Reservoir communication with a wellbore

This claims the benefit of U.S. Provisional Application Ser. Nos. 60/186,500, filed Mar. 2, 2000; 60/187,900, filed Mar. 8, 2000; and 60/252,754, filed Nov. 22, 2000.

TECHNICAL FIELD

The invention relates to improving reservoir communication within a wellbore.

BACKGROUND

To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in casing and to extend perforations into the surrounding formation.

The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties, and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.

One popular method of obtaining clean perforations is underbalanced perforating. The perforation is carried out with a lower wellbore pressure than the formation pressure. The pressure equalization is achieved by fluid flow from the formation and into the wellbore. This fluid flow carries some of the damaging rock particles. However, underbalance perforating may not always be effective and may be expensive and unsafe to implement in certain downhole conditions.

Fracturing of the formation to bypass the damaged and plugged perforation may be another option. However, fracturing is a relatively expensive operation. Moreover, clean, undamaged perforations are required for low fracture initiation pressure (one of the pre-conditions for a good fracturing job). Acidizing, another widely used method for removing perforation damage, is not effective for treating sand and loose debris left inside the perforation tunnel.

A need thus continues to exist for a method and apparatus to improve fluid communication with reservoirs in formations of a well.

SUMMARY

In general, according to one embodiment, a tool string for use in a wellbore extending from a well surface comprises a closure member adapted to be positioned below the well surface and a low pressure chamber defined at least in part by the closure member. At least a port is selectively openable to enable communication between the chamber and a wellbore region. The at least one port when opened creates a fluid surge into the chamber to provide a local low pressure condition in the wellbore region. A tool in the tool string is adapted to perform an operation in the local low pressure condition.

In general, according to one embodiment, a tool string for use in a wellbore comprises an assembly having at least a first chamber and a second chamber, and control elements to enable communication with the first chamber to create an underbalance condition in the wellbore and to enable communication with the second chamber to create a flow surge from a formation.

In general, according to another embodiment, a method for use in a wellbore comprises lowering a tool string having a first chamber into the wellbore proximal a formation and activating at least one explosive element to open communication with the chamber to create an underbalance condition in the wellbore proximal the formation.

In general, according to another embodiment, a tool string for use in a wellbore comprises a packer, a circulating valve, and an atmospheric chamber. The circulating valve, when open, is adapted to vent a lower wellbore region below the packer once the packer is set, and the atmospheric chamber is capable of being opened to create an underbalance condition below the packer.

In general, according to another embodiment, an apparatus for use with a wellbore comprises subsea wellhead equipment including a blow-out preventer, a choke line filled with a low density fluid, and a kill line filled with a heavy fluid. A downhole string is positioned below the subsea wellhead equipment, and the choke line is adapted to be open to create an underbalance condition in the wellbore.

In general, according to another embodiment, a method of creating an underbalance condition in a wellbore comprises controlling wellbore pressure at least in a perforating interval to achieve a target level and configuring a perforating gun to achieve a target detonation pressure in the perforating gun upon detonation. An underbalance condition in the perforating interval of the wellbore is created when the perforating gun is shot.

Other or alternative features will become apparent from the following description, from the drawings and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C

illustrate different embodiments of strings each employing an apparatus to generate a local low pressure condition.

FIGS. 2A and 2C

illustrate tool strings according to two embodiments for creating an underbalance condition in a wellbore for perforating.

FIG. 2B

illustrates a container including an atmospheric chamber, the container having ports that are explosively actuatable in accordance with one embodiment.

FIG. 3

is a flow diagram of a process of selecting characteristics of a fluid flow surge based on wellbore characteristics.

FIG. 4

illustrates a string having plural sections, each section including a perforating gun and an apparatus to create an underbalance condition or surge.

FIG. 5

illustrates a tool string according to another embodiment for creating an underbalance condition for a perforating operation followed by creating a flow surge from a target formation.

FIG. 6

is a timing diagram of a sequence of events performed by the tool string of FIG.

5

.

FIG. 7

illustrates a tool string according to a further embodiment for creating an underbalance condition for a perforating operation followed by creating a flow surge from a target formation.

FIG. 8

illustrates a tool string according to another embodiment for creating an underbalance condition in a wellbore.

FIG. 9

illustrates subsea well equipment that is useable with the tool string of FIG.

8

.

FIGS. 10 and 11

illustrate a perforating gun string positioned in a wellbore.

FIG. 12

is a graph illustrating the wellbore pressure during detonation of the perforating gun string.

FIG. 13

is a flow diagram of a process in accordance with an embodiment of the invention.

FIG. 14

illustrates an alternative embodiment of a tool string including a perforating gun and an apparatus to create a fluid surge.

FIG. 15

illustrates yet another embodiment of a tool string including a valve that is actuatable between open and closed positions to create desired pressure conditions during perforating and a subsequent surge operation.

