NO335817B1 - Procedure for removing filter cake in an open well - Google Patents

Abstract

An underbalanced perforation system is described for use in open (non-lined) well completions to maximize the efficiency of borehole and matrix cleaning, to connect natural fracture patterns, and / or to enable the application of new drilling fluid technology in difficult under-surface environments. The perforation system can be used for any hydrocarbon conducting formations with any lithology.

Description

The technical area

The present invention generally relates to improvements in the production of hydrocarbons from underground formations and more particularly relates to a system for perforating in an open borehole.

Background for the invention

US 2003/0089498 A1 relates to a method and an apparatus for contracting transient underbalance in a borehole.

For the extraction of hydrocarbons (for example oil, natural gas) it is of course necessary to drill a hole in the subsoil to come into contact with the hydrocarbon-bearing formation. In this way, hydrocarbons can flow from the formation, into the borehole and to the surface. Extraction of hydrocarbons from an underground formation is known as "production". In some productions, a casing is installed in the drilled wellbore to provide a structurally adapted conduit to recover hydrocarbons. In other productions, hydrocarbons are recovered from an unlined or "open" well.

When producing in an open well, a key parameter that affects the production loss is the permeability of the formation along the flow path that the hydrocarbons must pass to reach the borehole. Sometimes the formation rock has a naturally low permeability, while at other times the permeability is reduced during, for example, drilling of the well. When a well is drilled, a fluid is circulated into the borehole to contact the bit region for a number of reasons - including cooling the bit, moving cuttings away from the drill point, and maintaining a hydrostatic pressure on the formation wall to prevent production below drilling.

Drilling fluid is expensive, especially in light of the enormous quantities that must be used during drilling. Additionally, drilling fluid can be lost by leaking into the formation. To prevent this, drilling fluid is often intentionally modified so that small amounts leak away and form a coating or "filter cake" on the open borehole.

When the drilling is completed and production of the formation via the open borehole is desired, this filter cake must be removed to achieve the intended productivity. Current cleaning methodology includes applying a chemical treatment to dissolve the filter cake and any near-wellbore damage and/or applying a jet blast along the borehole to mechanically break down the filter cake. In long horizontal wells, these processes take a considerable time to complete. As a result, once a local section is cleaned, this leads to channel formation into which the treatment fluid can flow, leaving the majority of sections uncovered by the treatment fluid. This inability to uniformly clean the entire well is a major problem the oil industry faces when trying to produce from long, open wells. The other disadvantage of the current methodology is the inability to deliver the treatment fluid deep into the formation past the drilling damage. Maximum cleaning of filter cake is therefore not achieved even in the areas that receive the treatment fluid. Due to the combination of these problems - uneven coverage and shallow penetration of treatment fluid - borehole completions often do not meet expectations.

Consequently, there is a need within the drilling and completion industry for a reliable system to remove filter cake quickly, efficiently and completely to produce the well. This is the primary purpose of the present invention.

Summary of the invention

The main features of the invention appear from the independent patent claims. Further features of the invention are indicated in the independent claims.

According to one embodiment, the present invention generally provides a system for penetrating the formation in an open production well using perforating tools.

One embodiment of the perforating system according to the present invention includes, for example, the use of one or more shaped charges to penetrate the formation in an open borehole.

In a further embodiment, the perforating system of the present invention includes the use of one or more shaped charges to penetrate the formation in an open borehole in a transient underbalanced environment to facilitate more rapid removal of the filter cake from the borehole.

An object and feature of an embodiment of the present invention is to remove the filter cake from the intended production interval in a wellbore rapidly by perforating the wellbore interval with shaped charge detonation in a currently underbalanced environment.

A further purpose and a feature of an embodiment of the present invention is to facilitate the passage of perforation channels through the drilling damage.

Yet another object and feature of an embodiment of the present invention is to perforate an open borehole to overcome reservoir heterogeneity by detonating multiple perforations in low permeability well sections and less perforation in high permeability sections. "More" or "less" refers to the amount and/or energy of detonating charges.

Yet another object and feature of an embodiment of the present invention is to facilitate production in naturally fractured reservoirs by connecting fracture branches.

Other and alternative features will be apparent from the following description, from the drawings and from the patent claims.

