U.S. patent number 3,827,511 [Application Number 05/315,804] was granted by the patent office on 1974-08-06 for apparatus for controlling well pressure.
This patent grant is currently assigned to Cameron Iron Works, Inc.. Invention is credited to Marvin R. Jones.
United States Patent |
3,827,511 |
Jones |
August 6, 1974 |
APPARATUS FOR CONTROLLING WELL PRESSURE
Abstract
Apparatus is disclosed for controlling the bottom hole pressure
of a well into which a drill string extends by automatically
regulating a choke at the outlet of the well.
Inventors: |
Jones; Marvin R. (Houston,
TX) |
Assignee: |
Cameron Iron Works, Inc.
(Houston, TX)
|
Family
ID: |
23226132 |
Appl.
No.: |
05/315,804 |
Filed: |
December 18, 1972 |
Current U.S.
Class: |
175/25;
166/91.1 |
Current CPC
Class: |
E21B
21/08 (20130101) |
Current International
Class: |
E21B
21/08 (20060101); E21B 21/00 (20060101); E21b
007/00 () |
Field of
Search: |
;175/25,38,218,40,50
;166/75 ;340/18LD,18R ;137/515.7 ;73/151 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leppink; James A.
Attorney, Agent or Firm: Hyer; W. F. Eickenroht; Marvin
B.
Claims
The invention having been described, what is claimed is:
1. Apparatus for controlling the bottom hole pressure of a well,
wherein said well has a drill string therein and fluid is
circulated therethrough to an outlet from the well, comprising a
choke for regulating pressure at the outlet, means for sensing
ambient pressure near the bottom of the drill string, means for
comparing said sensed pressure with a desired pressure, and means
responsive to a difference between the sensed and desired pressures
for operating the choke in a manner to reduce said difference.
2. Apparatus for maintaining a predetermined pressure differential
between the bottom hole pressure and formation fluid pressure of a
well, wherein said well has a drill string therein and fluid is
circulated therethrough to an outlet from the well, comprising a
choke for regulating pressure at the outlet, means for sensing
ambient pressure near the bottom of the drill string, means for
comparing said sensed pressure with an ambient pressure which is
required to maintain said predetermined pressure differential, and
means responsive to a difference between the sensed and required
ambient pressures for operating the choke in a manner to reduce
said difference.
3. Apparatus for maintaining a predetermined pressure differential
between the bottom hole pressure and formation fluid pressure of a
well, wherein said well has a drill string therein and fluid is
circulated therethrough to an outlet from the well, comprising a
choke for regulating pressure at the outlet, means for sensing
ambient pressure near the bottom of the drill string, means for
producing a first signal representative of the sensed ambient
pressure, means for producing a second signal representative of an
ambient pressure which is required to maintain said predetermined
pressure differential, means for comparing said first and second
signals, and means responsive to a difference between the signals
for operating the choke in a manner to reduce said difference.
4. Apparatus for maintaining a predetermined pressure differential
between the bottom hole pressure and formation fluid pressure of a
well, wherein said well has a drill string therein and fluid is
circulated therethrough to an outlet from the well, comprising a
choke for regulating pressure at the outlet, means for sensing
ambient pressure near the bottom of the drill string, means for
producing a first signal representative of sensed ambient pressure,
means for producing a second signal representative of an ambient
pressure which is required to maintain said predetermined pressure
differential, means for comparing the first and second signals and
producing a control signal representative of the difference between
them, and means responsive to said control signal for operating the
choke in a manner to reduce said difference.
5. Apparatus of the character defined in claim 4, including means
for sensing the pressure of fluid at the outlet, and means for
producing a third signal representative of the sensed outlet
pressure, said choke operating means including means opposing said
control signal to said third signal, whereby the choke operates to
reduce said difference by increasing or decreasing the outlet
pressure in response to an imbalance between said control signal
and said third signal.
6. Apparatus of the character defined in claim 5, including means
for producing a fourth signal representative of a desired fluid
pressure at the outlet, said choke operating means also including
means for alternately comparing said fourth signal with said third
signal so as to operate the choke in a manner to maintain the third
signal equal to the fourth signal.
