized a pressure/flow control device as an instant
control of ECD during drilling for the purpose of preventing and
controlling influx or losses.
[0099] 5. Defined the use of a pressure/flow control device as a
pro-active method for adjusting ECD based on early detection of
influx/loss events.
[0100] The method and system of the invention will now be described
in more detail based on the appended FIGURES.
[0101] FIG. 3 illustrates a drilling method according to
state-of-the-art techniques. Thus, a drilling fluid is injected
through the drill string (1), down the wellbore through the bit (2)
and up the annulus (3). At the surface the fluid that is under
atmospheric pressure is directed to the shale shaker (4) for
solid/liquid separation. The liquid is directed to the mud tank (5)
from where the mud pumps (6) suck the fluid to inject it through
the drill string (1) and close the circuit. In case of a kick,
normally detected by mud tank volume variation indicated by level
sensors (7), the BOP (8) must be closed to allow kick control. At
this point the drilling operation is stopped to check pressure and
adjust the mud weight to avoid further influxes. Improvements in
state-of-the-art drilling methods are generally directed to, for
example, improve the measurement of volume increase or decrease in
tank (5). However, such improvements bring only minor changes to
the kick detection procedure; furthermore, no fundamental
modifications are known directed to the improvement of safety
and/or to keeping the drilling method continuous, this modification
being only brought about by the present invention.
[0102] On the contrary, according to FIG. 4 that illustrates one
embodiment of the invention, the drilling fluid is injected through
the drill string (1), going down towards the bottom hole through
the bit (2) and up the annulus (3) and is diverted by a pressure
containment device (26) through a closed return line (27) under
pressure. BOP (8) remains open during drilling. The fluid is made
to contact pressure and temperature sensors (9), fluid flow meter
(10), mass flow meter (11), flow/pressure control device (12),
degasser (13) then to the shale shaker (4). The shale shaker (4)
separates the cuttings (drill solids) from the liquid and the
solids have their mass/volume determined (19) while the liquid is
directed to the mud tank (5) having the mass/volume determined as
well (20). All standard drilling parameters are acquired by a
device (21) normally called mud logging. Downhole parameters are
acquired by a device (24) located close to the bit (2). The
mass/volume of gas separated in degasser (13) is measured by a
device (25).
[0103] The drilling fluid is injected with the aid of pump (6)
through an injection line (14) through which said fluid is made to
contact mass flow meter (15), fluid flow meter (16), pressure and
temperature sensors (17). Devices (7), (9), (10), (11), (15), (16),
(17), (19), (20), (21), (24), (25) all acquire data as signals that
are directed to a central data acquisition and control system (18).
System (18) sends a signal to the pressure/flow control device (12)
to open or close it. Whenever it is deemed necessary, a pump (23)
may send fluid directly to the annulus (3) through a dedicated
injection line (22) via a mass flow meter (28), fluid flow meter
(28) and pressure and temperature sensors (28). For figure
simplification these three devices are shown in just one piece of
equipment. This injection line may be incorporated as part of the
standard circulation system, or embodied in other ways, the purpose
being to provide an independent, of normal drilling circulation,
means of flow into wellbore. The central data acquisition and
control system (18) acquires data from device (28).
[0104] According to the concept of the present invention, as
illustrated in FIG. 4, a pressure containment device (26) diverts
the drilling fluid and keeps it under pressure. Device (26) may be
a rotating BOP or a rotating control head, but not limited to it.
The location of device (26) is not critical. It may be located at
the surface or at some point further down e.g. on the sea floor,
inside the wellbore, or at any other suitable location. The
drilling fluid is diverted to a closed pipe (27) and then to a
surface system. The type and design of the device (26) is not
critical and depends on each well being drilled. It is a standard
equipment that is commercially available or readily adapted from
existing designs.
[0105] As described hereinbefore, upon a signal received from
control system (18) the pressure/flow control device (12) opens or
closes to allow decrease or increase of the backpressure at the
well head so that the outflow can be restored to the predicted
value determined by system (18). Two or more of these pressure/flow
control devices (12) can be installed in parallel with isolation
valves to allow redundant operation. Devices (12) can be positioned
downstream of the pressure containment device (26) at any suitable
point in the surface system. Some surface systems may incorporate
two or more of such devices (12) at different nodes.
