U.S. patent application number 14/607974 was filed with the patent office on 2016-07-28 for look ahead pore pressure prediction.
This patent application is currently assigned to STATOIL GULF SERVICES LLC. The applicant listed for this patent is STATOIL GULF SERVICES LLC. Invention is credited to Ivar BREVIK, Giuseppe DE PRISCO, Alejandro Bello PALACIOS.
Application Number | 20160215565 14/607974 |
Document ID | / |
Family ID | 56433192 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160215565 |
Kind Code |
A1 |
DE PRISCO; Giuseppe ; et
al. |
July 28, 2016 |
LOOK AHEAD PORE PRESSURE PREDICTION
Abstract
A method for predicting jumps in pore pressure of a subsurface
includes the steps of obtaining a porosity and a resistivity log
value while drilling; dividing a cross plot of the porosity and the
resistivity log values into two regions where the split of the two
regions is based on the following (I): R=0.062/o.sup.1.5; averaging
the obtained porosity and resistivity log values in the subsurface
within a set interval to obtain a representative value of
resistivity and porosity for the subsurface within the set
interval; and giving a first warning of a high overpressure region
if the representative value of resistivity at the representative
value of porosity is lower than 0.062/o.sup.1.5. The method may
also include giving a second warning that a jump in pore pressure
is coming within 100-300 meters if the normalized values of
resistivity and porosity has a turning down point in its
trajectory.
Inventors: |
DE PRISCO; Giuseppe;
(Houston, TX) ; BREVIK; Ivar; (Trondheim, NO)
; PALACIOS; Alejandro Bello; (Bergen, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
STATOIL GULF SERVICES LLC |
Houston |
TX |
US |
|
|
Assignee: |
STATOIL GULF SERVICES LLC
Houston
TX
|
Family ID: |
56433192 |
Appl. No.: |
14/607974 |
Filed: |
January 28, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
E21B 7/04 20130101; E21B
47/10 20130101 |
International
Class: |
E21B 7/00 20060101
E21B007/00; E21B 47/06 20060101 E21B047/06 |
Claims
1. A method for predicting jumps in pore pressure of a subsurface,
comprising: obtaining a porosity and a resistivity log value while
drilling; dividing, using a processor, a cross plot of the porosity
and the resistivity log values into two regions where the split of
the two regions is based on the following (I): R=0.062/o.sup.1.5
(I) wherein R is the resistivity log value and o is the porosity
log value; averaging the obtained porosity and resistivity log
values in the subsurface within a set interval to obtain a
representative value of resistivity and porosity for the subsurface
within the set interval; rejecting intervals where the subsurface
is a sand formation that is not water saturated and rejecting
porosity values that are larger than a specified threshold; and
giving a first warning of a high overpressure region if the
representative value of resistivity at the representative value of
porosity is lower than 0.062/o.sup.1.5.
2. The method of claim 1, further comprising: normalizing the first
representative values of resistivity and porosity after the first
warning is given to obtain a resistivity reference and a porosity
reference; creating a new cross plot of normalized values of
resistivity and porosity to form a trajectory; giving a second
warning that a jump in pore pressure is coming within 100-300
meters if the normalized values of resistivity and porosity has a
turning point in their normalized trajectory; and defining a new
reference for resistivity and porosity to obtain a new normalized
plot.
3. The method of claim 1, further comprising: adjusting, using the
processor, a drilling operation associated with the drilling
location based on the predicted jump in pore pressure.
4. The method of claim 2, further comprising: adjusting, using the
processor, a drilling operation associated with the drilling
location based on the predicted jump in pore pressure.
5. The method of claim 3, wherein adjusting the drilling operation
comprises at least one selected from the group consisting of
adjusting a drilling fluid density, adjusting a drilling
trajectory, and optimizing a number of casing strings in a
borehole.
6. The method of claim 4, wherein adjusting the drilling operation
comprises at least one selected from the group consisting of
adjusting a drilling fluid density, adjusting a drilling
trajectory, and optimizing a number of casing strings in a
borehole.
7. The method of claim 1, wherein the drilling location comprises a
location below an operating drill bit in a borehole.
8. The method of claim 2, wherein the drilling location comprises a
location below an operating drill bit in a borehole.
9. The method of claim 1, wherein the first warning is displayed on
a graphical user interface.
10. The method of claim 2, wherein the second warning is displayed
on a graphical user interface.