7. Apparatus of the character defined in claim 6, wherein said
means for sensing ambient pressure near the lower end of the drill
string is adapted to sense such pressure on the inside of the drill
string.
8. Apparatus of the character defined in claim 4, wherein said
means for sensing ambient pressure near the lower end of the drill
string is adapted to sense such pressure on the outside of the
drill string.
9. Apparatus of the character defined in claim 4, wherein said
means for sensing ambient pressure near the lower end of the drill
string is adapted to sense such pressure on the inside of the drill
string.
10. Apparatus of the character defined in claim 4, including means
for damping said control signal.
Description
This invention relates generally to the control of the pressure of
drilling mud within a wellbore having a drill string extending into
the bore. More particularly, it relates to improvements in
apparatus for automatically controlling the well pressure by
regulating a choke to impose a desired back pressure on the outlet
of the well.
It has been customary to provide a choke in a manifold connecting
with the annulus of the well beneath a blow-out preventer so that,
upon closing the preventer about the drill string, the choke can
establish and maintain a back pressure on the drilling mud diverted
through the manifold. This back pressure, together with the
hydrostatic pressure of the drilling mud in the well, is intended
to contain the pressured fluids within formations penetrated by the
wellbore - i.e., prevent them from flowing into the wellbore. The
choke is preferably adjustable so that, in the case of a "kick," it
may be regulated in an attempt to maintain a predetermined pressure
differential between the bottom hole pressure of the mud and the
pressure of the formation fluid as heavier mud is circulated down
the drill string and up the annulus to "kill" the well. During this
time, it is desirable not only to contain such fluid, but also to
avoid excessive back pressure which might cause the drill string to
stick, or damage a formation, the well casing, or the wellhead
equipment.
In one use of prior apparatus for this purpose, when a "kick" is
encountered, the preventer and choke manifold are closed, the mud
pumps are stopped, and shut-in pressure is observed at the manifold
upstream of the choke. The pump is then started slowly as the choke
is gradually opened to maintain a back pressure at a level slightly
above the observed shut-in pressure. When the desired circulating
rate is reached, it is held constant and the choke is continuously
adjusted to maintain the pressure in a standpipe connected to the
upper end of the drill string at the level it has reached at such
circulating rate. The constant pressure in the standpipe is
maintained until the "kick" is circulated out of the annulus.
The mud density increase necessary to contain the formation fluid
is calculated, and the denser mud is pumped into the drill string
with the choke being adjusted to maintain the annulus back pressure
constant. When the heavier mud reaches bottom, the user begins to
adjust the choke to again maintain the standpipe pressure constant
as such mud circulates up through the annulus. Thus, in effect, the
user maintains a constant bottom hole pressure by controlling the
pressure in that portion of the well where the average density of
the fluid in it is known more closely. However, the presence of
evolving and expanding gas in the mud in the annulus, and the large
density-induced and rate-induced changes in circulating pressure
loss in the drill string, complicate the use of such apparatus.
U.S. Pat. Nos. 3,429,385 and 3,477,526, which are assigned to the
assignee of the present invention, disclose apparatus in which the
circulating pressure loss of the drilling mud is taken into account
in inferentially determining bottom hole pressure, so that it is
not necessary to maintain a constant circulating rate during
control of the well. For this purpose, a device is installed in the
standpipe or inlet to the drill string for continuously sensing the
circulating pressure loss, and the choke is regulated in such a
manner as to maintain a signal representative of the pressure in
the standpipe equal to a signal representative of the standpipe
pressure when the well is shut-in ("static pressure") plus the
circulating pressure loss and the desired differential.
However, since this prior apparatus senses the density of the
drilling mud at its input and assumes that the sensed density
exists throughout the drill string, it introduces an error when the
new drilling mud of increased density first reaches the sensor, and
this error must be corrected in most cases by manual adjustment of
the apparatus. Also, during the time that mud of a new, higher
density is displacing the original fluid within the drill string,
the hydrostatic pressure increases, so that the operator must
continually adjust the apparatus.
It is the primary object of this invention to provide relatively
simple and inexpensive apparatus of this type which avoids the
necessity of inferentially determining the bottom hole
pressure.