FIG. 16

illustrates a tool string for performing a perforate-surge-gravel pack operation, in accordance with another embodiment.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

As used here, the terms “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; “upstream” and “downstream”; “above” and “below” and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly described some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or other relationship as appropriate.

Generally, a method and apparatus is provided for creating a local low pressure condition in a wellbore. In some embodiments, the local low pressure condition is created by use of a chamber containing a relatively low fluid pressure. For example, the chamber is a sealed chamber containing a gas or other fluid at a lower pressure than the surrounding wellbore environment. As a result, when the chamber is opened, a sudden surge of fluid flows into the lower pressure chamber to create the local low pressure condition in a wellbore region in communication with the chamber after the chamber is opened.

In some embodiments, the chamber is a closed chamber that is defined in part by a closure member located below the surface of the well. In other words, the closed chamber does not extend all the way to the well surface. For example, the closure member may be a valve located downhole. Alternatively, the closure member includes a sealed container having ports that include elements that can be shattered by some mechanism (such as by the use of explosive or some other mechanism). The closure member may be other types of devices in other embodiments.

In accordance with a first embodiment, a method and apparatus provides for treatment of perforation damage and for the removal of perforation generated (charge and formation) debris from the perforation tunnels. In this first embodiment, a sealed atmospheric container is lowered into the wellbore after a formation has been perforated. After production is started, openings are created (such as by use of explosives, valves, or other mechanisms) in the housing of the container to generate a sudden underbalance condition or fluid surge to remove the damaged sand grains around the perforation tunnels and to remove loose debris.

Another application of creating a local low pressure condition or fluid surge in a wellbore region is to clean filter cake from open hole sections. Using an apparatus

52

(

FIG. 1A

) according to some embodiments of the invention, localized cleanup of a target open hole section

50

can be performed. The apparatus

52

includes one or more ports

53

that are selectively openable to enable communication with an inner, lower pressure chamber inside the apparatus

52

. The ports

53

can be actuated opened by use of a valve, an explosive, or some other mechanisms. In conventional global cleanup operations in which the entire well is treated, high permeability sections are preferentially treated, which may cause other open hole sections to be under-treated. By using local fluid surges to perform the cleanup, more focused treatment can be accomplished. The apparatus

52

is run to a desired depth on a carrier line

54

(e.g., coiled tubing, wireline, slickline, etc.).

Another drawback of global well treatments involving drawdown of the well is that the drawdown can be limited by surface equipment capacity to handle produced hydrocarbons. By using localized fluid surges according to some embodiments, a higher local drawdown in a given wellbore section can be achieved to enhance cleanup operations.

Yet another application of creating local low pressure conditions is the enhancement of the performance of jet cutter equipment. A jet cutter is a chemical cutter that uses chemical agents to cut through downhole structures. The performance of a jet cutter can be adversely affected if the jet cutter is operated in a relatively high fluid pressure environment. An apparatus

56

(

FIG. 1B

) according to some embodiments can be used to create a local low pressure condition proximal a jet cutter

58

to enhance jet cutter performance. The apparatus

56

includes one or more selectively openable ports

60

. In another embodiment, the jet cutter

50

can be substituted with a perforating gun, with the apparatus

56

used to create an underbalance condition to perform underbalance perforating. Alternatively, in the perforating gun example, the apparatus

56

can be used to create a fluid surge after perforating has been performed.

Another application of some embodiments is the use of a pressure surge apparatus

64

(

FIG. 1C

) as a fishing aid. The pressure surge apparatus

64

generates a local pressure surge when one or more ports

65

are opened to help remove a differential sticking force that causes a string to be stuck in a wellbore. The string includes a carrier line

62

, the pressure surge apparatus

64

, and a tool

66

, in one example. The creation of a pressure surge can cause application of an axial force on the string to help dislodge the string from its stuck position.

In each of the examples, and in other examples described below, various mechanisms can be used to provide the low pressure in a chamber. For example, tubing or control line can be used to communication the low pressure. Alternatively, the low pressure is carried in a sealed container into the wellbore. In a subsea application, the low pressure can be communicated through a choke line or kill line.

In accordance with other embodiments, a tool string including multiple chambers and a perforating gun is lowered into the wellbore. In these other embodiments, a first chamber is used to create an underbalance condition prior to perforating. The perforating gun is then fired, following which the perforating gun is released. After the perforating gun has dropped away from the perforated formation, a second chamber is opened to create a flow surge from the formation into the second chamber. After a surge of a predetermined volume of formation fluid into the second chamber, a flow control device may be opened to inject fluid in the second chamber back into the formation. Alternatively, the formation fluid in the second chamber may be produced to the surface.