[0106] One critical aspect of the present method is the accurate
measurement of the injected and returned mass and fluid flow rates.
The equipment used to carry out such measurement is mass flow
meters (11,15) and fluid flow meters (10,16). The equipment is
installed in the injected (14) and return (27) fluid lines. These
meters may also be installed at the gas outlet (25) of the degasser
(13) and somewhere (20) on the fluid line between shale shaker (4)
and tank (5). Also they may be installed on the independent
injection line (22). The mass and fluid flow meters are
commercially available equipment. Multi-phase meters are also
commercially available and may be used. The precision of this
equipment, allows accurate measurement, subsequent control and
safer drilling.
[0107] To further improve the accuracy of the method the cuttings
mass/volume rate can be measured by commercially available
equipment (19) to verify that the mass of cuttings being received
back at the surface is correlated with the rate of penetration and
wellbore geometry. This data allows correction of the mass flow
data and allows identification of trouble events.
[0108] The measurements of mass and fluid flow rates provide data
that are collected and directed to a central data acquisition and
control system (18).
[0109] The central data acquisition and control system (18) is
provided with a software designed to predict an expected, ideal
value for the outflow, said value being based on calculations
taking into account several parameters including but not restricted
to rate of penetration, rock and drilling fluid density, well
diameter, in and out flow rates, cuttings return rate, bottomhole
and wellhead pressures and temperatures.
[0110] Said software compares the said predicted ideal value with
the actual, return flow rate value as measured by the mass flow
meters (11,15) and fluid flow meters (10,16). If the comparison
yields any discrepancy, the software automatically sends a command
to a pressure/flow control device (12) designed to adjust the
return flow rate so as to restore the said return flow rate to the
predicted, ideal value.
[0111] Said software can also receive as input any early detection
parameters available or being developed or capable of being
developed. Such input will trigger a chain of investigation of
probable scenarios, checking of actual other parameter and any
other means (databased or software or mathematical) to ascertain
that an influx/loss event has occurred. Said software will in such
cases pre-emptively adjust backpressure to immediately control the
event.
[0112] Said software will allow for override of the standard
detection (state-of-the-art) by the early detection system of the
invention and will compensate and filter for any conflict in
fluid/mass flow indication.
[0113] Said software may have filters, databases, historical
learning and/or any other mathematical methods, fuzzy logic or
other software means to optimize control of the system.
[0114] The pressure/flow control device (12) used to restore the
ideal flow is an equipment chosen according to the well parameters
such as diameter of the return line, pressure and flow
requirements. The pressure/flow control device (12) is, as
previously stated, standard, commercially available equipment.
Alternatively, it may be specifically designed for the required
purpose.
[0115] According to the present method, the flow rates in and out
of the wellbore are controlled, and the pressure inside the
wellbore is adjusted by the pressure/flow control device (12)
installed on the return line (27) or further downstream in the
surface system.
[0116] Thus, if the drilling fluid volume returning from the
wellbore is increasing, after compensating for all possible factors
it is a sign that an influx is happening. In this case the surface
pressure should be increased to restore the bottomhole pressure in
such a way as to overcome the reservoir pressure.
[0117] On the other hand, if the fluid volume returning is
decreasing, after compensating for all possible factors it means
the pressure inside the wellbore is higher than the fracture
pressure of the rock, or that the sealing of the drilling mud is
not effective. Therefore, it is necessary to reduce the wellbore
pressure, and the reduction will take place by lowering the surface
back pressure sufficiently to restore the normal condition.
[0118] If an early detection signal is confirmed, control system
(18) will proactively adjust the backpressure by opening or closing
pressure/flow control device (12) to suit the occurred event.