Still further, in such prior apparatus, there is a tendency for the
choke to hunt or overcontrol due to the time lag between an
adjustment at the choke and a response at the inlet. In order to
reduce this tendency, it has been proposed, as shown in the
apparatus of U.S. Pat. No. 3,477,526, to provide means for damping
the signal to be transmitted to the choke operator, and it is
another object of this invention to provide apparatus of this type
in which there is less time lag between the choke and transducer,
and thus less need for correction.
These and other objects are accomplished, in accordance with the
illustrated embodiment of the invention, by apparatus which
includes a means for sensing and thus directly determining ambient
fluid pressure near the lower end of the drill string, a means for
comparing the sensed pressure with a desired pressure, and a means
responsive to a difference between the sensed and desired pressures
for operating the choke in a manner to reduce such difference. In
this way, it is possible to maintain a predetermined pressure
differential between the bottom hole pressure and formation
pressure without having to inferentially determine such bottom hole
pressure.
More particularly, this means for directly determining ambient
pressure near the lower end of the drill string includes means for
producing a first signal representative of such sensed pressure and
transmitting such signal back to the surface. This may be
accomplished in a number of different ways. Thus, SPE Paper 1843
and World Oil article, December 1967, "New Concept in Drilling Rigs
Designed to Lower Well Costs," describe non-rotating drilling pipe
having both power and signal conductors, which enable a pressure
sensing device to be installed on the inner or outer side of the
drill string near its lower end. In the more conventional well
drilling situations, where a rotating drill string is employed, a
number of prior patents, i.e., U.S. Pat. Nos. 3,205,477; 3,309,656;
and 3,588,804, suggest acoustic or sonic apparatus for telemetering
information signals, including signals providing pressure
information, from the bottom of the hole. In U.S. Pat. No.
2,370,818, a wire line communication system is illustrated which
can be adapted to be connected to a pressure sensing device or
transducer at the bottom of the hole being drilled. In the latter
case, it may be possible to employ additional apparatus for
inserting a cable into a well under pressure, such as shown in
Composite Catalog of Oilfield Equipment and Services, 23th
Revision, page 751, to aid in lowering of the transducer through
the drill string to a position in the drill string near its lower
end.
The apparatus of this invention also includes means for producing a
second signal representative of a desired fluid pressure near the
lower end of the drill string, which would include a pressure
differential, positive or negative, by which the bottom hole
pressure of the drilling fluid is to exceed the formation fluid
pressure. Still further, means are provided for comparing these
first and second signals and producing a control signal which
represents the difference between them, and for operating the choke
in response to the control signal so as to increase or decrease the
outlet fluid pressure (i.e., upstream of the choke) in such a
manner that it approaches a value at which the desired pressure
differential is maintained.
In the illustrated embodiment of the invention, the choke operating
means includes a reciprocable, pressure responsive actuator
connected to the choke, means for sensing the actual outlet fluid
pressure, means for producing a third signal representative of the
sensed outlet pressure, and means for opposing the third signal to
the control signal, whereby the third signal acts as a bias to
cause the choke to operate in the manner described by increasing or
decreasing the outlet pressure in response to an imbalance between
it and the control signal. Also, a means is provided for producing
a fourth signal representative of the desired fluid pressure at the
outlet, and the choke operating means includes means for
alternately comparing the third and fourth signals, so as to
operate the choke in such a manner as to maintain the actual outlet
pressure at the desired level.
Inasmuch as the effect of adjustment of the choke is detected by
the pressure sensing device near the lower end of the drill string,
the time lag and thus the tendency toward hunting or overcontrol is
less than that encountered in the prior apparatus. Nevertheless, in
order to reduce this tendency still further, a means is preferably
provided for damping the control signal.
In the drawings, wherein like reference characters are used
throughout to designate like parts:
FIG. 1 is a diagrammatic illustration of apparatus constructed in
accordance with a typical embodiment of the present invention, in
which a Kelly has been replaced by equipment for controlling a
"kick," and a pressure sensing device has been lowered into the
lower portion of a rotary drill string, the scale of the lower part
of the figure being smaller than that of the upper; and
FIG. 2 is a view of the lower portion of a non-rotating drill
string having the pressure sensing device on its outer side.