In accordance with yet another embodiment, an underbalance condition may be created by using a choke line and a kill line that are part of subsea well equipment in subsea wells. In this other embodiment, the choke line, which extends from the subsea well equipment to the sea surface, may be filled with a low density fluid, while the kill line, which also extends to the sea surface, may be filled with a heavy wellbore fluid. Once the tool string is run into the wellbore, a blow-out preventer (BOP), which is part of the subsea well equipment, may be closed, followed by opening of the choke line below the BOP and the closing of the kill line below the BOP. Opening of the choke line and closing of the kill line causes a reduction in the hydrostatic head in the wellbore to create an underbalance condition.

In yet another embodiment, a chamber within the gun can be used as a sink for wellbore fluids to generate the underbalance condition. Following charge combustion, hot detonation gas fills the internal chamber of the gun. If the resultant detonation gas pressure is less than the wellbore pressure, then the cooler wellbore fluids are sucked into the gun housing. The rapid acceleration through perforation ports in the gun housing breaks the fluid up into droplets and results in rapid cooling of the gas. Hence, rapid gun pressure loss and even more rapid wellbore fluid drainage occurs, which generates a drop in the wellbore pressure. The drop in wellbore pressure creates an underbalance condition.

Referring to

FIG. 2A

, a tool string having a sealed atmospheric container

10

(or container having an inner pressure that is lower than an expected pressure in the wellbore in the interval of the formation

12

) is lowered into a wellbore (which is lined with casing

24

) and placed adjacent a perforated formation

12

to be treated. The tool string is lowered on a carrier line

22

(e.g., wireline, slickline, coiled tubing, etc.). The container

10

includes a chamber that is filled with a gas (e.g., air, nitrogen) or other fluid. The container

10

has a sufficient length to treat the entire formation

12

and has multiple ports

16

that can be opened up using explosives.

As shown in

FIG. 2B

, the ports

16

may include openings that are plugged with sealing elements

18

(e.g., elastomer elements, ceramic covers, etc.). An explosive, such as a detonating cord

20

, is placed in the proximity of each of the ports

16

. Activation of the detonating cord

20

causes the sealing elements

18

to shatter or break away from corresponding ports

16

. In another embodiment, the ports

16

may include recesses, which are thinned regions in the housing of the container

10

. The thinned regions allow easier penetration by explosive forces.

In one embodiment, while the well is producing (after perforations in the formation

12

have been formed), the atmospheric chamber in the container

10

is explosively opened to the wellbore. This technique can be used with or without a perforating gun. When used with a gun, the atmospheric container allows the application of a dynamic underbalance even if the wellbore fluid is in overbalance just prior to perforating. The atmospheric container

10

may also be used after perforation operations have been performed. In this latter arrangement, production is established from the formation, with the ports

16

of the atmospheric container

10

explosively opened to create a sudden underbalance condition.

As discussed above, there are several potential mechanisms of damage to formation productivity and injectivity due to perforation. One may be the presence of a layer of low permeability sand grains (grains that are fractured by the shaped charge) after perforation. As the produced fluid from the formation may have to pass through this lower permeability zone, a higher than expected pressure drop may occur resulting in lower productivity. Underbalance perforating is one way of reducing this type of damage. However, in many cases, insufficient underbalance may result in only partial alleviation of the damage. The second major type of damage may arise from loose perforation-generated rock and charge debris that fills the perforation tunnels. Not all the particles may be removed into the wellbore during underbalance perforation, and these in turn may cause declines in productivity and injectivity (for example, during gravel packing, injection, and so forth). Yet another type of damage occurs from partial opening of perforations. Dissimilar grain size distribution can cause some of these perforations to be plugged (due to bridging, at the casing/cement portion of the perforation tunnel), which may lead to loss of productivity and injectivity.

To remedy these types of damage, two forces acting simultaneously may be needed, one to free the particles from forces that hold them in place and another to transport them. The fractured sand grains in the perforation tunnel walls may be held in place by rock cementation, whereas the loose rock and sand particles and charge debris in the tunnel may be held in place by weak electrostatic forces. Sufficient fluid flow velocity is required to transport the particles into the wellbore.

The explosively actuated container

10

in accordance with one embodiment includes air (or some other suitable gas or fluid) inside. The dimensions of the chamber

10

are such that it can be lowered into a completed well either by wireline, coiled tubing, or other mechanisms. The wall thickness of the chamber is designed to withstand the downhole wellbore pressures and temperatures. The length of the chamber is determined by the thickness of perforated formation being treated. Multiple ports

16

may be present along the wall of the chamber

10

. Explosives are placed inside the atmospheric container in the proximity of the ports. The explosives may include a detonating cord (such as

20

in

FIG. 2B

) or even shaped charges.

In one arrangement, the tool string including the container

10

is lowered into the wellbore and placed adjacent the perforated formation

12

. In this arrangement, the formation

12

has already been perforated, and the atmospheric chamber

10

is used as a surge generating device to generate a sudden underbalance condition. Prior to lowering the atmospheric container, a clean completion fluid may optionally be injected into the formation. The completion fluid is chosen based on the formation wettability, and the fluid properties of the formation fluid. This may help in removing particulates from the perforation tunnels during fluid flow.