[0119] Thus, upon any undesired event, the system acts in order to
adjust the rate of return flow and/or pressure thus increasing or
decreasing the backpressure, while creating the desired condition
downhole of no inflow from the exposed formation or no loss of
fluid to the same exposed formation. This is coupled with a
feedback loop to constantly monitor the reaction to each action, as
well as the necessary software design, and any necessary decision
system including but not limited to databases and fuzzy logic
filters to ensure consistent operation.
[0120] The system and method of drilling oil, gas and geothermal
wells according to the present invention is based on the principle
of mass conservation, a universal law.
[0121] While drilling a well, loss of fluid to the rock or influx
from the reservoir is common, and should be avoided to eliminate
several problems. By applying the principle of mass conservation,
the difference in mass being injected and returned from the well,
compensated for increase in hole volume, additional mass of rock
returning and other relevant factors, including but not limited to
thermal expansion/contraction and compressibility changes, is a
clear indication of what is happening downhole.
[0122] Therefore, the expression "mass flow" as used herein means
the total mass flow being injected and returned, comprised of
liquid, solids, and possibly gas.
[0123] In order to increase the accuracy of the method and to
expedite detection of any undesired event, the flow rates in and
out of the well are also monitored at all times. This way, the
calculation of the predicted, ideal return flow of the well can be
done with a certain redundancy and the detection of any discrepancy
can be made with reduced risks.
[0124] It should be understood that all the devices used in the
present system and method, such as flow metering system, pressure
containment device, pressure and temperature sensors, pressure/flow
control device are commercial devices and as such do not constitute
an object of the invention.
[0125] Further, it is within the scope of the application that any
improvements in mass/flow rate measurements or any other measuring
device can be incorporated into the method. Also comprised within
the scope of the application are any improvements in the accuracy
and time lag to detect influx or fluid losses as well as any
improvements in the system (18) to manipulate the data and make
decisions related to restore the predicted flow value.
[0126] Thus, improved detection, measurement or actuation tools are
all comprised within the scope of the application.
[0127] It has been shown that measurement of the flow rate only is
not accurate enough to provide a clear indication of losses or
gains while drilling. That is why the present method envisages the
addition of an accurate mass flow metering (11,15) system that
allows the present drilling method to be much safer than
state-of-the-art drilling methods.
[0128] This mass flow metering principle is extended to include
other subcomponents of the system where accuracy can be improved,
such as, but not limited to measuring the mass flux of cuttings
(19) being produced at the shakers (4) and mass outflow of gas (25)
from degasser (13), to allow verification and/or improvement of the
mass balance being continuously applied to the system.
[0129] Another very important device used in the method and system
of this invention is the pressure containment equipment (26), to
keep the well flowing under pressure at all times. By controlling
the pressure inside the well with a pressure/flow control device
(12) on the return line (27) the bottomhole pressure can be quickly
adjusted to the desired value so as to eliminate the losses or
gains being detected.
[0130] By having a pressure sensor (24) at the bottom of the string
(1) and another one (9) at the surface, the pore and fracture
pressures of the formations can be directly determined,
dramatically improving the accuracy of such pressure values.
[0131] The assessment of the pore and fracture pressures according
to the method of the invention is carried out the following way: if
the central data acquisition and control system (18) detects any
discrepancy and a decision to actuate the pressure/flow control
device (12) is made, it is a sign that either a fluid loss or
influx is occurring. The Applicant has thus ascertained that if
there is a fluid loss this means that the bottomhole pressure being
recorded is equivalent to the fracture pressure of the
formation.
[0132] On the contrary, if an influx is detected, this means that
the bottomhole pressure being recorded is equivalent to the pore
pressure of the formation.
[0133] Further, in case of the absence of the pressure sensor in
the bottomhole, the variables pore pressure and fracture pressure
can be estimated. Thus, the bottomhole pressure is not one of the
variables being recorded and only the wellhead or surface pressure
is the pressure variable being acquired. The pore pressure and the
fracture pressure can then be indirectly estimated by adding to the
obtained value the hydrostatic head and friction losses within the
wellbore.
[0134] The software pertaining to the central data and control
system (18) would include all the necessary algorithms, empirical
correlations or other method to allow accurate estimation of the
hydrostatic head and friction losses including any transient
effects like, but not limited to, changing temperature profile
along the wellbore.