With reference now to the details of FIG. 1 of the drawings, the
well on which the control apparatus is installed includes a casing
10 lining a portion of a wellbore 11 and a casing head 12 connected
to its upper end at the surface level 13. A blowout preventer 14
connected above the casing head 12 has a bore therethrough forming
a continuation of the wellbore and casing head and rams 15 mounted
for reciprocation between positions opening and closing the
bore.
A drill bit 16 is connected to the lower end of a drill string 17
extending through the wellhead and into the wellbore. The bit
normally has orifices in the bottom thereof through which drilling
fluid is jetted into the wellbore, and is rotated with the drill
string in any well known manner. A velocity valve 17A, which may be
similar to that shown in U.S. Pat. No. 3,027,914, is installed in
its lower end.
A Kelly (not shown), which occupies a position at the upper end of
the string during normal drilling operations, has been replaced by
a circulating head 18 of conventional construction connected to the
upper end of the string, and a cable inserting device 19 similar to
that above-mentioned installed above the head 18. Drilling mud is
circulated through a side inlet 18A in the head 18 for passage
downwardly through the drill string, out the orifices in the bit,
and upwardly through the annulus between the string and
wellbore.
A side outlet 20 connects with the bore of the casing head 12
beneath the blowout preventer, so that with the blowout preventer
rams closed about the drill string, as shown in the drawings,
drilling fluid within the annulus is diverted into the outlet. The
outlet forms part of conventional manifolding for conducting the
drilling fluid to one of a plurality of valves and chokes,
including an adjustable choke 21 forming part of the apparatus of
this invention. Thus, the well may be completely shut in, or
drilling fluid may be caused to flow through choke 21.
As shown in the drawing, the bit 16 has penetrated a formation F
containing fluid under pressure. Normally, this formation fluid is
contained by means of the drilling fluid circulated through the
wellbore. However, it may be found that the drilling fluid will not
contain the pressure formation fluid, in which case the well may
"kick," as indicated by the ability of the well to flow with the
pumps for circulating the drilling fluid shut down. When this
occurs, the operator may shut in the well, as above described, and
permit the pressure to build up in the annulus and thus in the
outlet 20 leading to the choke 21. After an initial build-up, the
pressure in the lower end of the wellbore, and known as "bottom
hole pressure," will have stabilized at a value required for
containing the formation fluid. It will be understood in this
respect that "bottom hole pressure" means the pressure opposite the
lower end of the bit 16 of the drill string, which need not
necessarily be at the lower end of the wellbore.
In like vein, "formation fluid pressure" is formation F pressure
corrected, if necessary, for the distance from the bit to such
formation.
The choke 21 is of a construction quite similar to that shown in
the above-mentioned prior patents. Thus, it includes a body 22
having a right angle flowway therethrough and a flow restricting
member 23 reciprocable therein between maximum and minimum flow
restricting positions with respect to opening 23a intermediate the
ends of the flowway. The member 23 is caused to reciprocate by
means of an actuator 24 mounted on the body 22 and including a
piston 25 reciprocable within a cylinder 26 and having a rod 27
extending from the operator and connecting with the restricting
member 23. The piston is caused to reciprocate in response to fluid
pressure differential across it, which is in turn responsive to
signals to be described.
As illustrated in the drawings, a transducer 28 of any suitable
construction is suspended within the lower end of the drill string
17 by means of an electrical cable 29 so as to produce an
electrical signal representative of the pressure of fluid at that
point and transmit it to a circuitry 29A at the surface. The
circuitry includes a source of electrical voltage and a signal
amplifier for amplifying the electrical signal, which is then
conducted by cable 29 from circuitry 29A to an
electric-to-pneumatic transmitting instrument 29B, to be described.
Alternatively, and as shown in FIG. 2, the drill string may be of
the non-rotating type above mentioned having a transducer 28A near
its lower end for sensing pressure on the outside of the string -
and thus bottom hole pressure - and transmitting such signal
through suitable conductors in the string to surface level.