After the atmospheric container

10

is lowered and placed adjacent the perforated formation

12

, the formation

12

is flowed by opening a production valve at the surface. While the formation is flowing, the explosives are set off inside the atmospheric container, opening the ports of the container

10

to the wellbore pressure. The shock wave generated by the explosives may provide the force for freeing the particles. The sudden drop in pressure inside the wellbore may cause the fluid from the formation to rush into the empty space left in the wellbore by the atmospheric container

10

. This fluid carries the mobilized particles into the wellbore, leaving clean formation tunnels. The chamber may be dropped into the well or pulled to the surface.

If used with a perforating gun, activation of the perforating gun may substantially coincide with opening of the ports

16

. This provides underbalanced perforation. Referring to

FIG. 2C

, use of an atmospheric container

10

A in conjunction with a perforating gun

30

, in accordance with another embodiment, is illustrated. In the embodiment of

FIG. 2C

, the container

10

A is divided into two portions, a first portion above the perforating gun

30

and a second portion below the perforating gun

30

. The container

10

A includes various openings

16

A that are adapted to be opened by an explosive force, such as an explosive force due to initiation of a detonating cord

20

A or detonation of explosives connected to the detonating cord

20

A. The detonating cord is also connected to shaped charges

32

in the perforating gun

30

. In one embodiment, as illustrated, the perforating gun

30

can be a strip gun, in which capsule shaped charges are mounted on a carrier

34

. Alternatively, the shaped charges

32

may be non-capsule shaped charges that are contained in a sealed container.

The fluid surge can be performed relatively soon after perforating. For example, the fluid surge can be performed within about one minute after perforating. In other embodiments, the pressure surge can be performed within (less than or equal to) about 10 seconds, one second, or 100 milliseconds, as examples, after perforating. The relative timing between perforation and fluid flow surge is applicable also to other embodiments described herein.

The characteristics (including the timing relative to perforating) of the fluid surge can be based on characteristics (e.g., wellbore diameter, formation pressure, hydrostatic pressure, formation permeability, etc.) of the wellbore section in which the local low pressure condition is to be generated. Generally, different types of wellbores having different characteristics. In addition to varying timing of the surge relative to the perforation, the volume of the low pressure chamber and the rate of fluid flow into the chamber can be controlled. Referring to

FIG. 3

, tests can be performed on wells of different characteristics, with the tests involving creation of pressure surges of varying characteristics to test their effectiveness. The test data is collected (at

70

), and the optimum surge characteristics for a given type of well are stored (at

71

) in models for later access.

When a target well in which a local surge operation is identified, the characteristics of the well are determined (at

73

) and matched to one of the stored models. Based on the model, the surge characteristics are selected (at

74

), and the operation involving the surge is performed (at

75

). As part of the operation, the pressure condition and other well conditions in the wellbore section resulting from the surge can be measured (at

75

), and the model is adjusted (at

76

) if necessary for future use.

The downhole pressure and other well conditions are measured using gauges or sensors run into the wellbore with the string. As a further refinement, the gauges or sensors can collect data at a relatively fast sampling rate. Based on the measurements, a different model may be selected (during the operation) to vary the relative timing of the perforation and surge.

Even though the described embodiments describe a single perforating operation followed by a single surge operation, other embodiments can involve multiple perforating and surge operations. For example, referring to

FIG. 4

, a string includes three sections that are activate at different times. Other examples can involve a lower number or greater number of sections. The string includes low pressure or surge apparatus

80

A,

80

B, and

80

C, and corresponding perforating guns

81

A,

81

B,

81

C. The first section (

80

A,

81

A) can be activated first, followed sequentially by activation of the second (

80

B,

81

B) and third (

80

C,

81

C) sections. The delay between activation of the different sections can be set to predetermined time delays. As discussed here, activation of a section can refer to activating the perforating gun

81

followed by opening the apparatus

80

to generate a surge. Alternatively, activation of a section can refer to opening the apparatus

80

to generate an underbalance condition followed by activation of the perforating gun

81

to perform underbalanced shooting.

Referring to

FIG. 5

, in accordance with another embodiment, a tool string with plural chambers may be employed. The tool string includes a perforating gun

100

that is attached to an anchor

102

. The anchor

102

may be explosively actuated to release the perforating gun

100

. Thus, for example, activation of a detonating cord

104

to fire shaped charges

106

in the perforating gun

100

will also actuate the anchor

102

to release the perforating gun

100

, which will then drop to the bottom of the wellbore.

