# Patent

## Title
Wellbore pressure control with optimized pressure drilling

## Abstract

A well system can include an accumulator in communication with a wellbore, whereby the accumulator applies pressure to the wellbore. A method of maintaining a desired pressure in a wellbore can include applying pressure to the wellbore from an accumulator in response to pressure in the wellbore being less than the desired pressure. Another well system can include a dampener in communication with a wellbore isolated from atmosphere, whereby the dampener mitigates pressure spikes in the wellbore.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit under 35 USC §119 of the filing date of International Application Serial No. PCT/US11/31790 filed 8 Apr. 2011. The entire disclosure of this prior application is incorporated herein by this reference.
BACKGROUND
The present disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides for wellbore pressure control with optimized pressure drilling.
It is important in drilling operations to control wellbore pressure. Excessive wellbore pressure can cause undesired fracturing of an earth formation penetrated by a wellbore being drilled, breakdown of casing shoes, and loss of valuable drilling fluids. Insufficient wellbore pressure can cause formation fluids to flow into the wellbore, and can cause wellbore instability.
Therefore, it will be appreciated that improvements are continually needed in the art of wellbore pressure control.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a representative partially cross-sectional view of a well system and associated method which can embody principles of this disclosure.
FIG. 2 is a representative block diagram of a process control system which may be used with the well system and method of FIG. 1, and which can embody principles of this disclosure.
FIG. 3 is a representative flowchart for a method which may be used with the well system, and which can embody principles of this disclosure.