11. A non-transitory computer readable medium comprising
instructions to perform a method for predicting jumps in pore
pressure of a subsurface, the instructions executable on a
processor and comprising functionality for: obtaining a porosity
and a resistivity log value while drilling; dividing a cross plot
of the porosity and the resistivity log values into two regions
where the split of the two regions is based on the following (I):
R=0.062/o.sup.1.5 (I) wherein R is the resistivity log value and o
is the porosity log value; averaging the obtained porosity and
resistivity log values in the subsurface within a set interval to
obtain a representative value of resistivity and porosity for the
subsurface within the set interval; rejecting intervals where the
subsurface is a sand formation that is not water saturated and
rejecting porosity values that are larger than a specified
threshold; and giving a first warning of a high overpressure region
if the representative value of resistivity at the representative
value of porosity is lower than 0.062/o.sup.1.5.
12. The non-transitory computer readable medium of claim 11,
wherein the instructions further comprise functionality for
adjusting a drilling operation associated with the drilling
location based on the predicted jump in pore pressure.
13. The non-transitory computer readable medium of claim 12,
wherein adjusting the drilling operation comprises at least one
selected from the group consisting of adjusting a drilling fluid
density, adjusting a drilling trajectory, and optimizing a number
of casing strings in a borehole.
14. The non-transitory computer readable medium of claim 11,
wherein the drilling location comprises a location below an
operating drill bit in a borehole.
15. The non-transitory computer readable medium of claim 11,
wherein the first warning is displayed on a graphical user
interface.
16. The non-transitory computer readable medium of claim 11,
wherein the instructions further comprise functionality for:
normalizing the first representative values of resistivity and
porosity after the first warning is given to obtain a resistivity
reference and a porosity reference; creating a new cross plot of
normalized values of resistivity and porosity to form a trajectory;
giving a second warning that a jump in pore pressure is coming
within 100-300 meters if the normalized values of resistivity and
porosity has a turning point in the trajectory; and defining a new
reference for resistivity and porosity to obtain a new normalized
plot.
17. The non-transitory computer readable medium of claim 16,
wherein the instructions further comprise functionality for
adjusting a drilling operation associated with the drilling
location based on the predicted jump in pore pressure.
18. The non-transitory computer readable medium of claim 17,
wherein adjusting the drilling operation comprises at least one
selected from the group consisting of adjusting a drilling fluid
density, adjusting a drilling trajectory, and optimizing a number
of casing strings in a borehole.
19. The non-transitory computer readable medium of claim 16,
wherein the drilling location comprises a location below an
operating drill bit in a borehole.
20. The non-transitory computer readable medium of claim 16,
wherein the second warning is displayed on a graphical user
interface.
21. A downhole tool configured to perform a method for predicting
jumps in pore pressure of a subsurface, the downhole tool
comprising: a processor; a memory comprising software instructions
for enabling the downhole tool under control of the processor to:
obtain a porosity and a resistivity log value while drilling;
divide a cross plot of the porosity and the resistivity log values
into two regions where the split of the two regions is based on the
following (I): R=0.062/o.sup.1.5 (I) wherein R is the resistivity
log value and o is the porosity log value; average the obtained
porosity and resistivity log values in the subsurface within a set
interval to obtain a representative value of resistivity and
porosity for the subsurface within the set interval; reject
intervals where the subsurface is a sand formation that is not
water saturated and reject porosity values that are larger than a
specified threshold; and give a first warning of a high
overpressure region if the representative value of resistivity at
the representative value of porosity is lower than
0.062/o.sup.1.5.
22. The downhole tool of claim 21, wherein the memory also enables
the downhole tool to adjust a drilling operation associated with
the drilling location based on the predicted jump in pore
pressure.
23. The downhole tool of claim 22, wherein adjusting the drilling
operation comprises at least one selected from the group consisting
of adjusting a drilling fluid density, adjusting a drilling
trajectory, and optimizing a number of casing strings in a
borehole.
24. The downhole tool of claim 21, wherein the drilling location
comprises a location below an operating drill bit in a
borehole.
25. The downhole tool of claim 21, wherein the first warning is
displayed on a graphical user interface.
26. The downhole tool of claim 21, wherein the memory also enables
the downhole tool to: normalize the first representative values of
resistivity and porosity to obtain a resistivity reference and a
porosity reference; create a new cross plot of normalized values of
resistivity and porosity to form a trajectory; give a second
warning that a jump in pore pressure is coming within 100-300
meters if the normalized values of resistivity and porosity has a
turning point in the trajectory; and defining a new reference for
resistivity and porosity to obtain a new normalized plot.