Brief description of the drawings

The manner in which these objects and other desirable characteristics can be achieved is explained in the following description and attached drawings in which: Figure 1 illustrates an embodiment of a perforating firing device positioned in an open borehole and includes a firing device system according to one of several embodiments. Figures 2, 3A and 3B illustrate embodiments of a perforating firing device system for use in generating a transient underbalanced condition in an open borehole. Figures 4A-4C illustrate embodiments of a hollow firing device carrier each including a charge tube in which shaped charges are mounted, with the charge tube filled with a porous material. Figures 5A-5B illustrate a perforating firing device system according to an embodiment of the present invention that includes a carrier tube containing shaped charges and a porous material. Figure 6 illustrates one embodiment of a perforating firing device system for ameliorating a transient underbalance in an open wellbore. Figures 7A-7D illustrate various embodiments of perforating firing devices with porous elements for use with the present invention. Figure 8 illustrates an embodiment of a sealed chamber for deployment in an open well. Figure 9 illustrates one embodiment of a perforating system in accordance with the present invention and depicts a perforating firing device string and a plurality of sealed chambers. Figure 10 illustrates an embodiment of a perforating firing device string connected to an anchoring device to selectively release the perforating firing device string. Figure 11 illustrates one embodiment of a perforating firing device string with a plurality of explosive-activated openings. Figures 12A-12B illustrate an embodiment of a valve-activated low-pressure chamber in accordance with the present invention.

However, it should be noted that the attached drawings only illustrate typical embodiments of this invention and should therefore not be considered as limiting the scope of the invention as the invention can be adapted to other equally effective embodiments.

Detailed description

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

In the description and appended claims, the terms "connect", "connection", "connected", "in connection with" and "connection" are used to indicate "in direct connection with" or "in connection with via a further element"; and the term "set" is used to denote "one element" or "more than one element". As used herein, the terms "up" and "down", "upper" and "lower", "upstream" and "downstream", "upstream" and "downstream" mean; "over and under"; and other similar designations indicating relative positions above or below a given point or element as used in this description to more clearly describe some embodiments of the invention. However, when used for equipment and methods for use in deviation wells or horizontal wells, such designations can be referred to a left-to-right indication, right-to-left indication, or other conditions in the individual case.

In general, tools, systems and methods for perforating in open hole completions are provided to maximize wellbore and matrix cleaning efficiency (by loosening and/or removing filter cake formed on the open hole, penetrating the underlying formation and enlarging the effective radius of the borehole past any drilling damage), connect natural fractures and/or enable the application of drilling fluid technology in difficult subsurface environments. The open hole perforation system according to the present invention can be used for any hydrocarbon bearing formations with any lithology. In some embodiments of the present invention, an open borehole may be perforated to remove the filter cake in an underbalanced, overbalanced, or near balanced well environment.

In some cases, it is desirable to lower the local pressure condition to improve transient underbalance during perforation. Treatment of filter cake, as well as removal of perforation damage and charge and formation waste from the perforation tunnels, can be accomplished by increasing the local pressure drop (that is, increasing the local transient under-equilibrium condition). Different methods and mechanisms can be used to achieve and control a transient underbalanced state in which perforation is to be performed. In one embodiment, for example, a perforating firing device with a special sealed firing device body and charge fill can be selected for introduction into the open borehole and generate a dynamic underbalance pressure. In this way, rapid removal of filter cake from the borehole can be achieved. The shaped charges can be selected to either penetrate both the sealed launcher body and the formation or alternatively just puncture the launcher body. The sealed launcher body includes an inner bore sealed at a particular pressure lower than the ambient borehole pressure. After puncture, a transient underbalanced condition is created by the pressure differential between the surrounding borehole and the exposed interior of the launcher body. This pressure differential creates a temporary rapid pressure change that facilitates the rapid removal of the filter cake from the borehole. In a further example, if penetration through the formation is not required, then a well-fast pressure tool can be used instead of a perforating shot to create the transient underbalanced condition.

In operation, a well operator identifies or determines a sought-after transient underbalance condition that is desirable in a borehole interval in an open well relative to a borehole pressure (which may be determined by the reservoir pressure). The desired temporary underbalance condition can be identified in one of several ways, such as based on empirical data from previous well operations or on simulations carried out with model software.

Based on the desired transient underbalance, the tool string (eg, perforating firing device string) is configured. For example, the firing device size, shot density, type of charge, arrangement of firing phases, orientation, explosive mass, fluid type (for example, slowly hydrolyzed acidic solutions, surface active agents, mutual solvents, chelating fluids or fluids whose viscosity is increased by means of a gelling agent) and transport method is configured appropriately to achieve the desired transient underbalance condition. The appropriate configuration can be based on empirical data from previous operations or from program modeling and simulations. Determining the appropriate configuration for use can be determined from software executable in a system, such as a calculator system. The software can be executed on one or more processors in the system. Various other configurations can be established to achieve an optimal result. In some embodiments, for example when completing a heterogeneous reservoir (ie a reservoir with varying degrees of permeability in different zones) the charge fill may be higher towards the low permeability zones to increase the flow area after perforation to overcome the preferred flow through the high permeability zone. In other embodiments, the perforation can be oriented in accordance with the reservoir's fracture network so that the perforations connect with the natural fracture branches.