DETAILED DESCRIPTION
Representatively illustrated in FIG. 1 is a well system 10 and associated method which can embody principles of this disclosure. In the system 10, a wellbore 12 is drilled by rotating a drill bit 14 on an end of a tubular drill string 16. The drill bit 14 may be rotated by rotating the drill string 16 and/or by operating a mud motor (not shown) interconnected in the drill string.
Drilling fluid 18, commonly known as mud, is circulated downward through the drill string 16, out the drill bit 14 and upward through an annulus 20 formed between the drill string and the wellbore 12, in order to cool the drill bit, lubricate the drill string, remove cuttings and provide a measure of bottom hole pressure control. A non-return valve 21 (typically a flapper-type check valve) prevents flow of the drilling fluid 18 upward through the drill string 16.
Control of bottom hole pressure is very important in managed pressure and underbalanced drilling, and in other types of optimized pressure drilling operations. Preferably, the bottom hole pressure is optimized to prevent excessive loss of fluid into an earth formation 64 surrounding the wellbore 12, undesired fracturing of the formation, undesired influx of formation fluids into the wellbore, etc.
In typical managed pressure drilling, it is desired to maintain the bottom hole pressure just greater than a pore pressure of the formation 64, without exceeding a fracture pressure of the formation. In typical underbalanced drilling, it is desired to maintain the bottom hole pressure somewhat less than the pore pressure, thereby obtaining a controlled influx of fluid from the formation 64.
Nitrogen or another gas, or another lighter weight fluid, may be added to the drilling fluid 18 for pressure control. This technique is especially useful, for example, in underbalanced drilling operations, or in segregated density (such as dual gradient) managed pressure drilling.
In the system 10, additional control over the bottom hole pressure is obtained by closing off the annulus 20 (e.g., isolating it from communication with the atmosphere and enabling the annulus to be pressurized at or near the surface) using a rotating control device 22 (RCD). The RCD 22 seals about the drill string 16 above a wellhead 24. Although not shown in FIG. 1, the drill string 16 would extend upwardly through the RCD 22 for connection to, for example, a standpipe line 26 and/or other conventional drilling equipment.
The drilling fluid 18 exits the wellhead 24 via a wing valve 28 in communication with the annulus 20 below the RCD 22. The fluid 18 then flows through a fluid return line 30 to a choke manifold 32, which includes redundant chokes 34. Backpressure is applied to the annulus 20 by variably restricting flow of the fluid 18 through the operative choke(s) 34.
The greater the restriction to flow through the choke 34, the greater the backpressure applied to the annulus 20. Thus, bottom hole pressure can be conveniently regulated by varying the backpressure applied to the annulus 20. A hydraulics model can be used, as described more fully below, to determine a pressure applied to the annulus 20 at or near the surface, which pressure will result in a desired bottom hole pressure. In this manner, an operator (or an automated control system) can readily determine how to regulate the pressure applied to the annulus at or near the surface (which can be conveniently measured) in order to obtain the desired bottom hole pressure.
It can also be desirable to control pressure at other locations along the wellbore 12. For example, the pressure at a casing shoe, at a heel of a lateral wellbore, in generally vertical or horizontal portions of the wellbore 12, or at any other location can be controlled using the principles of this disclosure.
Pressure applied to the annulus 20 can be measured at or near the surface via a variety of pressure sensors 36, 38, 40, each of which is in communication with the annulus. Pressure sensor 36 senses pressure below the RCD 22, but above a blowout preventer (BOP) stack 42. Pressure sensor 38 senses pressure in the wellhead below the BOP stack 42. Pressure sensor 40 senses pressure in the fluid return line 30 upstream of the choke manifold 32.
Another pressure sensor 44 senses pressure in the standpipe line 26. Yet another pressure sensor 46 senses pressure downstream of the choke manifold 32, but upstream of a separator 48, shaker 50 and mud pit 52. Additional sensors include temperature sensors 54, 56, Coriolis flowmeter 58, and flowmeters 62, 66.
Not all of these sensors are necessary. For example, the system 10 could include only one of the flowmeters 62, 66. However, input from the sensors is useful to the hydraulics model in determining what the pressure applied to the annulus 20 should be during the drilling operation.
In addition, the drill string 16 may include its own sensors 60, for example, to directly measure bottom hole pressure. Such sensors 60 may be of the type known to those skilled in the art as pressure while drilling (PWD), measurement while drilling (MWD) and/or logging while drilling (LWD) sensor systems. These drill string sensor systems generally provide at least pressure measurement, and may also provide temperature measurement, detection of drill string 16 characteristics (such as vibration, weight on bit, stick-slip, etc.), formation characteristics (such as resistivity, density, etc.) and/or other measurements. Various forms of telemetry (acoustic, pressure pulse, electromagnetic, optical, wired, etc.) may be used to transmit the downhole sensor measurements to the surface. The drill string 16 could be provided with conductors, optical waveguides, etc., for transmission of data and/or commands between the sensors 60 and the process control system 74 described below (see FIG. 2).