27. The downhole tool of claim 26, wherein the memory also enables
the downhole tool to adjust a drilling operation associated with
the drilling location based on the predicted jump in pore
pressure.
28. The downhole tool of claim 27, wherein adjusting the drilling
operation comprises at least one selected from the group consisting
of adjusting a drilling fluid density, adjusting a drilling
trajectory, and optimizing a number of casing strings in a
borehole.
29. The downhole tool of claim 26, wherein the drilling location
comprises a location below an operating drill bit in a
borehole.
30. The downhole tool of claim 26, wherein the second warning is
displayed on a graphical user interface.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to look ahead pore pressure
prediction. This system provides a warning if a high overpressure
regions is being drilled. The system then provides a second warning
when a high overpressure sand is approaching within 300 meters from
the drill bit.
[0003] 2. Description of Background Art
[0004] FIG. 1 shows an exemplary diagram of a drilling operation.
One of ordinary skill in the art will appreciate that the drilling
operation shown in FIG. 1 is provided for exemplary purposes only
and accordingly should not be construed as limiting the scope of
the present invention. For example, the drilling operation shown in
FIG. 1 is a seafloor drilling operation, but the drilling operation
may alternatively be a land drilling operation.
[0005] As shown in FIG. 1, a drilling rig 105 is configured to
drill into a formation (e.g., a formation below a seafloor 110)
using a drill bit (not shown) coupled to the distal end of a drill
string 125. Specifically, the drill bit is used to drill a borehole
130 extending to a target lithology 120. The target lithology 120
may be filled by hydrocarbon or a mineral resource targeted by a
drilling operation.
[0006] When a sediment is buried or compacted at a relatively high
depositional rate, fluid may be trapped in pores within the
resulting structure. Typically, compaction and dewatering of silts
or clays is much faster than compaction of sand lithology. The
compaction of silts or clays lowers the permeability of these
sediments and, in turn, delays further the dewatering of sand
layers that may be present below the clays. Fluid trapped in a sand
formation in this manner exerts pressure (defined as pore pressure)
on the surrounding formation. Formations in which pore pressure
exceeds hydrostatic pressure at a given depth are referred to as
overpressured. This is one example of a mechanism that gives an
overpressure sand formation. However, other mechanisms are possible
(e.g., cementation, aquathermal pressuring, etc.). The mechanism of
overpressure itself does not modify the scope of the present
invention. The only relevant factor is whether a sand is in an
overpressure condition. It is not important how the sand arrived to
this condition.
[0007] When drilling in an overpressured formation, the mud weight
(i.e., the weight of drilling fluids transmitted to the borehole)
must be high enough to prevent either the pore pressure from moving
formation fluids into the borehole in case of high enough
permeability formation (e.g., sand) or the pore pressure from
breaking down the formation and eventually causing borehole-walls
collapse in case of low enough permeability formation (e.g.,
shale). In the worst case of a high enough permeability formation,
formation fluids entering a borehole may result in loss of the well
and/or injury to personnel operating the drilling rig. Accordingly,
for safe and economic drilling, it is essential that the pore
pressure be predicted with sufficient accuracy. In particular, it
is beneficial to predict upcoming jumps in pore pressure at a
location that the drill bit has not yet reached.
[0008] Further, when drilling in overpressured formations, the
number of required casing strings (i.e., structural supports
inserted into the borehole) may be increased. Specifically, if a
sufficiently accurate pore pressure prediction is not available,
additional casing strings may be inserted prematurely to avoid the
possibility of well control problems (e.g., influx of formation
fluids, borehole collapse, etc.). Prematurely inserting casing
strings may delay the drilling operation and/or reduce the size of
the borehole and result in financial loss.
[0009] The knowledge of accurate pore pressure is crucial when
drilling a well in order to ensure the success of the drilling
operation. Pore pressure is also a controlling input parameter in
borehole stability modeling, well planning, design, and wellpath
optimization.
[0010] A problem often encountered when drilling wells in many
parts of the world is narrow drilling margins that require great
precision in pore pressure prediction in order to prevent any shale
instability problem resulting in risk of lost circulation and/or
gas kicks/blowouts.