In any event, inasmuch as the remaining portion of the control
apparatus to be described is pneumatic, the electrical signal is
converted to the above-mentioned pneumatic signal by
electric-to-pneumatic transmitting instrument. A suitable
instrument for this purpose is manufactured by Moore Products Co.,
of Spring House, Pa., and shown and described in FIG. 4, on page 3
of its Bulletin 7702. The thusly produced pneumatic signal, which
may be termed a first signal, is transmitted through line 29C to
relay 35, which will be described below. A gauge 30 is connected in
line 29C at a convenient location for observing the value of the
pressure of fluid within the drill string near its lower end.
An adjustable null balance regulator 32, such as that manufactured
by the said Moore Products Co. and shown on page 14 of its Bulletin
4003, is adapted to produce a pneumatic signal which represents the
pressure desired within the string near its lower end, including a
desired pressure differential, positive or negative, by which the
bottom hole pressure is to exceed the formation fluid. This signal,
which may be termed a second signal, is transmitted through line 33
leading from the regulator to relay 35, and a gauge 34 connected in
the line provides a visual indication of such desired pressure. Air
is supplied through line 31, which has a branch 31A leading to
regulator 32 and a branch 31B leading to the instrument 29B.
The relay 35, which compares the first and second signals
transmitted to it through lines 29C and 33 and produces a control
signal representative of the difference between them, may comprise
a pneumatic summing relay of the type manufactured by Moore
Products Co., and shown on page 22 of its Bulletin 68A-b 01. As
shown in the drawing, the relay 35 is adapted to receive air from a
branch 31C of supply line 31, or exhaust it to atmosphere through a
port 50, and supply a greater or lesser signal to outlet conduit
53, depending on the relative values of the first and second
signals. Thus, if the first signal is greater than the second
signal, the relay produces a lesser outlet signal, and if the first
signal is less than the second signal, the relay produces a greater
outlet signal.
A needle valve 52 is disposed in the conduit 53 to provide a damped
control signal for transmission through a line 36 to a hydraulic
relay 37 to be described below. The damped control signal is fed
back to relay 35 through a conduit 51, and internal porting (not
shown) in the relay transmits the undamped outlet signal to relay
35 to oppose the signal fed back, whereby the rate of change of the
control signal with respect to time becomes a function of the
difference between the first and second signals.
A transducer 38 is connected to the well outlet 20 for sensing the
outlet fluid pressure upstream from choke 21 and producing a
pneumatic signal representative thereof, which may be termed a
third signal. Air is transmitted to the transducer through a branch
31D of supply line 31, and the third signal is transmitted through
a line 39 to the hydraulic relay 37 to oppose the control signal,
in a manner to be described below. The value of outlet pressure
represented by the signal is indicated on a gauge 39A connecting
with the line 39.
An adjustable null balance regulator 41, similar to the regulator
32, is adapted to produce a pneumatic signal which represents the
pressure which may be desired within the outlet. The resulting
signal, which may be termed a fourth signal, is transmitted through
a line 42 from the regulator to a selector valve 55 disposed within
the line 36 leading to relay 37. A gauge 42A connected in the line
42 provides a visual indication of the desired outlet pressure. Air
is transmitted to the regulator 41 through a branch 31E from supply
line 31.
As shown in the drawing, valve 55 is in a position for transmitting
the control signal from relay 35 to the relay 37. However, as will
be understood from the drawing, such valve may be turned to an
alternate position for transmitting the fourth signal from
regulator 41 to the relay 37. The purpose of alternatively
transmitting either the control or fourth signal to the relay 37
will be understood from the description to follow.
The relay 37 includes a member 37A shiftable within a housing 37B
between alternate positions for admitting hydraulic fluid from a
suitable source to one side of the actuator 24 and exhausting it
from the other, and a neutral position, as shown in the drawing,
for interrupting flow between the source and operator. For this
purpose, hydraulic fluid is supplied to the relay by a conduit 44
leading from the source and returned therefrom back to the source
through a conduit 45. The relay is connected with the right-hand
side of the actuator piston 25 by means of a conduit 46 and to the
left-hand side thereof by means of a conduit 47. Thus, upon
shifting of the shiftable member to the right, hydraulic fluid will
be supplied from the line 44 through the line 46 to the right-hand
side of the piston 25 and exhausted from the left side thereof
through the line 47 to the line 45, for shifting the restricting
member 23 of the choke to the left and thus toward minimum flow
restricting position. On the other hand, upon shifting the
restricting member to the left, hydraulic fluid will be supplied
from the line 44 through the line 47 to the left-hand side of the
piston 25, and exhausted from the right side thereof through the
line 46 to the line 45, for shifting the restricting member 23 to
the right and thus toward its maximum flow restricting
position.