Additional sensors could be included in the system 10, if desired. For example, another flowmeter 67 could be used to measure the rate of flow of the fluid 18 exiting the wellhead 24, another Coriolis flowmeter (not shown) could be interconnected directly upstream or downstream of a rig mud pump 68, etc.
Fewer sensors could be included in the system 10, if desired. For example, the output of the rig mud pump 68 could be determined by counting pump strokes, instead of by using the flowmeter 62 or any other flowmeter(s).
Note that the separator 48 could be a 3 or 4 phase separator, or a mud gas separator (sometimes referred to as a “poor boy degasser”). However, the separator 48 is not necessarily used in the system 10.
The drilling fluid 18 is pumped through the standpipe line 26 and into the interior of the drill string 16 by the rig mud pump 68. The pump 68 receives the fluid 18 from the mud pit 52 and flows it to the standpipe line 26. The fluid 18 then circulates downward through the drill string 16, upward through the annulus 20, through the mud return line 30, through the choke manifold 32, and then via the separator 48 and shaker 50 to the mud pit 52 for conditioning and recirculation.
Note that, in the system 10 as so far described, the choke 34 cannot be used to control backpressure applied to the annulus 20 for control of the bottom hole pressure, unless the fluid 18 is flowing through the choke. In conventional overbalanced drilling operations, a lack of circulation can occur whenever a connection is made in the drill string 16 (e.g., to add another length of drill pipe to the drill string as the wellbore 12 is drilled deeper), and the lack of circulation will require that bottom hole pressure be regulated solely by the density of the fluid 18.
In the system 10, however, a desired pressure applied to the annulus 20 can be maintained, even though the fluid 18 does not circulate through the drill string 16 and annulus 20. Thus, pressure can still be applied to the annulus 20, without the fluid 18 necessarily flowing through the choke 34.
In the system 10 as depicted in FIG. 1, an accumulator 70 can be used to supply a flow of fluid to the return line 30 upstream of the choke manifold 32. In other examples, the accumulator 70 may be connected to the annulus 20 via the BOP stack 42, and in further examples the accumulator could be connected to the choke manifold 32.
The accumulator 70 can be used to maintain a desired pressure in the annulus 20, whether or not additional pressure sources (such as, a separate backpressure pump and/or the rig pump 68, etc.) are also used. Diversion of fluid 18 from the standpipe manifold (or otherwise from the rig pump 68) to the return line 30 is described in International Application Serial No. PCT/US08/87686, and in U.S. application Ser. No. 13/022,964. The use of a separate backpressure pump is described in International Application Serial No. PCT/US11/31767, filed Apr. 8, 2011.
The well system 10 can also (or alternatively) include a pressure dampener 72 connected to the return line 30 as depicted in FIG. 1. The dampener 72 could alternatively be connected to the annulus 20 via the BOP stack 42, o the dampener could be connected to the choke manifold 32.
The dampener 72 functions to dampen pressure spikes (positive or negative) which would otherwise be communicated to the annulus 20. Certain operations (such as recommencing drilling after making a connection in the drill string 16, the drill bit 14 penetrating different reservoir pressure regimes, variations in rig pump 68 output, etc.) can induce such pressure spikes in the wellbore 12. The dampener 72 mitigates pressure spikes, so that a relatively continuous desired wellbore pressure can be maintained.
Preferably, the dampener 72 includes a pressurized gas chamber 78 isolated from the fluid 18 by a flexible membrane 80 or a floating piston, etc. Compressible gas in the chamber 78 provides a “cushion” to dampen any pressure spikes. However, other types of dampeners may be used, in keeping with the principles of this disclosure.
If desired, the dampener 72 could be provided with sufficient volume that it also operates as an accumulator, suitable for supplying pressure to maintain the desired wellbore pressure, as described above for the accumulator 70. In that case, the separate accumulator 70 may not be used.
At this point it should be pointed out that the well system 10 is described here as merely one example of a well system which can embody principles of this disclosure. Thus, those principles are not limited at all to the details of the well system 10 as depicted in FIG. 1 or described herein.
Referring additionally now to FIG. 2, a block diagram of one example of a process control system 74 is representatively illustrated. The process control system 74 is described here as being used with the well system 10 of FIG. 1, but it should be understood that the process control system could be used with other well systems, in keeping with the principles of this disclosure. In other examples, the process control system 74 could include other numbers, types, combinations, etc., of elements, and any of the elements could be positioned at different locations or integrated with another element, in keeping with the scope of this disclosure.
As depicted in FIG. 2, the process control system 74 includes a data acquisition and control interface 118, a hydraulics model 120, a predictive device 122, a data validator 124 and a controller 126. These elements may be similar to those described in International Application Serial No. PCT/US10/56433 filed on 12 Nov. 2010.