The anchor

102

includes an annular conduit

108

to enable fluid communication in the annulus region

110

(also referred to as a rat hole) with a region outside a first chamber

114

of the tool string. The first chamber

114

has a predetermined volume of gas or fluid. As with the atmospheric container

10

of

FIGS. 2A

,

2

B, and

2

C, the housing defining the first chamber

114

may include ports

116

that can be opened, either explosively or otherwise. The volume of the first chamber

114

in one example may be approximately 7 liters or 2 gallons. This is provided to achieve roughly a 200 psi (pounds per square inch) underbalance condition in the annulus region

110

when the ports

116

are opened. In other configurations, other sizes of the chamber

114

may be used to achieve a desired underbalance condition that is based on the geometry of the wellbore and the formation pressure. A control module

126

may include a firing head (or other activating mechanism) to initiate a detonating cord

129

(or to activate some other mechanism) to open the ports

116

.

A packer

120

is set around the tool string to isolate the region

112

from an upper annulus region

122

above the packer

120

. Use of the packer

120

provides isolation of the rat hole so that a quicker response for the underbalance condition or surge can be achieved. However, in other embodiments, the packer

120

may be omitted. Generally, in the various embodiments described herein, use of a packer for isolation or not of the annulus region is optional.

The tool string of

FIG. 5

also includes a second chamber

124

. The control module

126

may also include a flow control device

127

(e.g., a valve) to control communication of well fluids from the first chamber

114

to the second chamber

124

. During creation of the underbalance condition, the flow control device

127

is closed.

Referring further to

FIG. 6

, operation of the tool string of

FIG. 5

is described. After the tool string is positioned downhole, the first chamber

114

may be opened (at

150

) to enable creation of an underbalance condition in the lower region

110

of the wellbore. Depending on the volume of the first chamber

114

and other factors (including the location of the chamber and length of the guns), the time to achieve a desired underbalance condition (at

152

) may vary. For example, to achieve about a 200 psi underbalance condition with a first chamber

114

having a volume of approximately 7 liters and the gun string having a length of approximately 150 ft., the time required may be greater than about 30 milliseconds (ms). The numbers given in the example are provided for illustration purposes only, and are not intended to limit the scope of the invention.

A delay is thus provided between the opening of the ports

116

of the first chamber

114

and firing of the perforating gun

100

. This delay may be provided by a downhole timer mechanism

131

or by independent control (in the form of commands such as elevated pressure or pressure pulse signals communicated through the annulus

122

, such as to a downhole control module coupled to the detonating cord

104

). Alternatively, sensors may be placed downhole to check for the underbalance condition.

Once the underbalance condition is achieved, the perforating gun

100

is fired (at

154

). If a check determines that the underbalance condition is not present, then firing of the gun

100

may be prevented. Firing of the perforating gun

100

may also activate the anchor

102

to release the gun

100

, which is then dropped (at

156

) to the bottom of the wellbore. The time to clear the formation depends on the length of the gun

100

and deviation of the well. For example, if the gun length is about 100 feet in a 60° deviated well, then it may take about 40 seconds for top of the gun to clear perforated formation. After the appropriate delay, the flow control device

127

in the control module

126

is opened (at

158

) to enable a fluid flow surge into the second chamber

124

. The volume of the second chamber

124

depends on the amount of surge desired. For example, the volume may be about 40 barrels (bbl). This may take about 120 seconds to fill.

Following the surge operation (at

160

) and after some predetermined delay set by a timer mechanism, surface control, or measurement of downhole condition, a valve (not shown) further up the wellbore may be opened and injection pressure applied to inject fluid (at

162

) in the second chamber

124

back into the formation. This is particularly useful in subsea applications, where production of fluid to the surface is undesirable. In an alternative embodiment, if the well is a land well, the fluid in the second chamber

124

may be produced to the surface. To produce fluid from the chamber

124

, the flow control device in the control module

126

may be closed to isolate the second chamber

124

from the formation.

Referring to

FIG. 7

, a tool string according to yet another embodiment is illustrated. The operations performed by the tool string are similar to those described above in connection with

FIGS. 5 and 6

. The tool string includes a perforating gun

200

attached below a tubing

202

. A packer

204

set around the tubing

202

isolates the annulus region

206

from the target formation

208

.

The tubing

202

may be attached to three valves

210

,

212

, and

214

. As illustrated, in one embodiment, the valves

210

,

212

, and

214

are ball valves. Alternatively, the valves may be sleeve valves, flapper valves, disk valves, or any other type of flow control device. When the valves

210

,

212

, and

214

are in the closed position (as illustrated), two chambers

220

and

222

are defined. The first and second chambers

220

and

222

correspond to the first and second chambers

114

and

124

, respectively, in the tool string of FIG.

5

. Both chambers

220

and

224

may be initially filled with a gas (e.g., air or nitrogen) or some other suitable compressible fluid. In one arrangement, the first chamber

220

is relatively small in volume, to create an underbalance condition prior to perforating, while the second chamber

222

is much larger to receive a fluid surge.