[0135] Usually, indirect estimation made before drilling, based on
correlations from logs, or during drilling using drilling
parameters are the best alternatives to determine the pore
pressure. Similarly, fracture pressure is also indirectly estimated
from logs before drilling. In some situations the fracture pressure
is determined at certain points while drilling, usually when a
casing shoe is set, not along the whole well.
[0136] Advantageously, when using the method and system of the
invention the pore and fracture pressure may be directly determined
while drilling the well. This entails great savings as regards
safety and time, two parameters of utmost importance in drilling
operations.
[0137] In state-of-the-art methods, the bottomhole pressure is
adjusted by increasing or reducing the mud weight. The increase or
reduction in mud weight is most of the time effected based on
quasi-empirical methods, which by definition implies inaccuracies,
which are handled by an iterative process of: --adjusting mud
weight, measuring mud weight--this process being repeated until the
desired value is reached. To further complicate the matter, due to
the time lag, caused by the circulation time (i.e., time for a full
loop movement of a unit element of mud), the adjustments must be
made in stages, e.g., in order to quickly contain an influx, a
higher density mud is introduced into the system to produce an
increase in ECD (Equivalent Circulating Density). At the point
where additional hydrostatic head of this higher density mud,
coupled with the hydrostatic head of lower density mud, initially
in circulation, becomes close to being sufficient to contain the
influx, another variation in density of mud must be executed in
order not to increase the ECD to the point of creating losses. This
is further complicated by the fact that such density adjustments
affect the rheology (viscosity, yield point, etc.) of the mud
system leading to changes in the friction component, which in turn
has a direct effect on the ECD. So, in practice, the adjustment of
mud weight is not always successful in restoring the desired
equilibrium of fluid circulation in the system. Inaccuracy,
depending on its extent, may lead to hazardous situations such as
blowouts.
[0138] On the contrary, the method and system of the invention
allows for a precise adjustment of increase or reduction in
bottomhole pressure. By using the pressure/flow control device (12)
to restore the equilibrium and pressures inside the wellbore, the
adjustment is much faster achieved, avoiding the hazardous
situation of well-known methods.
[0139] It should also be pointed out that in state-of-the-art
methods the needed bottomhole pressures needed to restore the
equilibrium are estimated under static conditions, since these
determinations are made without fluid circulation. However, the
influxes or fluid losses are events that occur under dynamic
conditions. This implies in even more errors and inaccuracies.
[0140] Also the speed of adjustment is much greater in the present
method, as opposed to the conventional situation, where increasing
the density (weighting up) or decreasing the density (cutting back)
is a very time consuming process. It has been cited before that
while drilling the ECD is the actual pressure that needs to
overcome the formation pressure to avoid influx. However, when the
circulation is stopped to make a connection, for example, the
friction loss is zero and thus the ECD reduces to the hydrostatic
value of the mud weight. In scenarios of very narrow mud window,
the margin can be as low as 0.2 ppg. In these cases, it is common
to observe influxes when circulation is interrupted, increasing
substantially the risks of drilling with the conventional drilling
system.
[0141] On the contrary, since the present method operates with the
well closed at all times which implies a back pressure at all
times, this back pressure may be adjusted to compensate for dynamic
friction losses when the mud circulation is interrupted, avoiding
the influx of reservoir fluids (kick). Thus the improved safety of
the method of the invention relative to the state-of-the-art
drilling methods may be clearly seen.
[0142] Replacement of the dynamic friction loss when the pump stops
can be accomplished by slowly reducing the circulation rate through
the normal flow path and simultaneously closing the pressure
flow/contol device and trapping a backpressure that compensates for
the loss in friction head.
[0143] This same purpose of keeping an unchanging pressure at the
bottomhole during circulation stops can be more readily achieved by
the following method: the back pressure adjustment can be applied
by pumping fluid, independent of the normal circulating flow path,
into the wellbore, to compensate for the loss in friction head, and
effecting a continuous flow that allows easy control of the back
pressure by adjustment of the pressure/flow control device (12).