When suitably configured, the tool string is then lowered to an open borehole interval, where the tool string is activated to detonate explosives in the tool string. Activation causes substantially (eg 70%) of the desired transient under-balance condition to be achieved. Penetration through the filter cake and formation and/or rapid removal of the filter cake is achieved.

Various embodiments of perforating firing devices and/or other tools are set forth below for use with the systems and methods of the present invention to create a transient underbalance condition in an open borehole to facilitate the rapid removal of filter cake.

Referring to Figure 1, according to one embodiment, a perforating firing device 10 (single firing device or firing device string) is positioned in an open (that is, unlined) borehole 20 with a producing formation 22 coated in the filter cake 24. In a further embodiment, the perforating firing device intended to be introduced through production tubing (not shown). The perforating firing device 10 may include a sealed firing device carrier 12 (or other sealed chamber) and one or more shaped charges 14 disposed therein. The firing device carrier 12 may be attached to an adapter 30 which in turn is connected to a carrier line 40 to suspend and carry the perforating firing device 10 into the open borehole 20. The carrier line 40 may include, but is not limited to, a wire or wire rope, a smooth steel wire, an e-wire, drill pipe or coil pipe. The carrier 12 is sealed to generate a pressure differential in which the internal pressure in the carrier is less than the surrounding borehole pressure.

In operation with reference to Figure 2, the perforating firing device 10 is lowered on a carrier line 40 through the borehole 20 and positioned next to or near the formation 22. To promote the removal of the filter cake 24, the perforating firing device 10 is fired. In one embodiment, the perforating firing device 10 is configured with shaped charges 14 (or other explosive charges) to penetrate the sealed firing device carrier 12 and the surrounding formation 22 (as illustrated in Figure 3A). In a further embodiment, the perforating projectile 10 is configured with shaped charges 14 (or other explosive charges) to penetrate only the sealed projectile carrier 12 and not the surrounding formation 22 (as illustrated in Figure 3B). In both embodiments, once the firing device carrier 12 is breached, the transient underbalanced pressure differential between the surrounding borehole and the volume inside the firing device carrier causes a sudden pressure change to break apart or otherwise remove the filter cake 24 from the borehole 20.

A further embodiment of the present invention includes a perforating firing device system provided with a porous material so that after firing the firing device system, the sealed volume of the porous material is exposed to the borehole pressure to temporarily reduce the borehole pressure to improve the local underbalance condition. Initially, the porous material (for example, a porous solid) contains sealed volumes containing gas, light liquids or a vacuum. When the explosives are detonated, the porous material is crushed or broken into pieces so that the volumes are exposed to the borehole. This effectively creates a new volume into which borehole fluids can flow and which creates a local, transient pressure drop. As a result, a transient underbalance condition is improved by using a porous material to facilitate filter cake removal in an open borehole.

For example, referring to Figures 2A-2B, one embodiment of a perforating firing device system 100A includes a linear strip 102 to which multiple capsule-shaped charges 106 are coupled. A detonating fuse 103 is connected to each of the shaped charges 106. The shaped charges 106 are mounted in corresponding support rings 104 of a support bracket 105. The support bracket 105 can be turned to provide a desired phase difference (eg 45°, 60 ° spiral, three-phase spiral etc). Alternatively, the support bracket 105 can be arranged in a non-phase pattern (eg 0° phase difference). In a further embodiment, the linear strip 102 can be omitted where the support bracket 105 provides the primary support for the capsule charges 106.

In one embodiment, carrier strip 102, support bracket 105, support rings 104, detonating fuse 103 and capsule charges 106 are encapsulated in a porous material 110. One example of the porous material includes a porous solid such as porous cement. An example of a porous cement includes "LITECRETE<®>". Porous cement is formed by mixing the cement with hollow structures, such as microspheres filled with a gas (such as air) or other types of gas- or vacuum-filled spheres or shells. Microspheres are generally thin-walled glass shells where a relatively large proportion is air.

Porous cement is one example of a porous solid containing a sealed volume. When the gas-filled or vacuum-filled hidden structures break apart in response to detonation of the shaped charges 106, additional volume is added to the borehole so that the pressure is temporarily reduced.

To provide structural support for the encapsulating means 110, a sleeve 112 is arranged around the encapsulating means 110. The sleeve 112 is formed from any type of material capable of providing structural support, such as plastic, metal, elastomer, and so on. The sleeve 112 is also designed to protect the encapsulant 110 when the firing device system 110A is introduced into the wellbore and it collides with other well structures. Alternatively, instead of a separate sleeve, a coating may be applied to the outer surface of the encapsulant 110. The coating adheres to the encapsulant when applied. The coating may be formed from a material selected to reduce fluid penetration. The material can also have a low friction.

In further embodiments, the encapsulation means 110 to provide higher pressure levels can be formed using a different type of material. For example, higher pressure grade cement with "S60" microspheres manufactured by 3M Corporation can be used. Alternatively, the encapsulant 110 may be epoxy (eg polyurethane) mixed with microspheres or other types of gas- or vacuum-filled spheres or shells. In yet another embodiment, the encapsulating agent 110 can have several layers. For example, one layer can be made of porous cement, while a further layer can be made of porous epoxy or other porous solid. Alternatively, the encapsulant 110 may be a liquid or gel-based material, with the sleeve 112 providing a sealed container for the encapsulant 110.

In some embodiments, the porous material is a composite material

which includes a hollow filler material (for porosity), a heavy powder (for density) and a binder/matrix. The binder/matrix can be a liquid, a solid or a gel. Examples of solid binder/matrix materials include polymer (eg castable thermosetting plastic such as epoxy, rubber etc, or an injection/castable thermoplastic), a chemically bonded ceramic (eg a cementitious compound), a metal or a highly compressible elastomer. A non-solid binder/matrix material includes a gel (which is more shock-compressible than a solid) or a liquid. The hollow filler for the anti-shock material may be a fine powder, each particle including an outer shell surrounding a volume of gas or vacuum. In an exemplary embodiment, the hollow filler can include up to approximately 60% by volume of the total

bond volume, where each hollow filler particle includes 70 to 80% air by volume. The shell in the hollow filler is non-permeable and has high strength to prevent collapse at typical borehole pressures (of the order of about 700 kg/cm<2>in one example). An alternative to using hollow fillers is to produce and maintain stable air bubbles directly inside the matrix by means of mixing, surfactants and the like.

In an exemplary embodiment, the heavy filler powder can constitute up to 50% by volume of the total connection volume, the powder being a metal such as copper, iron, tungsten or any other high-density material. Alternatively, the heavy filler can be sand. In other embodiments, the heavy powder may comprise up to about 10, 25 or 40% by volume of the total compound volume. The shape of the high-density powder particles is chosen to produce the correct mixture rheology to achieve a segregation-free final mixture.

Using sand as the heavy filler instead of metal provides one or more advantages. For example, field personnel will be familiar with sand and can be handled more easily. In addition, by increasing the volume of sand, the volume of matrix/binder is reduced, which reduces the amount of waste that constitutes the matrix/binder after detonation.

In some examples, the bulk density of the shock absorbing material is in the range of about 0.5 g/cm<3> (grams per cubic centimeter) to about 10 g/cm<3>, with a porosity of the compound in the range of between about 2% and 90%.

Other exemplary porous solids include a 10 g/cm<3>, 40% porous material, such as tungsten powder mixed with hollow microspheres, 50% by volume of each. A further exemplary compound includes 53% by volume low viscosity epoxy, 42% by volume hollow glass beads and 5% by volume copper powder. The mix density is about 1.3 g/cm<3> and the porosity is about 33%. An additional mix includes approximately 39% by volume water, 21% by volume Lehigh Class H cement, 40% by volume glass beads and sports additives to optimize rheology and setting rate. The density of this mixture is about 1.3 g/cm<3> and the porosity is about 30%.

To form the encapsulant 110, the porous material (in liquid or slurry form) can be poured around the carrier strip 102 contained within the sleeve 112. The porous material is then allowed to harden. With porous cement, cement in powder form can be mixed with water and other additives to form a cement slurry. During mixing of cement, microspheres are added to the mixture. The mixture is then poured in slurry form into the sleeve 112 and allowed to harden. The equipment used to create the desired mix can be any conventional cement mixing equipment. Fibers (for example glass fibres, carbon fibers etc) can also be added to increase the strength of the encapsulant.

The encapsulant 110 can also be preformed. For example, the encapsulation means may be divided into two sections, with suitable contours molded into the inner surfaces of the two sections to receive a firing device or one or more charges. The firing device can then be placed between the two sections which are attached to each other to provide the encapsulation means 110 shown in Figure 4B. In yet another example, the porous material can be molded into shape between two charges and filled as the charges are introduced.

In a further embodiment, as shown in Figure 4C, the linear strip 102 is omitted, with the support bracket 105 and the encapsulation means 110 providing the necessary support.

Referring to Figures 5A-5B in accordance with the further embodiment, instead of the carrier strip 102 shown in Figures 4A and 4B, a similar concept can be extended to a hollow carrier firing device 100B to generate a transient underbalanced condition in an open borehole for to facilitate the removal of filter cake. In the hollow carrier firing device 100B, a charge tube 120 is positioned inside a hollow carrier 122. The charge tube 120 provides openings 124 through which shaped charges 126 can emerge. The shaped charges 126 may be non-encapsulated charges as the shaped charges are protected from the environment by the hollow carrier 122, which is typically sealed. After the shaped charges 126 are mounted inside the charging tube 120 during assembly, a porous material (for example, porous cement) which is initially in liquid or slurry form can be poured through the top or bottom opening 130 of the charging tube. The material is then allowed to solidify to provide a porous material filler 125 inside the charging tube 120. Figure 5B shows a cross section of the firing device 100B.

The porous material filler may also fill the interior of the hollow carrier 122 to provide a larger volume. In addition to increasing the local transient under-equilibrium state, a further advantage of the porous material is that it is an energy-absorbing agent that reduces charge-to-charge interference. The porous material can also provide structural support for the hollow carrier so that a thinner-walled hollow carrier can be used. The porous material provides support within the hollow carriers against forces generated due to borehole pressure. With thinner hollow carriers, a perforating firing device is provided with lighter weight and which makes handling and operation more advantageous. A layer 123 formed of a porous material can also be arranged around the outer surface of the hollow carrier 122. The combination of the porous material inside and outside the hollow carrier 122 provides a volume that can receive borehole fluids after detonation.

Referring to Figure 6, yet another embodiment, a perforating firing device system 100C includes a tubular carrier 202 that can be used to carry shell charges 204 mounted near openings 206 in the tubular carrier 202 to generate a transient underbalanced condition to facilitate the removal of filter cake from a borehole. The tubular carrier 202 may be arranged in a manner similar to the charging tube 120 in the hollow carrier firing device 100B, except that the tubular carrier 202 is not contained within a hollow carrier. As a result, capsule charges 204 are used in place of the non-capsule charges 106 in Figure 5A. In one arrangement, a detonating fuse 208 may be routed along the outside of the tubular carrier 202 and connected to the capsule charges 206. In a further arrangement, the detonating fuse 208 may be routed inside the tubular carrier 202. As with the charge tube 120 in Figure 5A, a porous material (for example, porous cement) originally in liquid or slurry form is poured through a top or bottom opening 210 of the tubular carrier 202. The porous material solidifies within the tubular carrier 202 to form the porous material for shock and interference reduction. An advantage of using the tubular carrier 202 is that damage to the porous material is less likely due to the porous material being protected by the tubular carrier 206, which is typically a strong and rigid structure.

Referring to Figure 7A, in accordance with yet another includes

embodiment a strip firing device 100D multiple shaped charges arranged in a phased pattern (for example spiral, three-phase and so on) on a linear strip 302. Alternatively, a non-phased arrangement of the charges may be used. The 0°-

phased shaped charges (referred to as 304) can be mounted directly to the strip 302. The other charges (not shown) are mounted inside the tubes 306 attached to the strip 302. Openings 308 are provided in each tube 306 for corresponding shaped charges. A porous material, which may be one of the porous materials discussed above, is disposed in each tube 306.

The tube 306 can be made of a metal or other suitable rigid material. Alternatively, the pipe 306 can also be formed from a porous material, such as a porous solid (for example, porous cement, porous epoxy, etc.).

In Figures 7B to 7D, in a further embodiment, instead of a hollow tube 306, a fixed rod 306A with a cavity 308A (for the shaped charges) is used. Figures 7B-7D show three views of three different parts of rod 306A without the charges mounted therein. The rod 306A can be made of a porous material, such as a porous solid. As shown in Figures 5B and 5D, first and second grooves 310 and 312 are formed at the ends of the rod 306A to receive the 0° phased shaped charges 304. Slots 314 are also formed on the outer surface of the rod 306A between the openings 308A to receive a detonating fuse ballistically coupled to each of the shaped charges in rod 306A.

To further increase the underbalance effect, a larger amount of the porous solid can be arranged around each firing device. For example, a cylindrical block of the porous solid may have a maximum diameter that is slightly smaller than the smallest restriction (eg, production pipe string) that the firing device must pass through.

Alternatively, a porous slurry can be pumped down and around the firing device; in such a scenario, the restriction on size is not a limitation on how much porous material can be placed around the firing device. In Figure 1, for example, the borehole annulus 20 around the firing device 10 is thus filled with the porous slurry that has been pumped down through the production pipe and around the firing device system 10.

Other embodiments to increase transient pressure drops and thus transient underbalance conditions are described in the following. In a further embodiment, a sealed atmospheric container is lowered into the borehole after a formation has been perforated. After production is started, openings (for example, using explosives, valves or other mechanisms) are created in the casing of the reservoir to generate a sudden underbalance or fluid pressure shock to remove the damaged filter cake around the perforation tunnels in the formation.

In yet another embodiment, a chamber with the firing device can be used as a collection container for borehole fluids to generate the underbalance condition. After charge detonation, hot detonation gases fill the inner chamber of the firing device. If the resulting detonation gas pressure is less than the borehole pressure, then the colder borehole fluids are sucked into the housing part of the firing device. The rapid acceleration through perforation openings in the housing part of the firing device breaks the fluid up into droplets and results in rapid cooling of the gas. Rapid loss of firing device pressure and even more rapid drainage of borehole fluid therefore occurs and this generates a drop in borehole pressure. The drop in borehole pressure creates an underbalance condition.

Referring to Figure 8, a tool string with a sealed atmospheric container 510 (or container with an internal pressure lower than an expected downhole pressure in the interval of the formation 512) is lowered into an open wellbore 524 and placed adjacent to a perforated formation 512 to be processed. The tool string is lowered onto a carrier line 522 (for example wire/cable, smooth steel wire, e-line, coil or extension pipe, drill pipe, etc.). The container 510 includes a chamber that is filled with a gas (eg air, nitrogen) or other fluid. The container 510 has a sufficient length to treat the entire formation 512 and has several openings 516 which can be opened by the use of explosives.

In one embodiment, while the well is producing (after perforations in the formation 512 have been formed) the atmospheric chamber in the container 510 is explosively opened to the wellbore. This method can be used with or without a perforating firing device. When used with a firing device, the atmospheric container allows the use of a dynamic underbalance even if the borehole fluid is in overbalance immediately before perforating. The atmospheric container 510 can also be used after perforation operations have been carried out. In this latter arrangement, production is established from the formation with the openings 516 in the atmospheric vessel 510 explosively opened to create a sudden under-equilibrium condition.

The explosively activated container 510 in accordance with one embodiment includes internal air (or other suitable gas or fluid). The dimensions of the chamber 510 are such that it can be lowered into a completed well either by means of a wire/cable, smooth beam wire, e-line, coil or joint pipe, drill pipe or other mechanisms. The wall thickness of the chamber is designed to withstand pressures and temperatures in the open borehole. The length of the chamber is determined by the thickness of the perforated formation being treated. Multiple openings 516 may be present along the wall of chamber 510. Explosives are placed within the atmospheric container in proximity to the openings.

In one arrangement, the tool string including the container 510 is lowered into the borehole and positioned until the perforated formation 512.1 this arrangement the atmospheric chamber 510 is used as a pressure shock generating device to generate a sudden underbalance condition. Before the immersion of the atmospheric container, a clean completion fluid or treatment fluid can optionally be used for injection into the formation or otherwise fill the borehole and allow natural leakage into the formation. The completion fluid is selected based on the wettability of the formation and the fluid properties of the formation fluid. This can help remove the filter cake and/or other particulate matter from the perforation tunnels during fluid flow.

After the atmospheric container 510 has been lowered and brought up to the perforated formation 512, the formation 512 is placed under the flow by opening a production valve at the surface. While the formation is set under flow, the explosives are fired inside the atmospheric container and this opens the openings in the container 510 to the borehole pressure. The shock wave generated by the explosives can provide the force to release the filter cake and/or other particles. The sudden drop in pressure inside the borehole can cause fluid from the formation to quickly enter the void left in the borehole from the atmospheric container 510. This fluid carries the now mobile particles into the borehole and leaves clean formation tunnels and borehole surface. The chamber can be lowered into the well or retrieved to the surface.

When used with a perforating firing device, activation thereof may substantially coincide with opening of the apertures 516. This provides underbalanced perforation. Referring to Figure 9, use of an atmospheric container 51 OA in connection with a perforating firing device 530 is illustrated in accordance with the further embodiment. In the embodiment in fig. 7, the container 51 OA is divided into two parts, a first part above the perforating firing device 530 and a second part below the perforating firing device 530. The container 51 OA includes various openings 516A adapted to be opened by an explosive force, such as an explosive force resulting from the initiation of a detonating fuse 520A or the detonation of explosives connected to the detonating fuse 520A. The detonating fuse is also connected to shaped charges 532 in the perforating firing device 530. In one embodiment, as illustrated, the perforating firing device 530 may be a strip firing device in which shaped capsule charges are mounted on a carrier 534. Alternatively, the shaped charges 532 may be non-capsulated charges contained in a sealed container.

The sudden fluid flow can be carried out relatively quickly after perforation. For example, the sudden fluid flow can be accomplished within about one minute of perforation. In other embodiments, the sudden fluid flow may be conducted within (less than or equal to) about 10 seconds, one second, or 100 milliseconds as examples after perforation. The relative timing between perforation and the sudden fluid flow is also applicable to other embodiments described herein.

With reference to Figure 10, in accordance with a further embodiment, a tool string with several chambers can be used for use in an open borehole. The tool string includes a perforating firing device 600 which is attached to an anchor 602. Activation of a detonating fuse 604 to fire shaped charges 606 in the perforating firing device 600 will thus, for example, also activate the anchor 602 to release the perforating firing device 600, which will then fall to the bottom of the borehole.

The anchor 602 includes an annular conduit 608 to enable fluid communication in the annulus region 610 (also referred to as a "rat hole") with a region outside a first chamber 614 of the tool string. The first chamber 614 has a predetermined volume of gas or fluid. The housing portion defining the first chamber 614 may include apertures 614 that can be opened, either explosively or otherwise. The volume of the first chamber 614 in one example may be approximately 7 liters. This is arranged to achieve an approximately 14 kg/cm<2> underbalance condition in the annulus region 610 when the openings 616 are opened. In other configurations, other sizes of chamber 614 may be used to achieve a desired underbalance condition based on the geometry of the wellbore and formation pressure. A control module 626 may include a firing head (or other actuating mechanism) to initiate a detonating fuse 629 (or to actuate another mechanism) to open the apertures 616.

A gasket 620 is mounted around the tool string to isolate the region 612 from an upper annulus region 622 above the gasket 620. Use of the gasket 620 provides isolation of the "rat hole" so that a faster response to the underbalance condition or sudden fluid flow can be achieved. In other embodiments, however, the gasket 620 may be omitted. In the various embodiments described herein, the use of a gasket for insulation or not of the annulus region is generally possible.

Referring to figure 11, there is illustrated yet another embodiment for creating an underbalance condition during a perforating operation in an open borehole. A perforating firing device string includes a perforating firing device 702 and a carrier line 704 which may be a smooth steel wire, an electrical wire, a wire/steel rope or coiled or extension pipe, or drill pipe. In one embodiment, the perforating firing device 702 is a hollow carrier firing device with shaped charges 714 inside a chamber 718 in the sealed housing 716. In the arrangement in Figure 11, the perforating firing device 702 is lowered through a production pipe 706. A gasket is arranged around the production pipe 706 to isolate the interval 712 in which the perforating firing device 702 is to be fired (referred to as the "perforating interval 712"). A pressure Pwer present in the perforation interval 712.

During detonation of the shaped charges 714, perforating openings 720 are formed as a result of perforating jets produced by the shaped charges 714. During detonation of the shaped charges 714, hot gas fills the inner chamber 718 of the firing device 702. If the resulting detonation gas pressure, PG , is less than the borehole pressure, Pw, by a given amount, then the colder borehole fluids will be sucked into the chamber 718 of the firing device 702. The rapid acceleration of well fluids through the perforation openings 720 will break up the fluid into droplets resulting in rapid cooling of the gas inside in chamber 718. The resulting rapid firing device pressure loss and even more rapid draining of the wellbore fluid into chamber 718 causes the wellbore pressure P to be reduced. Depending on the absolute pressures, this pressure drop can be sufficient to generate a relatively large underbalance condition (for example, more than 140 kg/cm<2>), even in a well that begins with a significant overbalance (for example, about 35 kg/cm< 2>). The underbalance condition is dependent on the level of the detonation pressure Pg compared to the borehole pressure pw.

When a perforating firing device is fired, the detonation gas is substantially hotter than the borehole fluid. If cold borehole fluids that are sucked into firing devices produce rapid cooling of the hot gas, then the gas volume will shrink relatively quickly and this reduces the pressure to promote that even more borehole fluids are sucked into the firing device. The gas cooling can take place during a period of a few milliseconds in an example. Drainage of borehole fluids (which have little compressibility) out of the perforating interval 712 can cause the borehole pressure Pw to drop to a relatively large extent (several hundred kg/cm<2>).

According to some embodiments, various parameters are controlled to achieve the desired difference in values between the two pressures Pwog Pg- For example, the level of the detonation gas pressure PG can be regulated by the explosive charge or by regulating the volume of the chamber 718. The level of the borehole pressure Pw can be regulated by pumping up the entire well or an isolated section of the well, or dynamically increasing the borehole pressure at a local level.

Instead of perforating firing devices, other embodiments may use other types of devices containing explosive components.

With reference to figures 12A and 12B, in still further embodiments, a local low pressure drop is promoted by using a chamber 850 (or other closing device element) containing a relatively low fluid pressure. For example, the chamber 850 includes (1) a sealed bore 852 containing a gas, liquid, or other fluid at a lower pressure than the surrounding borehole 820; and (2) a valve 854 to establish communication between the bore 852 and the wellbore 820. As a result, when the valve 854 in the chamber 850 is opened a sudden flow of fluid flows into the lower pressure chamber to create a local low pressure condition in the borehole region in communication with the chamber . The transient low pressure condition and resulting sudden fluid flow serves to remove the filter cake 824 from the wellbore 820. In some embodiments, the chamber may be a closed chamber defined in part by means of a closure device element located below the surface of the well. In other words, the closed chamber does not extend all the way to the well surface. Alternatively, in other embodiments, the closure device element includes a sealed container with apertures that include elements that can be broken by a mechanism (such as by the use of an explosive

or some other mechanism).

The various embodiments of the mechanisms and processes for perforating described above serve several purposes in the open hole. Firstly, by pressure control during perforation, the borehole wall can be subjected to a high instantaneous underbalance to uniformly remove the filter cake quickly from the entire borehole. Second, perforation generates flow channels past the drilling damage. Third, perforation allows production profile control to overcome reservoir heterogeneity. This is achieved by shooting more perforations in low-permeability sections and fewer in high-permeability sections. Fourth, perforating can be beneficial in naturally fractured reservoirs by connecting multiple fracture branches.

In other embodiments, the perforating work is carried out while there is a reactive fluid in the borehole. In such embodiments, an overbalanced perforation is laid up so that the pressure is recovered to overbalanced pressure after a dynamic underbalance to allow unused reactive fluid to penetrate into the formation.

Claims (1)

1. Method for removing filter cake (24; 824) in a selected perforation interval (712) in an open well (20; 820), characterized by the following steps: determination, from one of empirical data and modeling software, of characteristics of the selected perforation interval (712 ) in the open well (20; 820), of a transient underbalance target condition in the selected perforating interval (712) sufficient to remove filter cake (24, 824) from the perforating interval (712); determining a configuration of a perforating firing device (10; 530; 600; 702) based on the intended transient underbalance target condition; configuring the perforating firing device (10; 530; 600; 702) according to the transient underbalance target condition; lowering the perforating gun (10; 530; 600; 702) into the well (20; 820) to the selected perforating interval (712); and firing or detonation of the perforating firing device (10; 530; 600; 702) to thereby generate the transient underbalance target condition in the selected perforating interval (712) and to remove the filter cake (24, 824). 2. Method according to claim 1, further comprising steps where: filter cake (24, 824) is fractured from the intended perforation interval (712) of the open well (20; 820) at the borehole interval; and filter cake (24, 824) is removed from the open borehole (20; 820) at the borehole interval via the transient underbalance condition. 3. Method according to claim 1 or 2, wherein configuring the perforating firing device (10; 530; 600; 702) comprises one or more steps selected from a group consisting of: determining a size for the perforating firing device (10; 530; 600; 702), a shot density for the perforating firing device (10; 530; 600; 702) is determined, an explosive charge type is determined for the perforating firing device (10; 530; 600; 702), a phase pattern of an explosive charge is determined for the perforating firing device (10; 530; 600; 702), an orientation before the perforating firing device (10; 530; 600; 702) is determined, a fluid type for injection into the open well (20; 820) is determined, and a transport method for deploying the perforating firing device (10; 530; 600; 702) in the open well (20; 820) is determined. 4. Method for use in an open well (20; 820), characterized by the following steps: configuring a sudden pressure change tool to create a transient underbalanced target condition determined from one of empirical data and modeling software for characteristics of an interval in the open well (20 ; 820) to be sufficient to remove filter cake (24, 824) from the selected interval; insertion of the sudden pressure change tool into the open well (20; 820) to an intended interval; the sudden pressure change tool is activated to generate the transient underbalanced target condition and thereby remove the filter cake (24, 824).