The valves

210

,

212

, and

214

are controlled by operators

216

,

218

, and

219

, respectively. In one embodiment, the operators are activated by pressure communicated in the annulus region

206

. The operators may thus be responsive to elevated pressures or to predetermined numbers of pressure cycles. Alternatively, the operators are responsive to low-level pressure pulse signals of predetermined amplitudes and periods. The operators

216

,

218

, and

219

are thus controllable from the surface. In yet other embodiments, other types of actuators can be used to control the operators

216

,

218

, and

219

. Such other actuators include electrical actuators or mechanical actuators. The sequence of events shown in

FIG. 6

may be performed with the tool string of FIG.

7

.

When the tool string of

FIG. 7

is run in, the valves

210

,

212

, and

214

are closed. Before shooting the gun

200

, the first valve

210

is opened to enable communication with the first chamber

220

to create an underbalance condition. Fluid flows from the rat hole through ports

209

into the inner bore of the tubing

202

and to the first chamber

220

. The gun

200

is then fired, with the gun dropped by an anchor

205

after firing. Thereafter, the second valve

212

may be opened to create a fluid surge from the formation

208

into second chamber

222

. After the second chamber

222

has filled up, or after some predetermined time period, the third valve

214

may be opened to enable either production to the surface or application of injection pressure to inject the second chamber fluid back into the formation

208

.

Using either the embodiments of

FIGS. 5 and 7

, the various events are achievable in a single trip. This avoids costs that may be incurred if multiple runs are needed. By performing the underbalance perforating in conjunction with subsequent surge, improved perforation tunnel characteristics may be achieved. Tool strings according to some embodiments employ at least two chambers initially at some low pressure (e.g., atmospheric pressure), with a first chamber to create the underbalance condition and a second chamber to provide the fluid surge.

Referring to

FIG. 8

, a tool string

300

in accordance with another embodiment is illustrated. Similar to the tool string of

FIG. 7

, an atmospheric chamber

304

is defined between a first valve

302

(e.g., a ball valve) and a second valve

306

(e.g., a ball valve). A circulating valve

307

is also provided to enable communication between an inner bore of the tool string

300

and an annulus region

324

above a packer

310

. The circulating valve

307

may include a sleeve valve, a disk valve, or any other type of valve to control fluid communication between the inside and outside of the tool string

300

.

A pressure monitoring device

308

may also be attached to the tool string

300

. The pressure monitoring device

308

is used to sense pressure conditions in the wellbore and to communicate the sensed pressure to the well surface. This may be accomplished by using electrical cabling. Alternatively, the pressure monitoring device

308

may include a storage device to store collected pressure data which may be accessed once the tool string

300

is retrieved to the surface.

The packer

310

may be attached below the pressure monitoring device. A pressure feed port

312

in the tool string below the packer

310

is provided to enable communication between a rat hole

326

(below the packer

310

) and the inner bore of the tool string

300

. If the circulating valve

307

is open, then fluid pressure in the rat hole

326

is communicated through the feed ports

312

to the annulus region

324

.

In the example embodiment, the tool string

300

also includes a full bore firing head

314

, a ballistic swivel

316

, and an anchor

318

that may be explosively activated to release a perforating gun

314

. Orienting weights

320

and

322

may be attached to the perforating gun

314

to orient the gun

314

in a desired azimuthal direction.

In accordance with some embodiments, the circulating valve

307

allows pressure in the rat hole

326

to be vented to a known level after the packer

310

is set. When setting a packer on a closed bottom hole (such as in a subsea well), the compression of setting the packer can pump up the well by up to about 800 psi. This may give uncertainty in the pressure below the packer

310

and hence in the perforating pressure. By opening the circulating valve, the rat hole

326

below the packer

310

may be vented to a known pressure level after the packer

310

is set and a BOP is set at the well surface.

After the circulation valve

307

is closed, the ball valve

306

may be opened to open the atmospheric chamber

304

to create an underbalance condition in the rat hole

326

. A perforating or other operation may then be performed in the underbalance condition.

One aspect of some of the embodiments described above is that the formation that is being perforated remains isolated by a valve and/or a sealing element from a conduit that is in communication with the well surface. After perforation, the isolating device is removed to perform the surge. Such isolation is performed to prevent unwanted production of hydrocarbons to the well surface. For example, in

FIG. 5

, the flow control device

127

remains closed so that formation pressure does not escape up the tubing connected above the second chamber. The packer

120

prevents fluid communication up the annulus

122

. In the example of

FIG. 7

, the valve

212

remains closed during perforation. In the example of

FIG. 8

, the valve

302

remains closed during perforation.

FIG. 14

shows another embodiment, which includes a string having a tubing

722

, three valves

702

,

704

, and

706

, and a perforating gun

720

. A packer

708

is set around the string to isolate an annulus

710

. A chamber

712

between the valves

702

and

704

is initially at a relatively low pressure (lower than the surrounding wellbore pressure). The low pressure may be, for example, atmospheric pressure. The valves

702

and

704

may be mechanically, electrically, or hydraulically operable.