This fluid flow may be achieved completely independent of the
normal circulating path by means of a mud pump (23) and injection
line (22).
[0144] Therefore, a circulation bypass composed of a pump (23) and
a dedicated injection line (22) to the wellbore annulus allows
keeping a constant pressure downhole during circulation stops and
continuously detecting any changes in the mass balance indicative
of an influx or loss during the circulation stop.
[0145] By using the method and system of the invention, the errors
from estimating the required mud weight based on static conditions
are avoided since the measurements are effected under the same
dynamic conditions as those when the actual events occur.
[0146] This method also renders possible to run the mud density at
a value slightly lower than that required to balance the formation
pressure and using the backpressure on the well to exert an
extremely controllable ECD at the bottomhole that has the
flexibility to be instantaneously adjusted up or down. This will be
the preferred method in wells with very narrow pore
pressure/fracture pressure margins as occur in some drilling
scenarios.
[0147] In this case one of the parameters mentioned in Table 1,
which is the advantage of having three safety barriers is negated.
However, the current technical limit on some ultra-deep water
wells, due to the narrow margin, when drilling with the
state-of-the art method, leads to a sequence of fluid
influxes/losses due to the inaccuracies in manually controlling the
mud density and subsequent ECD as described above, that can lead to
loss of control of the drilling situation and has resulted in the
abandonment of such wells due to the safety risks and technical
inability to recover from the situation.
[0148] However, the method of the invention allows, by creating an
instant control mud weight window, controlling the ECD by
increasing or decreasing the backpressure, controlled by the
positioning of the pressure/flow control device, to create the
conditions for staying within the narrow margin. This results in
the technical ability to drill wells in very adverse conditions as
in narrow mud weight window, under full control with the consequent
improvement in safety as the well is at all times in a stable
circulating condition, while still retaining two barriers ie. the
BOP (blow-out preventer), and the pressure containment device.
[0149] The central data acquisition and control system (18) has a
direct output for actuation of the pressure/flow control device(s)
(12) downstream the wellhead opening or closing the flow out of the
well to restore the expected value. At this point, if an action is
needed, the bottomhole pressure is recorded and associated to the
pore or fracture pressure, if a gain or loss is being observed,
respectively.
[0150] In case an influx of gas occurs, the circulation of the gas
out of the well is immediately effected. By closing the
pressure/flow control device (12) to restore the balance of flow
and the predicted value, the bottomhole pressure reaches back a
value that avoids any further influx. At this point no more gas
will enter the well and the problem is limited to circulating out
the small amount of gas that might have entered the well. Since the
well that is being drilled is closed at all times, there is no need
to stop circulation, check if the well is flowing, shut-in the BOP,
measure the pressures, adjust the mud weight, and then circulate
the kick out of the well as in standard methods. The mass flow
together with the flow rate measurements provide a very efficient
and fast way of detecting an inflow of gas. Also, the complete
removal of the gas from the well is easily determined by the
combination of the mass flow and flow rates in and out of the
well.
[0151] Also the incorporation of early detection of influx/loss
devices, which can preemptively result in opening or closing the
pressure/flow control device (12), as part of the system, will
allow pro-active reaction to influx/losses not achieved by state-of
the-art systems.
[0152] The function of the rotating pressure containment device
(26) is to allow the drill string (1) to pass through it and
rotate, if a rotating drilling activity is carried on. Thus, the
drill string (1) is stripped through the rotating pressure
containment device; the annulus between the outside of the drill
pipe and the inside of the wellbore/casing/riser is closed by this
equipment. The rotating pressure containment device (26) can be
replaced by a simplified pressure containment device such as the
stripper(s) (a type of BOP designed to allow continuous passage of
non-jointed pipe) on coiled tubing operations. The return flow of
drilling fluid is, therefore, diverted to a closed pipe (27) to the
surface treatment package. This surface package should be composed
of at least a degasser (13) and shale shaker (4) for solids
separation. This way the influxes can be automatically handled.
[0153] In a more appropriate configuration, a closed 3-phase
separator (liquid, solid and gas) could be installed replacing the
degasser (13). In this case a fully closed system is achieved. This
may be desirable when dealing with hostile fluids or fluids posing
environmentally risks.