[0011] There is a great need in the art for a method that makes it
possible to accurately predict pore pressure in real time
measurements at the rig site. If such data were available, it would
also be useful for identifying high risk shallow water zones,
optimizing mud weight, detecting shallow hazard zones, detecting
abnormal pressure zones, determining formation strength for
wellpath optimization, and, in general, for obtaining the most
trouble-free, cost effective drilling.
[0012] Conventional technology only addresses and infers pressure
magnitude in a shale lithology looking at the data available. As
such, conventional technology does not predict ahead of the drill
bit if a high overpressure sand is coming or only predicts the
presence of a high overpressure sand within tens of meters, see for
example US 2007/0127314 A1.
[0013] Therefore, there is an industry-wide need for a method and
system of more accurately predicting pore pressures.
SUMMARY OF THE INVENTION
[0014] The first embodiment of the present invention is directed to
a method for predicting jumps in pore pressure of a subsurface,
comprising obtaining at least two logs value while drilling (e.g.,
density and resistivity, density and shear velocity, resistivity
and shear velocity, etc.), or a combination of multiple variables
like resistivity, density, velocities, etc. in, for example, a
multivariate way. The following description looks at the values of
porosity (from bulk density) and resistivity logs, just two
variables, in order to explain in a simplistic way the idea of the
present invention. However, any two or multiple values described
above could be used. A cross plot of porosity--resistivity is
divided in two regions where the split of the two regions is based
on the following (I):
R=0.062/o.sup.1.5 (I)
wherein R is the resistivity log value and is the porosity log
value. The measured log values of resistivity and porosity are
averaged in the shale formation (rejecting values from any other
formation) within 1 [m], or 5 [m], or 10 [m], or 15 [m], or 20 [m]
or any other interval to obtain a representative value of
resistivity and porosity for that formation within the specified
interval. When a sand formation that is not water saturated up to
85% of its volume or any other water saturation that does not make
the sand properly saturated with water is present in the selected
interval, the interval should be discarded from the analysis. The
presence of hydrocarbon in that section could compromise the
resistivity measurement within the neighbor shale. The next step is
giving a first warning of a high overpressure region if the
measured averaged resistivity value at a specific measured averaged
porosity value is lower than 0.062/o.sup.15 . In this case, the
expected overpressure value in the specific region will be higher
than 5000 PSI. In order to avoid a false alarm due to noise of the
measured averaged resistivity and porosity, different thresholds
are defined to quantify the degree of risk for a high overpressure
region. For example, measured and averaged resistivity value (at a
specific measured and averaged porosity value): lower than
0.08/o.sup.15 but higher than 0.075/o.sup.1.5, it is not a
dangerous situation (green); lower than 0.075/o.sup.15 but higher
than 0.07/o.sup.1.5, it is a low dangerous situation (light green);
lower than 0.07/o.sup.1.5 but higher than 0.066/o.sup.1.5, it is a
mild dangerous situation (orange); lower than 0.066/o.sup.1.5 but
higher than 0.06/o.sup.1.5, it is a dangerous situation (dark
orange); lower than 0.06/o.sup.1.5 but higher than 0.055/o.sup.1.5,
it is a high dangerous situation (red); and lower than
0.055/o.sup.1.5, it is a very high dangerous situation (dark red).
For the very dangerous situation, the expected overpressure is
higher than 5000 [PSI]
[0015] The method may also include the detection of an upcoming
sudden pressure increase (e.g., overpressure sand). In order to do
this, resistivity and porosity are normalized by the resistivity
and porosity value obtained in the previous step when the
measurements cross the threshold of a dangerous situation. From
that point, a new non-dimensional cross plot is made that starts
from the value on the normalized resistivity and porosity (the
value being one and one). All the subsequent measured values of
resistivity and porosity are normalized and followed while
drilling: the normalized values while drilling are going to make a
trajectory that is followed in the new non-dimensional crossplot.