The hydraulics model 120 is used to determine a desired pressure in the annulus 20 to thereby achieve a desired pressure at a certain location in the wellbore 12. The hydraulics model 120, using data such as wellbore depth, drill string rpm, running speed, mud type, etc., models the wellbore 12, the drill string 16, flow of the fluid through the drill string and annulus 20 (including equivalent circulating density due to such flow), etc.
The data acquisition and control interface 118 receives data from the various sensors 36, 38, 40, 44, 46, 54, 56, 58, 60, 62, 66, 67, together with rig and downhole data, and relays this data to the hydraulics model 120 and the data validator 124. In addition, the interface 118 relays the desired annulus pressure from the hydraulics model 120 to the data validator 124.
The predictive device 122 can be included in this example to determine, based on past data, what sensor data should currently be received and what the desired annulus pressure should be. The predictive device 122 could comprise a neural network, a genetic algorithm, fuzzy logic, etc., or any combination of predictive elements, to produce predictions of the sensor data and desired annulus pressure.
The data validator 124 uses these predictions to determine whether any particular sensor data is valid, whether the desired annulus pressure output by the hydraulics model 120 is appropriate, etc. If it is appropriate, the data validator 124 transmits the desired annulus pressure to the controller 126 (such as a programmable logic controller, which may comprise a proportional integral derivative (PID) controller), which controls operation of the choke 34, the accumulator 70 and various flow control devices (such as, a valve 82 of the standpipe manifold, etc.).
In this manner, the choke 60, accumulator 70 and various flow control devices (such as, the standpipe valve 82, etc.) can be automatically controlled to achieve and maintain the desired pressure in the annulus 20. Actual pressure in the annulus 20 is typically measured at or near the wellhead 24 (for example, using sensors 36, 38, 40), which may be at a land or subsea location.
For example, if there is no circulation of the fluid 18 through the drill string 16 and annulus 20, and pressure in the wellbore 12 falls below the desired pressure setpoint, a valve 84 of the accumulator 70 can be opened by the controller 126 to supply the requisite pressure to the annulus, so that the desired pressure is maintained in the annulus and the remainder of the wellbore 12. This situation could occur, for example, when making connections in the drill string 16, when tripping the drill string into or out of the wellbore 12, if there is a malfunction of the rig pump 68, etc.
Referring additionally now to FIG. 3, a method 90 of maintaining a desired pressure in the wellbore 12 is representatively illustrated in flowchart form. The method 90 may be used with the well system 10 of FIG. 1, or it may be used with other well systems without departing from the principles of this disclosure.
The method 90 as depicted in FIG. 3 is used for when a connection is made in the drill string 16, but it will be appreciated that the method, with appropriate modifications, can be used when tripping the drill string into or out of the wellbore 12, when another pressure source is otherwise not available to supply pressure to the wellbore, etc.
The method 90 example of FIG. 3 begins with a starting step 92 and ends at step 94 with drilling ahead. Although not shown in FIG. 3, throughout the method 90 the hydraulics model 120 continues to output a desired pressure setpoint, and if fluid 18 flows through the choke 34, the choke is operated as needed to maintain the desired pressure in the wellbore. However, in a portion of the method 90, there is no flow through the choke 34, and so the controller 126 will maintain the choke closed in that portion of the method, as described more fully below.
In step 96, the accumulator 70 is charged (e.g., pressurized). The accumulator 70 may be charged before or after the method 90 begins. Preferably, the accumulator 70 is maintained in a charged state throughout the optimized pressure drilling operation, and is charged prior to starting the method 90, but step 96 is included in the method to indicate that, at this point, the accumulator should be in a charged state.
In preparation for making the connection in the drill string 16, the output of the rig pump 68 is gradually decreased (step 98), the desired pressure setpoint output by the hydraulics model 120 changes (step 100), and the choke 34 is adjusted accordingly (step 102). These steps 98, 100, 102 are depicted in FIG. 3 as being performed in parallel, because each one depends on the others, and the steps can be performed simultaneously.
For example, as the rig pump 68 output decreases, equivalent circulating density also decreases, due to reduced flow of the fluid 18 through the wellbore 12. This situation is detected by various sensors, and is input to the hydraulics model 120, which updates the desired wellbore pressure setpoint accordingly. The choke 34 is adjusted as needed to maintain the updated desired pressure in the wellbore.
Eventually, flow from the rig pump 68 ceases, and the choke 34 is fully closed. The standpipe valve 82 is also closed to thereby trap the desired pressure in the wellbore 12 (step 104).
In step 106, the accumulator valve 84 is opened, so that the accumulator 70 can supply pressure to the annulus 20, if needed. Alternatively, the accumulator valve 84 could be opened only when and if pressure in the wellbore 12 falls below the desired pressure setpoint.
In step 108, pressure in the standpipe 26 is bled off in preparation for disconnecting a kelly or top drive, etc. A standpipe 26 bleed valve (not shown) is used for this purpose in conventional drilling operations.