[0154] The central data acquisition and control system (18)
receives all the signals of different drilling parameters,
including but not limited to injection and return flow rates,
injection and return mass flow rates, back-pressure at the surface,
down-hole pressure, cuttings mass rates, rate of penetration, mud
density, rock lithology, and wellbore diameter. It is not necessary
to use all these parameters with the drilling method herein
proposed.
[0155] The central data acquisition and control system (18)
processes the signals received and looks for any deviation from
expected behavior. If a deviation is detected, the central data
acquisition and control system (18) activates the flow
pressure/flow control device (12) to adjust the back-pressure on
the return line (27). This is coupled with a feedback loop to
constantly monitor the reaction to each action, as well as the
necessary software design, and any necessary decision system
including but not limited to databases and fuzzy logic filters to
ensure consistent operation.
[0156] In spite of the fact that some early-detection means have
been described, it should be understood that the present method and
system is not limited to the described items. Thus, an influx may
be detected by other means including but not limited to downhole
temperature effects, downhole hydrocarbon detection, pressure
changes, pressure pulses; said system pre-emptively adjusting
backpressure on the wellbore based on influx or loss indication
before surface system detection.
[0157] The drilling of the well is done with the rotating pressure
containment device (26) closed against the drill string. If a
deviation outside the predicted values of the return flow and mass
flow rates is observed, the control system (18) sends a signal
either to open the flow, reducing the back-pressure or restricting
the flow, increasing the back-pressure.
[0158] This deviation may also be a signal from an early detection
device.
[0159] The first option (flow opening) is applied in case a fluid
loss is detected and the second one (flow restriction), if a fluid
gain is observed. The changes in flow are done in steps previously
defined steps. These step changes can be adjusted as the well is
drilled and the effective pore and fracture pressures are
determined.
[0160] The whole drilling operation is continuously monitored so
that a switch to a manual control can be implemented, if anything
goes wrong. Any adjustments and modifications can also be
implemented as the drilling progresses. If at all desired,
restoring to the state-of-the-art drilling method is easily done,
by not using anymore the rotating pressure containment device (26)
against the drill string (1), allowing the annulus to be open to
the atmosphere again.
[0161] A block diagram of the method described in the present
invention is shown in FIG. 5.
[0162] In fact, the present system and method implies many
variations and modifications within its scope and as such it can be
applied to all kinds of wells, onshore as well as offshore, and the
equipment location and distribution can vary according to the well,
risks, application and restrictions of each case.
[0163] FIG. 6 is a flowchart illustrating the drilling method of
the invention in a schematic mode, with the decision-making process
that leads to the restoration of the predicted flow as determined
by the central data acquisition and control system.
[0164] It has been mentioned before that in the conventional
drilling methods the hydrostatic pressure exerted by the mud column
is responsible for keeping the reservoir fluids from flowing into
the well. This is called a primary safety barrier. All drilling
operations should have two safety barriers, the second one usually
being the blow-out preventer equipment, which can be closed in case
an influx occurs. The drilling method and system herein described
introduces for the first time three safety barriers during
drilling, these being the drilling fluid, the blow-out preventer
equipment, and the rotating pressure containment device.
[0165] In underbalanced drilling (UBD) operations, there are just
two barriers, the rotating pressure containment device and the
blow-out preventer, since the drilling fluid inside the wellbore
must exert a bottomhole pressure smaller than the reservoir
pressure to allow production while drilling.
[0166] As noted before, there are three other main methods of
closed system drilling, known as underbalanced drilling (UBD),
mud-cap drilling, and air drilling. All three methods have
restricted operating scenarios applicable to small portions of the
wellbore, with mud-cap drilling and air drilling only usable under
very specific conditions, whereas the method herein described is
applicable to the entire length of the wellbore.
[0167] TABLE 1 below shows the key differences among the
traditional drilling system (Conv.), compared with the
underbalanced drilling system (UBD) and the present drilling method
herein proposed. It can be seen that the key points addressed by
the present application are not covered or considered by either the
traditional conventional drilling system or by the underbalanced
drilling method currently used by the industry.