The trajectory of the subsequent normalized resistivity and
porosity is expected to go to values larger than the resistivity
reference and smaller than the porosity reference with increasing
depth. Thus, in the normalized plot, the value of normalized
resistivity is expected to be larger than 1 with increasing depth,
and the value of normalized porosity is expected to be lower than 1
with increasing depth. The resistivity value is larger than the
reference at a shallow point because compaction is acting with
increasing depth. Porosity is expected to be smaller than the
reference for the same reason. The normalization eliminates any
other effects, like salinity, that could push the resistivity lower
than the reference. In fact, salinity is expected to increase with
depth. As such, the normalization is going to make salinity a
second order effect within the analyzed interval deeper than the
reference, if the surrounding pore pressure is high enough. In
fact, the overpressure is high at this point in depth because a
first warning was sent (previous point) that the region is
characterized by high overpressure. Note that in this context,
normalization has also the meaning of a zoom in the porosity
resistivity cross plot. A threshold should be set for the measured
averaged porosity to be small enough: a value of porosity lower
than 24%, preferably lower than 20%, more preferably lower than 17%
has given good results in all the cases analyzed. If the porosity
is larger than this value, other mechanisms could be driving the
porosity resistivity cross plot (for example salinity) even with a
large enough pore pressure so that a low value of resistivity with
depth could be obtained due to mechanisms other than high pore
pressure. When an upcoming high over pressure sand is approaching,
the resistivity and porosity measurements are going to be affected
by the large pressure in the sand that is communicated in the shale
above. In this case, resistivity should decrease compared with the
reference point, and porosity should slightly increase. This trend
is reflected in the normalized plot with a turning point of the
trajectory resistivity versus porosity followed with a normalized
resistivity decrease and normalized porosity increase, which means
that an upcoming pore pressure jump must be preset in the deepest
section. The change in the direction of the trajectory should
persist for some depth (10 [m], 20 [m], 50 [m] or any other
interval) long enough to make sure that the turning point is not
due to noise in the data. Note that the use of two variables
(resistivity and porosity) also has the purpose of enhancing the
signal to noise ratio in order to extract better information than
with only one variable. An improvement, which is easy to understand
by one of ordinary skill in the art, is to use multiple variables
(for example resistivity, density, and velocity) in a multivariate
manner in order to improve resolutions. Thus, a second warning is
sent that a jump in pore pressure is coming within 100-300 meters.
After the second warning, and so after the drill bit goes through
the pore pressure jump, a new reference has to be defined for
resistivity and porosity so that a new normalized plot can be
obtained. Changing the reference value is also useful when the
drill bit goes from one lithology to another that, for example,
were deposited with different depositional attributes (different
initial porosity, depositional rate and so on). The method may also
include adjusting, using the processor, a drilling operation
associated with the drilling location based on the predicted jump
in pore pressure. The step of adjusting the drilling operation may
include at least one selected from the group consisting of
adjusting a drilling fluid density, adjusting a drilling
trajectory, and optimizing a number of casing strings in a
borehole. The drilling location may include a location below an
operating drill bit in a borehole. The first and second warning may
be displayed on a graphical user interface.
[0016] The second embodiment of the present invention is directed
to a non-transitory computer readable medium comprising
instructions to perform a method for predicting jumps in pore
pressure, the instructions executable on a processor and comprising
functionality for obtaining a porosity and a resistivity log value
while drilling; dividing a cross plot of the porosity and the
resistivity log values into two regions where the split of the two
regions is based on the following (I):
R=0.062/o.sup.1.5
wherein R is the resistivity log value and o is the porosity log
value; averaging the obtained porosity and resistivity log values
in the subsurface within a set interval to obtain a representative
value of resistivity and porosity for the subsurface within the set
interval; and rejecting intervals where sand formations are not
water saturated and porosity values that are larger than a
specified threshold; and giving a first warning of a high
overpressure region if the representative value of resistivity at
the representative value of porosity is lower than 0.062/o.sup.1.5.
The instructions may also include functionality for normalizing the
first representative values of resistivity and porosity after the
first warning is given to obtain a resistivity reference and a
porosity reference; creating a new cross plot of normalized values
of resistivity and porosity and following their normalized path or
trajectory; and giving a second warning that a jump in pore
pressure is coming within 100-300 meters if the normalized values
of resistivity and porosity has a turning point of the trajectory
of resistivity versus porosity followed by a normalized resistivity
decrease and a normalized porosity increase. After the second
warning, and so after the drill bit goes through the pore pressure
jump, a new reference has to be defined for resistivity and
porosity so that a new normalized plot can be obtained. Changing
the reference value is also useful when the drill bit goes from one
lithology to another that, for example, were deposited with
different depositional attributes (different initial porosity,
depositional rate and so on). The instructions may also include
functionality for adjusting a drilling operation associated with
the drilling location based on the predicted jump in pore pressure.
The step of adjusting the drilling operation may include at least
one selected from the group consisting of adjusting a drilling
fluid density, adjusting a drilling trajectory, and optimizing a
number of casing strings in a borehole. The drilling location may
include a location below an operating drill bit in a borehole. The
first and second warning may be displayed on a graphical user
interface.