In step 110, the connection is made in the drill string 16. This step 110 could comprise threading a stand of drill pipe to the drill string 16 after disconnecting the kelly or top drive, etc. After the connection is made, the kelly or top drive, etc. is reconnected to the drill string 16, and the standpipe 26 bleed valve is closed.
In step 112, the standpipe valve 82 is opened, and the choke 34 is opened, to thereby reestablish circulation through the drill string 16 and annulus 20. This step is preferably performed gradually to minimize pressure spikes, for example, by slowly filling the added drill pipe stand and the standpipe 26 with the fluid 18 from the rig pump 68. Any resulting pressure spikes can be mitigated by the dampener 72.
In steps 114, 130, 132, the output of the rig pump 68 is gradually increased, the setpoint pressure output by the hydraulics model 120 is updated, and the choke 34 is adjusted as needed to maintain the updated desired pressure in the wellbore 12. These steps are similar to the steps 98, 100, 102 described above, except in reverse (e.g., the output of the pump 68 is increased in step 114, instead of being decreased as in step 98).
When circulation of the fluid 18 through the drill string 16 and annulus 20 has been reestablished (steps 112, 114, 130, 132), the accumulator valve 84 can be closed (step 134), since at that point the choke 34 can be used to maintain the desired pressure in the wellbore 12. However, in other examples it may be desired to leave the accumulator 70 available to apply pressure the wellbore before and/or after the method 90 is performed.
Although FIG. 3 indicates that the accumulator valve 84 is opened at a particular point in the method 90 (step 106), and is closed at a particular point in the method (step 134), it should be clearly understood that the accumulator 70 may only supply pressure to the annulus 20 when and if pressure in the wellbore 12 falls below the desired pressure setpoint. The controller 126 could automatically control operation of the accumulator valve 84 (or another type of flow control device, e.g., a pressure regulator, etc.), so that pressure is supplied from the accumulator 70 to the wellbore 12 only when needed.
It may now be fully appreciated that the above disclosure provides significant advancements to the art of wellbore pressure control for optimized pressure drilling operations. The accumulator 70 can provide for application of pressure to the annulus 20, for example, when the fluid 18 is not flowing through the choke 34. The dampener 72 can be used to mitigate pressure spikes during the drilling operation and, if provided with sufficient volume, can serve as an accumulator itself.
The above disclosure provides to the art a well system 10. The well system 10 can include an accumulator 70 in communication with a wellbore 12, whereby the accumulator 70 applies pressure to the wellbore 12.
The wellbore 12 may be isolated from atmosphere by a rotating control device 22.
The well system 10 may also include a hydraulics model 120 which outputs a desired wellbore pressure. The accumulator 70 can apply pressure to the wellbore 12 in response to actual wellbore pressure being less than the desired wellbore pressure.
The accumulator 70 may be in communication with an annulus 20 formed between a drill string 16 and the wellbore 12. The accumulator 70 can be connected to a fluid return line 30 between a blowout preventer stack 42 and a choke manifold 32.
The well system 10 can include a choke 34 which variably restricts flow of fluid 18 from the wellbore 12, with the accumulator 70 applying pressure to the wellbore 12 in an absence of flow of the fluid 18 through the choke 34.
The well system 10 can also include a dampener 72 in communication with the wellbore 12.
The above disclosure also describes a method 90 of maintaining a desired pressure in a wellbore 12. The method 90 can include applying pressure to the wellbore 12 from an accumulator 70 in response to pressure in the wellbore 12 being less than the desired pressure.
Applying pressure may be performed concurrently with an absence of fluid 18 flow through a choke 34 which variably restricts flow of the fluid 18 from the wellbore 12.
The method 90 can also include providing communication between the wellbore 12 and a dampener 72.
The method 90 can include isolating the wellbore 12 from atmosphere with a rotating control device 22.
The method 90 can include outputting the desired pressure from a hydraulics model 120.
The method 90 can include providing communication between the accumulator 70 and an annulus 20 formed between a drill string 16 and the wellbore 12.
The method 90 can include performing the applying pressure while making or breaking a connection in a drill string 16.
Applying pressure may be performed in absence of fluid 18 circulating through a drill string 16 and an annulus 20 formed between the drill string 16 and the wellbore 12.
Also described above is a well system 10 which can include a dampener 72 in communication with a wellbore 12 isolated from atmosphere. The dampener 72 mitigates pressure spikes in the wellbore 12.
The wellbore 12 may be isolated from atmosphere by a rotating control device 22.
The dampener 72 may be in communication with an annulus 20 formed between a drill string 16 and the wellbore 12.
It is to be understood that the various embodiments of the present disclosure described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present disclosure. The embodiments are described merely as examples of useful applications of the principles of the disclosure, which is not limited to any specific details of these embodiments.
Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the disclosure, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to the specific embodiments, and such changes are contemplated by the principles of the present disclosure. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

