Perforating gun assembly and method for creating perforation cavities

[0001] This invention relates, in general, to perforating a cased wellbore that traverses a subterranean hydrocarbon bearing formation and, in particular, to a perforating gun assembly having collections of shaped charges that are detonated to discharge jets that interact together to form perforation cavities.

[0002] Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation with a perforating gun assembly, as an example.

[0003] After drilling a section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic openings or perforations must be made through the casing string, the cement and a short distance into the formation.

[0004] Typically, these perforations are created by detonating a series of shaped charges that are disposed within the casing string and are positioned adjacent to the formation. Specifically, one or more charge carriers are loaded with shaped charges that are connected with a detonator via a detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, electric line, coil tubing or other conveyance. Once the charge carriers are properly positioned in the wellbore such that the shaped charges are adjacent to the interval to be perforated, the shaped charges may be fired. Upon detonation, each shaped charge generates a high-pressure stream of metallic particles in the form of a jet that penetrates through the casing, the cement and into the formation.

[0005] The goal of the perforation process is to create openings through the casing to form a path for the effective communication of fluids between the reservoir and the wellbore. It has been found, however, that a variety of factors associated with the perforating process can significantly influence the productivity of the well. For example, during the drilling phase of well construction, drilling mud particles build up a filter cake on the side of the wellbore. While the filter cake prevents additional leaching of drilling mud into the reservoir, this filtrate may impair production from the reservoir. Accordingly, effective perforations must not only be formed through the casing and cement, but also through this filter cake and into virgin rock.

[0006] As another example, the pressure condition within the wellbore during the perforation process has a significant impact on the efficiency of the perforations. Specifically, perforating may be performed in an overbalanced or underbalanced pressure regime. Perforating overbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is greater than the reservoir pressure. Overbalanced perforating has the tendency to allow the wellbore fluid to flow into the reservoir formation. Perforating underbalanced involves creating the opening through the casing under conditions in which the hydrostatic pressure inside the casing is less than the reservoir pressure. Underbalanced perforating has the tendency to allow the reservoir fluid to flow into the wellbore. It is generally preferable to perform underbalanced perforating as the influx of reservoir fluid into the wellbore tends to clean up the perforation tunnels and increase the depth of the clear tunnel of the perforation.

[0007] It has been found, however, that even when perforating is performed underbalanced, the effective diameter of the perforation tunnels is small as the jet of metallic particles that creates the perforation tunnels is highly concentrated. Due to the small diameter of the perforation tunnels, the volume of the perforation tunnels is also small. In addition, it has been found that even when perforating is performed underbalanced, the surface of the perforation tunnels has reduced permeability compared to the virgin rock.

[0008] Therefore a need has arisen for a perforating gun assembly having shaped charges that produce jets that are capable of penetrating through the casing, the cement, the filter cake and into the virgin rock of the reservoir formation. A need has also arisen for such a perforating gun assembly that is not limited to creating small volume perforation tunnels behind the casing. Further, a need has arisen for such a perforating gun assembly that is not limited to creating perforation tunnels having a surface with reduced permeability compared to the virgin rock.

[0009] The present invention disclosed herein comprises a perforating gun assembly having shaped charges that produce jets that are capable of penetrating through the casing, the cement, the filter cake and into the virgin rock of the reservoir formation. In addition, the perforating gun assembly of present invention is not limited to creating small volume perforation tunnels behind the casing. Further, the perforating gun assembly of present invention is not limited to creating perforation tunnels having a surface with reduced permeability compared to the virgin rock.

[0010] The perforating gun assembly of present invention comprises a housing, a detonator positioned within the housing and a detonating cord operably associated with the detonator. A plurality of shaped charges forming a substantially axially oriented collection are operably associated with the detonating cord. Upon detonation, the shaped charges in the collection form jets that interact with one another to create a perforation cavity in the formation.

[0011] In one embodiment, the jets formed upon detonating the shaped charges in the collection are directed substantially toward a focal point. In this embodiment, the jets may progress to a location short of the focal point, to a location past the focal point or may converge at the focal point. Accordingly, the jets formed upon detonating the shaped charges in the collection may or may not intersect. The interaction of the jets may be achieved by converging adjacent shaped charges in the collection toward one another. For example, adjacent shaped charges in the collection may converge toward one another at an angle between about 1 degree and about 45 degrees. This configuration may include a center shaped charge and two outer shaped charges, wherein the center shaped charge is oriented substantially perpendicular to an axis of the housing and the outer two shaped charges are oriented to converge toward the center shaped charge.

[0012] In another embodiment, the perforating gun assembly of the present invention may include a plurality of collections of shaped charges. In this embodiment, each collection of shaped charges in the plurality of collections of shaped charges may be circumferentially phased relative to adjacent collections of shaped charges. For example, adjacent collections of shaped charges may be circumferentially phased at an angle of between about 15 degrees and about 180 degrees.

[0013] In another aspect, the present invention comprises a method for creating a perforation cavity in a formation behind a wellbore casing. The method includes positioning a perforating gun assembly within the wellbore casing, the perforating gun assembly including a plurality of shaped charges that form a substantially axially oriented collection and detonating the collection of shaped charges to form jets that interact with one another, thereby creating the perforation cavity in the formation. The method may also include sequentially detonating the collection of shaped charges and performing a treatment operation following detonating the collection of shaped charges. The method may be performed in an underbalanced pressure condition or when an underbalanced pressure condition does not exist.

[0014] In another aspect, the present invention comprises a completion including a subterranean formation, wellbore that traverses the formation and a casing disposed within the wellbore, wherein the formation has a perforation cavity formed therein as a result of an interaction of jets created upon the detonation of a collection of shaped charges within the wellbore.

[0028] While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention.

[0029] Referring initially to FIG. 1, a perforating gun assembly adapted for use in a wellbore operating from an offshore oil and gas platform is schematically illustrated and generally designated 10. A semi-submersible platform 12 is centered over a submerged oil and gas formation 14 located below sea floor 16. A subsea conduit 18 extends from deck 20 of platform 12 to wellhead installation 22 including blowout preventers 24. Platform 12 has a hoisting apparatus 26 and a derrick 28 for raising and lowering pipe strings.

[0030] A wellbore 36 extends through the various earth strata including formation 14. Casing 38 is cemented within wellbore 36 by cement 40. When it is desired to perforate casing 38 adjacent to formation 14, a perforating gun assembly 42 is lowered into casing 38 via conveyance 44 such as a wireline, electric line or coiled tubing. Perforating gun assembly 42 includes a housing 46 which encloses one or more detonators and associated detonating cords as well as a plurality of shaped charges. The shaped charges are axially and circumferentially oriented behind scallops 48 in housing 46 which are areas of housing 46 having a reduced thickness. As illustrated, scallops 48 are formed in groups of three axially oriented scallops with adjacent groups of scallops being circumferentially phased. Alternatively, housing 46 may include a series of ports having port plugs positioned therein instead of scallops 48.

[0031] Once perforating gun assembly 42 is positioned adjacent to formation 14, an electric or other triggering signal is sent to the detonator which initiates the detonation of the shaped charges that are disposed within perforating gun assembly 42. Upon detonation, each of the shaped charges generate a high-pressure stream of metallic particles in the form of a jet that penetrates casing 38, cement 40 and into formation 14. In the present invention, certain of the jets interaction with one another such that perforation cavities are created in formation 14 that are large regions of high permeability surrounding wellbore 36 that significantly enhance the productivity of the well.

[0032] Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the perforating gun assembly of the present invention is equally well-suited for use in wells having other geometries such as deviated wells, inclined wells or horizontal wells. Accordingly, use of directional terms such as up, down, above, below, upper, lower and the like are with reference to the illustrated embodiments in the figures. Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the perforating gun assembly of the present invention is equally well-suited for use in onshore operations. Additionally, even though FIG. 1 depicts a single perforating gun assembly, the principles of the present invention are applicable to gun systems utilizing strings of perforating gun assemblies as well as gun systems utilizing select fire techniques.

[0033] Referring now to FIG. 2, therein is depicted a perforating gun assembly 60 positioned in a wellbore 62 that traverses formation 64. A casing 66 lines wellbore 62 and is secured in position by cement 68. A conveyance 70 is coupled to perforating gun assembly 60 at a cable head 72. A collar locator 74 is positioned below cable head 72 to aid in the positioning of perforating gun assembly 60 in wellbore 62. As noted above, during the drilling phase of well construction, a drilling mud is used to contain formation pressure. Accordingly, the hydrostatic pressure of the drilling mud exceeds the reservoir pressure causing portions of the drilling mud to leach into formation 64. As part of this leaching process, a filter cake 76 builds up near the surface of wellbore 64 which helps to prevent additional leaching but may impair production from formation 64.

[0034] A fluid such as drilling fluid (not shown) fills the annular region between perforating gun assembly 60 and casing 66. In the illustrated embodiment, perforating gun assembly 60 includes a plurality of shaped charges, such as shaped charge 78. Each of the shaped charges includes an outer housing, such as housing 80 of shaped charge 78, and a liner, such as liner 82 of shaped charge 78. Disposed between each housing and liner is a quantity of high explosive. The shaped charges are retained within a charge carrier housing 84 by a support member (not pictured) that maintains the shaped charges in the unique orientation of the present invention.

[0035] Disposed within housing 84 is a detonator 86 that is coupled to an electrical energy source via electrical wire 88. Detonator 86 may be any type of detonator that is suitable for initiating a detonation in a detonating cord as the present invention is detonator independent, such detonators being of the type that are well known in the art or subsequently discovered. Detonator 86 is coupled to a detonating cord 90, such as a primacord. Detonating cord 90 is operably coupled to the initiation ends of the shaped charges allowing detonating cord 90 to initiate the high explosive within the shaped charges through, for example, an aperture defined at the apex of the housings of the shaped charges. In the illustrated embodiment, once detonator 86 is operated, the detonation will propagate down detonating cord 90 to sequentially detonate the shaped charges from the top to the bottom of perforating gun assembly 60. It should be noted, however, by those skilled in the art that other firing sequences could alternatively be used including, for example, a bottom up sequence or simultaneously firing shaped charges at multiple axial levels using multiple detonators, multiple detonating cords, timing devices or the like.

[0036] In the illustrated embodiment, perforating gun assembly 60 includes four collections of shaped charges, namely collections 92, 94, 96, 98. Each collection 92, 94, 96, 98 includes three individual shaped charges such as shaped charges 100, 102, 104 of collection 94. The shaped charges within each collection 92, 94, 96, 98 are. positioned axially relative to one another such that the shaped charges within each collection 92, 94, 96, 98 generally point in the same circumferential direction of housing 84. Accordingly, as used herein the term axially oriented will be used to describe the relationship of shaped charges within a collection of shaped charges wherein adjacent shaped charges are generally axially displaced from one another and generally point in the same circumferential direction.

[0037] In the illustrated embodiment, the shaped charges within each collection 92, 94, 96, 98 are oriented to converge toward one another. For example, collection 94 includes, outer shaped charge 100, center shaped charge 102 and outer shaped charge 104. Center shaped charge 102 is oriented substantially perpendicular to the axis of housing 84. Outer shaped charges 100, 104 are oriented to converge toward center shaped charge 102. In one preferred orientation, the angle of convergence between adjacent shaped charges in each collection 92, 94, 96, 98 is between about 5 degrees and about 10 degrees. Other preferred orientations include angles of convergence between about 1 degree and about 45 degrees. It should be noted that the desired angle of convergence for a particular perforating gun assembly being used to perforate a particular wellbore will be dependent on a variety of factors including the size of the shaped charges, the diameter of the perforating gun assembly and wellbore casing, the expected depth of penetration into the formation and the like.

[0038] In the illustrated embodiment, the shaped charges in adjacent collections are circumferentially phased relative to one another. Specifically, the shaped charges in collection 92 are circumferentially phased ninety degrees from the shaped charges in collection 94. Likewise, the shaped charges in collection 94 are circumferentially phased ninety degrees from the shaped charges in collection 96, the shaped charges in collection 96 are circumferentially phased ninety degrees from the shaped charges in collection 98 and the shaped charges in collection 98 are circumferentially phased ninety degrees from the shaped charges in the next adjacent collection (not pictured) which are circumferentially aligned with the shaped charges in collection 92. Importantly, other circumferential phasing increments may be desirable when using the perforating gun assembly of the present invention such other circumferential phasing increments being within the scope of the present invention. Specifically, circumferential phasing in increments of between about 15 degrees and about 180 degrees are suitable for use in the present invention.

[0039] Even though FIG. 2 has depicted all of the shaped charges as having a uniform size, it should be understood by those skilled in the art that it may be desirable to have different sized shaped charges within a collection such as having larger or smaller outer shaped charger than the center shaped charge. Also, even though FIG. 2 has depicted a uniform axial distance between each of the shaped charges, it should be understood by those skilled in the art that it may be desirable to have different axial spacing between shaped charges such as having the axial distance between adjacent shaped charges in adjacent collections being greater than or less than the axial distance between adjacent shaped charges within a collection.

[0040] Referring next to FIG. 3, therein is depicted a portion of a perforating gun assembly 110 positioned in a wellbore 112 that traverses formation 114. A casing 116 lines wellbore 112 and is secured in position by cement 118. Wellbore 112 includes a filter cake 120 near the surface of wellbore 112. The portion of perforating gun assembly 110 shown includes a substantially axially oriented collection of shaped charges 122, 124, 126. In the illustrated embodiment, shaped charges 122, 124, 126 are oriented to converge toward one another. Specifically, center shaped charge 124 is oriented substantially perpendicular to the axis of perforating gun assembly 110 while outer shaped charges 122, 126 are oriented to converge toward center shaped charge 124. More specifically, shaped charges 122, 124, 126 are each oriented toward a focal point 128 in formation 114 as indicated by dashed lines 130, 132, 134, respectively. In this orientation, upon detonating the collection of shaped charges 122, 124, 126 with detonating cord 136, a perforation cavity will be created in formation 14.

[0041] As best seen in FIG. 4, when the collection of shaped charges 122, 124, 126 is detonated, shaped charge 122 discharges jet 140, shaped charge 124 discharges jet 142 and shaped charge 126 discharges jet 144, each of which is directed toward focal point 128. In the illustrated embodiment, jets 140, 142, 144 do not reach focal point 128 and do not intersect. Nonetheless, as best seen in FIG. 5, jets 140, 142, 144 interact together within formation 114. Specifically, jets 140, 142, 144 not only create perforation tunnels 146, 148, 150, respectively, but also create a pulverized zone represented by dotted line 152 in formation 114. The interaction of jets 140, 142, 144 substantially rubblizes, pulverizes or otherwise breaks down or fragments the structure of the rock in pulverized zone 152.

[0042] As best seen in FIG. 6, due to the interaction of jets 140, 142, 144, a perforation cavity 154 is created in formation 114 behind casing 116 which has a volume significantly larger than the volume of conventional perforation tunnels. Using the present invention to create perforation cavities, such as perforation cavity 154, establishes large volume regions of high permeability into which formation fluid drain, increasing the productivity of a well as compared to wells having only conventional perforation tunnels. In addition, the need to perforate underbalanced is reduced by the use of the present invention as perforation cavity 154 is not as easily plugged by debris or rock structure as are conventional perforation tunnels. As discussed below, however, operating the present invention in underbalanced pressure condition will aid in cleaning up perforation cavity 154 and further increase the volume of perforation cavity 154.

[0043] Even though FIGS. 3-6 have depicted a substantially axially oriented collection of three shaped charges that are oriented to converge toward a focal point in the formation and that form jets that interact but do not reach the focal point and do not intersect, the present invention is not limited to such a configuration. For example, as best seen in FIG. 7, a portion of a perforating gun assembly 160 is depicted as being disposed in a wellbore 112 that traverses formation 114. The portion of perforating gun assembly 160 shown includes a substantially axially oriented collection of shaped charges 162, 164, 166 that are oriented to converge toward one another and more specifically toward focal point 128 in formation 114. In this orientation, upon detonating the collection of shaped charges 162, 164, 166 with detonating cord 168, jets 170, 172, 174 are formed. In the illustrated embodiment, jets 170, 172, 174 penetrate through casing 116, cement 118, filter cake 120 and into formation 114 past focal point 128 such that jets 170, 172, 174 intersect substantially at focal point 128. This interaction of jets 170, 172, 174 substantially rubblizes, pulverizes or otherwise breaks down or fragments the structure of the rock behind casing 116 such that a perforation cavity similar to perforation cavity 154 of FIG. 6 is created.

[0044] As another example, as best seen in FIG. 8, a portion of a perforating gun assembly 180 is depicted as being disposed in a wellbore 112 that traverses formation 114. The portion of perforating gun assembly 180 shown includes a substantially axially oriented collection of shaped charges 182, 184, 186, 188 that are oriented to converge toward one another and more specifically toward focal point 128 in formation 114. In this orientation, upon detonating the collection of shaped charges 182, 184, 186, 188 with detonating cord 190, jets 192, 194, 196, 198 are formed. In the illustrated embodiment, jets 192, 194, 196, 198 penetrate through casing 116, cement 118, filter cake 120 and into formation 114 converging at focal point 128. This interaction of jets 192, 194, 196, 198 substantially rubblizes, pulverizes or otherwise breaks down or fragments the structure of the rock behind casing 116 such that a perforation cavity similar to perforation cavity 154 of FIG. 6 is created.

[0045] It should be understood by those skilled in the art that while the preceding figures have depicted each of the shaped charges with a collection of shaped charges as being oriented toward a focal point, this configuration is not required by the present invention. For example, some of the shaped charges in a collection of shaped charges may be directed toward one location in the formation while other of the shaped charges in the same collection may be directed toward another location in the formation. As another example, there may be some circumferential offset or phasing between adjacent shaped charges in an axially oriented collection of shaped charges. In either of these configurations, the jets generated from the shaped charges in the collection are able to interact and create a perforation cavity of the present invention.

[0046] Use of the perforating gun assembly of the present invention enables the creation of large volume perforation cavities in the formation behind the casing that enhances the productivity of a well when compared to a conventionally perforating system that creates small volume perforation tunnels. Nonetheless, following the creation of the perforation cavities of the present invention, it may be desirable to stimulate or otherwise treat the producing interval. Treatment processes such as gravel packs, frac packs, fracture stimulations, acid treatments and the like may be preformed. In fact, the perforation cavities of the present invention allow for improved sand control as the sand, gravel, proppants or the like used in gravel pack and frac pack slurries fills the perforation cavities, thereby preventing the migration of formation fines into the wellbore. Additionally, the large volume of the perforation cavities helps to enhance the propagation of fractures deep into the formation during frac pack and fracture stimulation operations.

[0047] In tests comparing conventional perforating systems with the perforating gun assembly of the present invention, significant volumetric differences between conventional perforation tunnels and the perforation cavities of the present invention have been shown. Tests were performed using 3⅜ inch Millennium 25 g HMX shaped charges fired through a 0.5 inch 4140 steel plate, 0.75 inches of cement and into a confined 60 mD Berea Sandstone target.

[0048]

	TABLE 1
	Single Charge	Three Charge Collection
Entrance Hole (in)	0.35	2.25 × 0.5
Penetration Depth (in)	13.22	13.51
Clear Depth (in)	10.12	11.15
Hole Volume (in3)	0.6	6.43
Cleaned Up Volume (in3)	3.80	11.63

Table 1 shows that the use of a collection of three shaped charges that are oriented to converge toward one another and form jets that interact together, create a perforation cavity having a volume that is significant larger than the volume of a conventional perforation tunnel. Specifically, the entrance hole into the target created by the conventional single charge was 0.35 inches in diameter while the entrance hole created by the three charge collection had a height of 2.25 inches and a width of 0.5 inches. The depth of penetration into the target for the conventional single charge was 13.22 inches and for the three charge collection was 13.51 inches with the clear depth for the conventional single charge being 10.12 inches and for the three charge collection being 11.15 inches.

[0049] Most importantly, the hole volume for the conventional single charge was only 0.6 cubic inches while the hole volume for the three charge collection was 6.43 cubic inches. FIG. 9 depicts a volumetric representation designated 200 of the 0.6 cubic inch perforation tunnel created by the conventional single charge. FIG. 10 depicts a volumetric representation designated 202 of the 6.43 cubic inch perforation cavity created by the three charge collection. As should be appreciated by those skilled in the art, the volume of perforation cavity 202 is more than ten times greater than the volume of perforation tunnel 200.

[0050] FIG. 11 depicts a volumetric representation designated 204 of a 3.80 cubic inch perforation tunnel created by the conventional single charge under simulated underbalanced conditions to completely clean up perforation tunnel 200 of FIG. 9. Likewise, FIG. 12 depicts a volumetric representation designated 208 of an 11.63 cubic inch perforation cavity created by the three charge collection under simulated underbalanced conditions to completely clean up perforation cavity 202 of FIG. 10. After clean up, the volume of perforation cavity 208 is more than three times greater than the volume of perforation tunnel 204.

[0051] Importantly, as noted above, even after complete clean up, conventional perforation tunnels have a skin or region near the surface with reduced permeability as compared to the permeability of virgin rock. This skin surrounds the entire perforation tunnel and reduces the productivity of the well. In FIG. 11, the affected surface of perforation tunnel 204 has been designated 206. Unlike conventional perforation tunnels, the perforation cavities of the present invention are not surrounded by a reduced permeability skin. Instead, perforation cavities created using the present invention only have a reduced permeability skin at their uppermost and lowermost regions, which have been designated 210, 212 in FIG. 12. The sides portions of perforation cavity 208, designated 214 in FIG. 12, do not have this reduced permeability skin due in part to tension waves ablating the rock. These tension waves arise from the interaction of compression waves between the tunnels which are created during the formation of the perforation cavities. This improved permeability further enhances the productivity of wells having perforation cavities created using the perforating gun assembly of the present invention.

[0052] While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.