[0017] The third embodiment of the present invention is directed to
a downhole tool configured to perform a method for predicting jumps
in pore pressure, the downhole tool comprising a processor; a
memory comprising software instructions for enabling the downhole
tool under control of the processor to obtain a porosity and a
resistivity log value while drilling; divide a cross plot of the
porosity and the resistivity log values into two regions where the
split of the two regions is based on the following (I):
R=0.062/o.sup.1.5
wherein R is the resistivity log value and o is the porosity log
value; average the obtained porosity and resistivity log values in
the subsurface within a set interval to obtain a representative
value of resistivity and porosity for the subsurface within the set
interval; and rejecting intervals where sand formations are not
water saturated, and porosity values that are larger than a
specified threshold; and give a first warning of a high
overpressure region if the representative value of resistivity at
the representative value of porosity is lower than 0.062/o.sup.15.
The memory may also enable the downhole tool to normalize the first
representative values of resistivity and porosity after the first
warning is given to obtain a resistivity reference and a porosity
reference; create a new cross plot of normalized values of
resistivity and porosity to form a trajectory; and give a second
warning that a jump in pore pressure is coming within 100-300
meters if the normalized values of resistivity and porosity has a
turning point of the trajectory resistivity versus porosity
followed with a normalized resistivity decrease and normalized
porosity increase, so it differs from the normal compaction
trajectory. After the second warning, and so after the drill bit
goes through the pore pressure jump, a new reference has to be
defined for resistivity and porosity so that a new normalized plot
can be obtained. Changing the reference value is also useful when
the drill bit goes from one lithology to another that, for example,
were deposited with different depositional attributes (different
initial porosity, depositional rate and so on). The memory may also
enable the downhole tool to adjust a drilling operation associated
with the drilling location based on the predicted jump in pore
pressure. The step of adjusting the drilling operation may include
at least one selected from the group consisting of adjusting a
drilling fluid density, adjusting a drilling trajectory, and
optimizing a number of casing strings in a borehole. The drilling
location may include a location below an operating drill bit in a
borehole. The first and second warning may be displayed on a
graphical user interface.
[0018] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to one of ordinary skill in the art
from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows an exemplary diagram of a drilling
operation.
[0020] FIG. 2 shows a diagram of a system in accordance with one or
more embodiments of the present invention.
[0021] FIG. 3 shows a flowchart in accordance with one or more
embodiments of the present invention.
[0022] FIG. 4 shows a diagram of a computer system in accordance
with one or more embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention will now be described with reference
to the accompanying drawings.
[0024] The present invention is directed to a predictable
relationship between resistivity and porosity (derived from
density) that occurs up to 300 meters above an overpressured
sandstone in the overlying shales. Because high overpressure from a
sand body is communicated to the shale above, a variation from the
normal trend of the resistivity/porosity (or any other combination
of variables, like resistivity and velocity) field is expected when
approaching the high overpressured sand.
[0025] The present invention is also directed to a two-warning
system based on this information. First, an early warning alerts
that there is an overpressured region below. Then, a second warning
occurs when the overpressured layer is approaching in order to
better locate it.
[0026] As a drill bit drills, porosity and resistivity are measured
and plotted on a graph with porosity on the x-axis and resistivity
on the y-axis. The graph also includes a line that corresponds to
the equation 0.062/(porosity).sup.1.5. From this plot, the first
warning is given if the drill bit will enter a high overpressure
region. In this regard, if the porosity-resistivity value is to the
left of the line, the first warning is given because the drill is
in a high overpressure region. If the porosity-resistivity value is
to the right of the line, a warning is not given because the drill
is not in a high overpressure region.
[0027] In other words, porosity and resistivity are measured as the
drill bit drills in order to obtain a resistivity-porosity value
that is above or below the following (I):
R=0.062/o.sup.1.5 (I)
wherein R is the resistivity log value and o is the porosity log
value. If the actual measured log value of resistivity is lower
than the resistivity obtained according to (I), then the drill is
in a high overpressure region, and the first warning is given. If
the actual measured value of resistivity is the same as or higher
than the resistivity obtained according to (I), then the drill is
not in a high overpressure region, so no warning is given.