The valve

706

, in one embodiment, may be operated by sending pressure pulse commands down the annulus

710

. In addition to the valves

702

,

710

, and

712

, a circulation valve

714

(which may include a sleeve

716

) is included in the string illustrated in FIG.

14

.

During run-in, the valves

702

,

704

, and

714

are closed, while the valve

706

is open. Once run to the desired depth, the packer

708

is set. The valve

704

is then opened, which causes a surge of pressure from the rat hole (beneath the packer

708

) into the low pressure chamber

712

. This causes the rat hole pressure to decrease to a target underbalance condition. The perforating gun

720

is then fired in the underbalance condition to create perforations in formation

726

.

As a result of the fluid surge through the valve

704

as it is opening, the sealing elements of the valve

704

may be damaged. Consequently, the valve

704

may be rendered unusable. To maintain isolation of the formation, the valve

706

is used as a backup after the valve

704

has been opened.

After the surge and perforation operations, the valve

706

is closed (in response to signals sent down the annulus

710

). Once closed, the valve

706

serves to isolate the formation

726

. The valve

702

is then opened to enable communication with the inner bore of the tubing

722

. The circulation valve

714

is then opened to enable reverse circulation of hydrocarbons in the string up to the well surface (the reverse circulation flow is indicated by the arrows

724

).

Referring to

FIG. 15

, in an alternative embodiment, a single valve

804

(e.g., a ball valve) is used. The ball valve

804

is part of a string that also includes a tubing or other conduit

802

, a packer

808

, and a perforating gun

810

.

When run-in, the valve

804

is in the closed position. Once the string is lowered to the proper position, the valve

804

is opened, and the packer

808

is set to isolate an annulus region

806

above the packer

808

from a rathole region

812

below the packer

808

. The internal pressure of the tubing

802

is bled to a lower pressure such that an underbalance condition is created in the rathole

802

proximal the perforating gun

810

. After the tubing pressure has been bled to achieve a desired rathole pressure, the valve

804

is closed, and the perforating gun

810

is fired. Since the rathole

812

at this point has been bled to an underbalance condition, an underbalanced perforation is performed. Because the valve

804

is closed, the formation is isolated during perforation. The pressure inside the tubing is bled down further, such as to an atmospheric pressure. After the gun

810

is fired, the valve

804

is opened, which causes a surge of fluid from the rathole

812

into the inner bore of the tubing

802

.

Referring to

FIG. 9

, a portion of subsea well equipment

400

is illustrated. The subsea well equipment

400

is connected to casing

403

and tubing

404

that extend into a subsea well. The wellhead equipment

400

includes a BOP

402

above the sea bed or mudline

406

. The tubing

404

may extend through the BOP

402

. The BOP

402

includes sealing rams that close on the tubing

404

to create a seal so that the wellbore below the BOP

402

is closed off from the surface. In a subsea well, the BOP

402

is used to prevent wellbore fluids from escaping to the well surface, which may pose environmental hazards. Above the BOP

402

, the tubing

404

is enclosed within a marine riser

408

. Both the marine riser

408

and the tubing

404

extend to the sea surface

410

.

Various fluid communications lines extend from the subsea well equipment

400

to the sea surface

410

. Examples of such fluid communications lines include a choke line

412

and a kill line

414

. As illustrated, both the choke and kill lines

412

and

414

extend to a point below the BOP

402

.

The subsea well equipment

400

may be used in conjunction with the tool string

300

(FIG.

8

). As noted above, after the tool string

300

is run into the subsea wellbore, the packer

310

is set downhole. Setting of the packer

310

can pump up pressure in the well to an unknown level. To vent such pressure buildup, the circulating valve

307

may be opened to vent the pressure in the rat hole

326

before the BOP

402

is closed. The circulation valve

307

is then closed followed by closing of the BOP

402

on the tubing

404

. Next, the atmospheric chamber

304

can be opened to create the underbalance condition in the rat hole

326

. Following that, an underbalance perforating operation may be performed.

In accordance with another embodiment, an alternative procedure for creating an underbalance condition may be performed using the components of

FIGS. 8 and 9

. In this alternative procedure, the choke line

412

may be filled with a low density fluid (e.g., about 8.5 ppg). The kill line

412

may be filled with a heavy wellbore fluid (e.g., about 11.2 ppg). The tool string

300

can then be run into the wellbore on the tubing

404

with the circulation valve

307

in the open position. After the tool string

300

is lowered to a desired depth, the packer

310

is set. Since the circulating valve

307

is open, this prevents an unknown pressure buildup in the rat hole

326

below the packer

310

. Thus, in one example, in an 11,000 feet well, the bottom hole pressure may be around 6,400 psi. After the packer

310

is set, the BOP

402

is closed on the tubing

404

. The choke line

412

at this point is in its closed position while the kill line

414

is in its open position.

After the BOP

402

is closed, the choke line

412

can be opened below the BOP

402

while the kill line

414

is closed below the BOP

402

. This reduces the wellbore pressure below the BOP

402

. Since the circulating valve

307

is open, the rat hole pressure is also reduced. In one example, if the well is in 4,000 feet of water, the hydrostatic head may be reduced by up to 560 psi. The actual drop may be slightly less due to heavy fluid flowing into the choke line but the correction may be of second order.

An underbalance condition is thus created in the rat hole

326

below the packer

310

. Next, the circulating valve

307

may be closed, followed by closing the choke line

412

below the BOP

402

and opening the kill line below the BOP. This restores the overbalance condition in the wellbore above the packer

310

. Next, the perforating gun

314

may be perforated underbalance.

Referring to

FIG. 10

, yet another embodiment for creating an underbalance condition during a perforating operation is illustrated. A perforating gun string

400

includes a perforating gun

402

and a carrier line

404

, which can be a slickline, a wireline, or coiled tubing. In one embodiment, the perforating gun

402

is a hollow carrier gun having shaped charges

414

inside a chamber

418

of a sealed housing

416

. In the arrangement of

FIG. 10

, the perforating gun

402

is lowered through a tubing

406

. A packer

410

is provided around the tubing

406

to isolate the interval

412

in which the perforating gun

402

is to be shot (referred to as the “perforating interval

412

”). A pressure P

W

is present in the perforating interval

412

.

Referring to

FIG. 11

, during detonation of the shaped charges

414

, perforating ports

420

are formed as a result of perforating jets produced by the shaped charges

414

. During combustion of the shaped charges

414

, hot detonation gas fills the internal chamber

418

of the gun

416

. If the resultant detonation gas pressure, P

G

, is less than the wellbore pressure, P

W

, by a given amount, then the cooler wellbore fluids will be sucked into the chamber

418

of the gun

402

. The rapid acceleration of well fluids through the perforation ports

420

will break the fluid up into droplets, which results in rapid cooling of the gas within the chamber

418

. The resultant rapid gun pressure loss and even more rapid wellbore fluid drainage into the chamber

418

causes the wellbore pressure P

W

to be reduced. Depending on the absolute pressures, this pressure drop can be sufficient to generate a relatively large underbalance condition (e.g., greater than 2000 psi), even in a well that starts with a substantial overbalance (e.g., about 500 psi). The underbalance condition is dependent upon the level of the detonation gas pressure P

G

, as compared to the wellbore pressure, P

W

.

When a perforating gun is fired, the detonation gas product of the combustion process is substantially hotter than the wellbore fluid. If cold wellbore fluids that are sucked into the gun produce rapid cooling of the hot gas, then the gas volume will shrink relatively rapidly, which reduces the pressure to encourage even more wellbore fluids to be sucked into the gun. The gas cooling can occur over a period of a few milliseconds, in one example. Draining wellbore liquids (which have small compressibility) out of the perforating interval

412

can drop the wellbore pressure, P

W

, by a relatively large amount (several thousands of psi).

In accordance with some embodiments, various parameters are controlled to achieve the desired difference in values between the two pressures P

W

and P

G

For example, the level of the detonation gas pressure, P

G

, can be adjusted by the explosive loading or by adjusting the volume of the chamber

418

. The level of wellbore pressure, P

W

, can be adjusted by pumping up the entire well or an isolated section of the well, or by dynamically increasing the wellbore pressure on a local level.

Referring to

FIG. 12

, a graph illustrates a simulated perforating operation over time. In the graph, the wellbore pressure is initially at 4000 psi, as indicated by curve

502

, with the pore or formation pressure at 3500 psi, as indicated by curve

500

. This represents an overbalance condition of about 500 psi. Upon detonation, the gas pressure in the gun

402

is about 2700 psi. The rapid influx of fluid into the gun cools the gas, which results in rapid filling of the gun chamber

418

and a relatively large wellbore pressure drop, as indicated by the curve

502

. Initially, the overbalance was about 500 psi. However, shortly after detonation of the gun, the wellbore pressure drops relatively sharply, creating an underbalance of more than about 2000 psi.

For the system illustrated in

FIGS. 10 and 11

to be effective, the pre-detonation wellbore pressure must be greater than the detonation gas pressure, and the post-detonation wellbore must be below the pore or formation pressure by the level required to generate underbalance cleanup.

Referring to

FIG. 13

, a process of controlling parameters to achieve the underbalance in the perforating interval is illustrated. The pressure of the perforating interval is controlled (at

602

). The wellbore pressure can be controlled by pumping up from the surface or pumping up under a packer. If the desired wellbore pressure cannot be attained by a regular hydrostatic or pump-up mechanisms, then a transient pressure adjustment can be used using a local pressure generating device. For example, a small pyrotechnic or ballistic charge can be used to raise the pressure in a similar manner to opening an atmospheric chamber. The pyrotechnic or ballistic charge can be detonated slightly before the main charges within the gun

402