## Claims

1. A well system, comprising:
an accumulator in communication with a wellbore, whereby the accumulator applies pressure to the wellbore,
wherein the accumulator is connected to a fluid return line between a blowout preventer stack and a choke manifold.

2. The well system of claim 1, wherein the wellbore is isolated from atmosphere by a rotating control device.

3. The well system of claim 1, further comprising a hydraulics model which outputs a desired wellbore pressure, and wherein the accumulator applies pressure to the wellbore in response to actual wellbore pressure being less than the desired wellbore pressure.

4. The well system of claim 1, wherein the accumulator is in communication with an annulus formed between a drill string and the wellbore.

5. The well system of claim 1, further comprising a choke which variably restricts flow of fluid from the wellbore, and wherein the accumulator applies pressure to the wellbore in an absence of flow of the fluid through the choke.

6. The well system of claim 1, further comprising a dampener in communication with the wellbore.

7. A method of maintaining a desired pressure in a wellbore, the method comprising:
providing an accumulator connected to a fluid return line between a blowout preventer stack and a choke manifold; and
applying pressure to the wellbore from the accumulator in response to pressure in the wellbore being less than the desired pressure.

8. The method of claim 7, wherein applying pressure is performed concurrently with an absence of fluid flow through a choke which variably restricts flow of the fluid from the wellbore.

9. The method of claim 7, further comprising providing communication between the wellbore and a dampener.

10. The method of claim 7, further comprising isolating the wellbore from atmosphere with a rotating control device.

11. The method of claim 7, further comprising outputting the desired pressure from a hydraulics model.

12. The method of claim 7, further comprising providing communication between the accumulator and an annulus formed between a drill string and the wellbore.

13. The method of claim 7, further comprising performing the applying pressure while making a connection in a drill string.

14. The method of claim 7, further comprising performing the applying pressure while breaking a connection in a drill string.

15. The method of claim 7, wherein applying pressure is performed in an absence of fluid circulating through a drill string and an annulus formed between the drill string and the wellbore.

16. A well system, comprising:
a dampener in communication with a wellbore isolated from atmosphere, whereby the dampener mitigates pressure spikes in the wellbore; and
an accumulator connected to a fluid return line between a blowout preventer stack and a choke manifold, the accumulator being in communication with and configured to apply pressure to the wellbore.

17. The well system of claim 16, wherein the wellbore is isolated from atmosphere by a rotating control device.

18. The well system of claim 16, wherein the dampener is in communication with an annulus formed between a drill string and the wellbore.

19. The well system of claim 16, further comprising an accumulator in communication with the wellbore, whereby the accumulator applies pressure to the wellbore.

20. The well system of claim 19, further comprising a hydraulics model which outputs a desired wellbore pressure, and wherein the accumulator applies pressure to the wellbore in response to actual wellbore pressure being less than the desired wellbore pressure.

21. The well system of claim 19, further comprising a choke which variably restricts flow of fluid from the wellbore, and wherein the accumulator applies pressure to the wellbore in an absence of flow of the fluid through the choke.