[0028] A region is considered a high overpressure region if the
overpressure is higher than 5,000 psi. A region is considered a
medium overpressure region if the overpressure is between 1,400 and
5,000 psi. A region is considered a low overpressure region if the
overpressure is less than 1,400 psi.
[0029] For the second warning, the first representative values of
resistivity and porosity after the first warning is given are
normalized to obtain a resistivity reference and a porosity
reference. Then, a new cross plot of normalized values of
resistivity and porosity is created to form a trajectory while
drilling. The trajectory is supposed to increase while drilling
(normalized resistivity larger than the normalized reference (1)
and normalized porosity smaller than the normalized reference (1))
with increasing depth. If the normalized values of resistivity and
porosity differ from the trajectory, so that a turning point is
detected followed by a persisting decreasing trajectory, then a
jump in pressure is coming within 100-300 meters, and the second
warning is given. As the ratio of pore pressure to effective stress
increases, the trajectory is expected to decrease and a jump in
pore pressure can be expected. Accordingly, the second warning is
given. Effective stress is the stress on the matrix frame.
[0030] After the second warning, and so after the drill bit goes
through the pore pressure jump, a new reference has to be defined
for resistivity and porosity so that a new normalized plot can be
obtained. Changing the reference value is also useful when the
drill bit goes from one lithology to another that, for example,
were deposited with different depositional attributes (different
initial porosity, depositional rate and so on).
[0031] With the pore pressure calculations explained, the different
embodiments of the present invention can be further explained. In
general, embodiments of the present invention provide a method and
system for pore pressure prediction according to (I) discussed
above. Then, a drilling operation associated with the drilling
location is adjusted based on the predicted pore pressure.
[0032] FIG. 2 shows a diagram of a system in accordance with one or
more embodiments of the present invention. Specifically, FIG. 2
shows a diagram of a computing environment 205 in accordance with
one or more embodiments of the present invention.
[0033] In one or more embodiments of the present invention, the
computing environment 205 may include one or more computer systems
(e.g., computer system A 210, computer system N 215) configured to
perform drilling-related tasks. In one or more embodiments of the
present invention, the computer system(s) (e.g., 210, 215) may be
web servers, embedded systems (e.g., a computer located in a
downhole tool), desktop computers, laptop computers, personal
digital assistants, any other similar type of computer system, or
any combination thereof.
[0034] Specifically, in one or more embodiments of the present
invention, one or more of the computer systems (e.g., 210, 215) may
include a pore pressure calculator 235. In one or more embodiments
of the present invention, the pore pressure calculator 235 may be
located in a single computer system (e.g., 210, 215), distributed
across multiple computer systems (e.g., 210, 215), or any
combination thereof In one or more embodiments of the present
invention, the pore pressure calculator 235 may include one or more
software modules, one or more hardware modules, or any combination
thereof Further, in one or more embodiments of the present
invention, the pore pressure calculator may be configured to
communicate with each other via function calls, application program
interfaces (APIs), a network protocol (i.e., a wired or wireless
network protocol), electronic circuitry, any other similar type of
communication and/or communication protocol, or any combination
thereof.
[0035] In one or more embodiments of the invention, the pore
pressure calculator 235 may be configured to predict a jump in pore
pressure in 200-300 meters and in 100-300 meters as set forth
above.
[0036] FIG. 3 shows a flowchart in accordance with one or more
embodiments of the present invention. Specifically, FIG. 3 shows a
flowchart of a method for predicting a jump in pore pressure in
accordance with one or more embodiments of the present
invention.
[0037] In one embodiment of the present invention, a drilling
location corresponds to a location that has not yet been drilled.
In other words, the drill bit has not reached the drilling
location. However, the drilling location is in the intended path of
the drill bit and, unless the trajectory of the borehole changes,
the drill bit will eventually reach the drilling location. In one
embodiment of the present invention, the method described in FIG. 3
may be performed while drilling.
[0038] Turning to FIG. 3, porosity and resistivity log values at
reference depths are obtained (Step 305) for a depth interval of
specified length (e.g., 1 [m], 5[m]. 10 [m], 20 [m] or any other
length) long enough to obtain a representative averaged value for
such interval.