1TABLE 1 Feature UBD Conv. INVENTION Well closed at all times Yes
No Yes Production of reservoir fluids while Yes No No drilling Flow
rates measured in and out Yes Yes Yes Mass flow measured in No No
Yes Mass flow measured out Yes No Yes Pressure/flow control device
on the Yes No Yes return line Return flow adjusted automatically No
No Yes according to mass balance Degasser device on the return line
Yes No Yes Kick detection accurate and fast N/A No Yes Real
time.sup.1 kick/loss detection while No No Yes drilling Can
instantly utilize input from early N/A No Yes detection of
kick/loss Bottom-hole pressure instantly.sup.2 adjusted No No Yes
from surface with small action Three safety barriers while drilling
No No Yes Accurate pore and fracture pressure No No Yes
determination while drilling Can keep a constant pressure at bottom
No No Yes hole during connections and trips Immediate control of
the well in case of N/A No Yes kick Can be used to drill the entire
well No Yes Yes Can be used to drill safely within a very No No Yes
narrow pore/fracture pressure margin Where N/A = not applicable
.sup.1real time is the determination of the pore and fracture
pressure at the moment the influx of fluid loss occurs, rather than
by means of calculation after some period of time. .sup.2the
underbalanced drilling case here considers a two-phase flow, the
most common application of this type of drilling system.
[0168] The present method is applicable to the whole wellbore from
the first casing string with a BOP connection, and to any type of
well (gas, oil or geothermal), and to any environment (land,
offshore, deep offshore, ultra-deep offshore). It can be
implemented and adopted to any rig or drilling installation that
uses the conventional method with very few exceptions and
limitations.
[0169] Thus the present method can be called INTELLIGENT SAFE
DRILLING, since the response to influx or losses is nearly
immediate and so smoothly done that the drilling can go on without
any break in the normal course of action, this representing an
unusual and unknown feature in the technique.
[0170] Therefore, the present system and method of drilling makes
possible:
[0171] i) accurate and fast determination of any difference between
the in and out flow, detecting any fluid losses or influx;
[0172] ii) easy and fast control of the influx or losses;
[0173] iii) strong increase of drilling operations safety in
challenging environments, such as when drilling in narrow margin
between pore and fracture pressures;
[0174] iv) strong increase of drilling operations safety when
drilling in locations with pore pressure uncertainty, such as
exploration wells;
[0175] v) strong increase of drilling operations safety when
drilling in locations with high pore pressure;
[0176] vi) easy switch to underbalanced or conventional drilling
modes;
[0177] vii) drilling with minimum overbalance, increasing the
productivity of the wells, increasing the rate of penetration and
thus reducing the overall drilling time;
[0178] viii) direct determination of both the pore and fracture
pressures;
[0179] ix) a large reduction in time and therefore cost spent
weighting (increasing density) and cutting back (decreasing
density) mud systems;
[0180] x) a large reduction in the cost of wells by reduction in
the number of necessary casing strings;
[0181] xi) a significant cost reduction in the cost of wells by
significantly reducing or eliminating completely the time spent on
the problems of differential sticking, lost circulation;
[0182] xii) significantly reducing the risk of underground
blow-outs;
[0183] xiii) a significant reduction of risk to personnel compared
to conventional drilling due to the fact that the wellbore is
closed at all times, e.g., exposure to sour gas;
[0184] xiv) a significant cost reduction due to lowering quantities
of mud lost to formations;
[0185] xv) a significant improvement in productivity of producing
horizons by reduction of fluid loss and consequential permeability
reduction (damage);
[0186] xvi) a significant improvement in exploration success as
fluid invasion due to overweighted mud is limited. Such fluid
invasion can mask the presence of hydrocarbons during evaluation by
electric logs;
[0187] xv) to drill wells in ultra deep water that are reaching
technical limit with conventional state-of-the art method;
[0188] xvi) to economically drill ultra-deep wells onshore and
offshore by increasing the reach of casing strings.
* * * * *