The signal representing actual outlet well pressure is transmitted
through the line 39 to the left end of shiftable member 37A to urge
it to the right, and either the control signal or the fourth signal
is transmitted through line 36 to the right end of the shiftable
member to urge it to the left. Relay 37 includes centering springs
48 on opposite ends of the shiftable member for normally urging
such member to its neutral position shown in the drawing.
With the valve 55 positioned as shown, the relay 37 will shift with
a change in the control signal in line 36, so as to adjust the
choke in such a manner as to cause the first signal to be equal to
the second signal. Thus, in the event the control signal decreases
due to an increase in the first signal, shiftable member 37A moves
to the right to cause reciprocable member 23 to move away from flow
restricting position and thereby decrease the first signal; and, in
the event the control signal increases due to a decrease in the
first signal, shiftable member 37A moves to the left to cause the
reciprocable member to move toward flow restricting position and
thereby increase the first signal. In either event, the choke
operates to maintain the control signal constant and thus the
pressure within the lower end of the drill string at its desired
value. On the other hand, with the valve 55 turned to its alternate
position, the relay will shift depending on an imbalance between
the third and fourth signals, so as to maintain the outlet pressure
at its desired value.
The outlet pressure signal has an additional function in that it
will cause the relay to respond to abnormal pressure fluctuations
in the annulus, even though such changes may not be immediately
reflected by a change in the signal representing pressure near the
lower end of the drill string. Thus, for example, in the event of
an abnormal increase in annulus pressure, the shiftable member of
the relay will be caused to move to the right and thus to cause
hydraulic fluid to urge the flow restricting member 23 toward
minimum flow restricting position, whereby outlet pressure is
quickly reduced.
An accumulator 56 is preferably connected in line 36 intermediate
the valve 55 and the relay 37. As described in the previously
mentioned U.S. Pat. No. 3,477,526, this too is useful in preventing
hunting and overcontrol.
In one manner of using the apparatus, upon indication of a kick,
the operator shuts in the well, as previously described, by closing
the choke manifold, shutting the pumps down, raising the drill
string so as to bring the Kelly above the rotary table, and closing
the blowout preventer around the drill string. He then observes the
pressures indicated by gauge 30 and/or by gauge 39A to confirm kick
by pressure buildup on either, and, if this is the case, performs
the following steps:
1. Replaces the Kelly with circulating head 18 and cable inserting
device 19,
2. Runs pressure transducer 28 on cable 29 to as near to bottom in
the drill string as possible,
3. Sets regulator 41 so that pressure gauge 42A reads the same as
gauge 39A,
4. Sets selector valve 55 to transmit the signal produced by
pressure regulator 41 to relay 37,
5. Places outlet 20 in open communication with choke 21,
6. Starts the pumps slowly and gradually brings them up to the
speed desired for the killing operation,
7. Observes the pressure indicated at gauge 30 and sets regulator
32 so that gauge 34 shows the same pressure as gauge 30,
8. Moves selector valve 55 so as to transmit the output signal from
relay 35 to relay 37, and
9. Pumps the wellbore free of formation fluid, displacing the
original drilling fluid with one of increased density, as
desired.
In another manner of using the apparatus, when the transducer is on
the outside of the drill string, so as to sense bottom hole
pressure directly as illustrated in FIG. 2, the operator would
merely have to perform the fifth through the ninth steps.
From the foregoing it will be seen that this invention is one well
adapted to attain all of the ends and objects hereinabove set
forth, together with other advantages which are obvious and which
are inherent to the apparatus.
It will be understood that certain features and subcombinations are
of utility and may be employed without reference to other features
and subcombinations. This is contemplated by and is within the
scope of the claims.
As many possible embodiments may be made of the invention without
departing from the scope thereof, it is to be understood that all
matter herein set forth or shown in the accompanying drawing is to
be interpreted as illustrative and not in a limiting sense.
* * * * *