[0039] In Step 320, a cross plot of the porosity and the
resistivity log values is divided into two regions where the split
of the two regions is based on (I). In Step 322, the obtained
porosity and resistivity log values in the subsurface within a set
interval are averaged to obtain a representative value of
resistivity and porosity for the subsurface within the set
interval. An averaged value of resistivity and porosity belonging
to an interval with the presence of a sand that is not water
saturated would be rejected. All values of porosity lower than a
specific threshold, for example 20%, would also be rejected. If the
representative value of resistivity at the representative value of
porosity is not lower than 0.062/o.sup.15 (via Step 330), then
porosity and resistivity log values are measured again at the new
depth in Step 305, and the process begins again. If the
representative value of resistivity at the representative value of
porosity is lower than 0.062/o.sup.1.5 (via Step 330), then a first
warning is given that the drill bit is in a high overpressure
region in Step 335. Then, in Step 340, the drilling operation is
adjusted based on the predicted high pore pressure. Specifically,
in one or more embodiments of the present invention, adjusting the
drilling operation may involve adjusting a drilling fluid density
(i.e., increasing or decreasing the drilling fluid density as
appropriate), adjusting a drilling trajectory (e.g., to avoid an
overpressured area, to pass through a low-pressure area, etc.),
optimizing the number of casing strings in the borehole (i.e.,
adding a casing string, delaying addition of a casing string,
etc.), or any other similar type of adjustment.
[0040] In Step 310, the first representative values of resistivity
and porosity after the first warning is given is used as reference
to normalize all the upcoming deeper resistivity and a porosity
measurements. In Step 312, a new cross plot of normalized values of
resistivity and porosity is created to follow their path or
trajectory while drilling. If the normalized values of resistivity
and porosity make a monotonically increasing trajectory, filtering
out the noise, (via Step 315), then the cross plot of normalized
values of resistivity and porosity continues to be created at the
new depth in Step 312, and the process begins again. If the
normalized values of resistivity and porosity has a turning down
point followed by a decreasing path or trajectory (via Step 315),
then a second warning is given that a jump in pore pressure is
coming within 100-300 meters in Step 325. Then, in Step 340, the
drilling operation is adjusted based on the predicted jump in pore
pressure. Specifically, in one or more embodiments of the present
invention, adjusting the drilling operation may involve adjusting a
drilling fluid density (i.e., increasing or decreasing the drilling
fluid density as appropriate), adjusting a drilling trajectory
(e.g., to avoid an overpressured area, to pass through a
low-pressure area, etc.), optimizing the number of casing strings
in the borehole (i.e., adding a casing string, delaying addition of
a casing string, etc.), or any other similar type of
adjustment.
[0041] After the second warning, and so after the drill bit goes
through the pore pressure jump, a new reference is defined for
resistivity and porosity in Step 345 so that a new normalized cross
plot can be obtained in Step 312 to determine whether another jump
in pore pressure is coming. Changing the reference value is also
useful when the drill bit goes from one lithology to another that,
for example, were deposited with different depositional attributes
(different initial porosity, depositional rate and so on).
[0042] The processes of the first warning system and the second
warning system can occur simultaneously or the process of the
second warning system can start after the first warning is
given.
[0043] One or more embodiments of the present invention provide a
means for accurately predicting a jump in pore pressure.
Accordingly, one or more embodiments of the present invention may
prevent formation fluids from entering a borehole, thereby
preventing damage to the well and/or personnel operating a drilling
rig. Further, one or more embodiments of the present invention may
prevent the financial overhead of prematurely inserting casing
strings.
[0044] The present invention may be implemented on virtually any
type of computer regardless of the platform being used. For
example, as shown in FIG. 4, a computer system 400 includes a
processor 402, associated memory 404, a storage device 406, and
numerous other elements and functionalities typical of today's
computers (not shown). The computer 400 may also include input
means, such as a keyboard 408 and a mouse 410, and output means,
such as a monitor 412. The computer system 400 may be connected to
a network 414 (e.g., a local area network (LAN), a wide area
network (WAN) such as the Internet, or any other similar type of
network) via a network interface connection (not shown). One of
ordinary skill in the art will appreciate that these input and
output means may take other forms.
[0045] Furthermore, one of ordinary skill in the art will
appreciate that one or more elements of the aforementioned computer
system 400 may be located at a remote location and connected to the
other elements over a network. Further, software instructions to
perform embodiments of the present invention may be stored on a
non-transitory computer readable medium such as a compact disc
(CD), a diskette, a tape, a file, or any other non-transitory
computer readable storage device. In addition, in one embodiment of
the present invention, the predicted jump in pore pressure
(including all the calculations using the method described in FIG.
3) may be displayed to a user via a graphical user interface (e.g.,
a display device).
[0046] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *