[0001] The present invention relates to a method for performing chemical treatments in wellbores. In particular, the present invention relates to a method for performing chemical treatments in wellbores (and particularly oil and gas wellbores including vertical, deviated and horizontal wellbores) with perforated or slotted production casing or liner sections that increases chemical treatment placement efficiency behind the casing and reduces or eliminates the invasion of chemicals into the rock structure.

BACKGROUND

[0002] Chemical treatments are performed in oil and gas wells for many different reasons. For instance, chemical treatments may be used to stimulate hydrocarbon production, to shut-off undesired water or gas production, reinforce the wellbore or to reduce solids migration into the wellbore from the surrounding rock structure,

[0003] There are a number of current methods of introducing chemicals into a wellbore and the annular cavity or space between a perforated casing and the surrounding rock structure, including but not limited to using jointed pipe or coiled tubing, in combination with different types of downhole tools, such as rotating jetting nozzle tools, cup to cup tools, packers, etc. However, these current methods of placing chemicals in the wellbore and/or annular cavity suffer from drawbacks.

[0004] In the case of chemical treatments for wellbore solid mitigation in coal seam gas wells, chemicals can flow beyond the annular cavity and penetrate the coal seams resulting in water and gas production impairment. Also, the placement of chemical treatments in coal seam gas wells with pre-perforated or slotted casing or liner can result in cross flow problems between the wellbore casing and the annular cavity resulting in treatment failure. Furthermore, chemical treatments are regularly placed using a downhole tool with an upper frac cup facing down which effectively diverts the treatment fluid into the annular space. Unfortunately, however, these frac cups are often difficult to run in hole because they are designed to prevent flow upwards inside the casing. Thus, a considerable snubbing force, with a relatively large sized pipe is required to push them down and they easily get stuck or cannot be ran past small casing obstructions. This can prevent an optimal placement of the chemical treatment and result in leaving the bottom section of the well untreated.

[0005] Some attempts have been made to overcome these problems. For instance, the applicant’s own Australian innovation patent no. 2017101559 describes a method of reducing the solids migration into a wellbore by performing a chemical treatment in the wellbore. In this case, the chemical treatment involves introducing a settable foamed material into a wellbore such that the settable material is located between a perforated production casing and the surrounding rock structure, and subsequently creating one or more passages in the settable material to reconnect the coal seams in the rock to the wellbore.

[0006] While this method is an effective one, the applicant has discovered that, except in new and stable wells without any production time, bridges of solid material form between the perforated casing and the surrounding rock structure. In these circumstances, the presence of the bridges will reduce or preclude the ability of the chemical treatment pumped into the annular space between the perforated casing and the surrounding rock structure from at or adjacent the bottom of the well to rise to the top of the annular space.

[0007] In addition, a loss of the chemical treatment into coal seams in more depleted wells has been observed. It has been postulated that this loss of the chemical treatment may occur when, due to the removal of water and gas during well production, coal seams become relatively weak and brittle, allowing fractures to appear in the coal seams into which the chemical treatment flows.

[0008] Thus, there would be an advantage if it were possible to provide a method for performing chemical treatments in wellbores (and particularly a wellbore in which a pre perforated casing is present) that could be used in wells in which bridges of solid material have been formed between the perforated casing and the surrounding rock structure, and which reduced or eliminated the loss of the chemical treatment into the coal seams in more depleted wells.

[0009] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

SUMMARY OF INVENTION

[0010] Embodiments of the present invention provide a method for performing chemical treatments in wellbores, which may at least partially address one or more of the problems or deficiencies mentioned above or which may provide the public with a useful or commercial choice.

[0011] With the foregoing in view, the present invention in one form, resides broadly in a method for performing chemical treatments in wellbores, the method comprising the steps of: introducing a settable fluid into a mixing apparatus located within a wellbore; separately introducing a setting additive into the mixing apparatus; forming a mixture of the settable fluid and the setting additive in the mixing apparatus; introducing the mixture into the wellbore through an injection tool located, initially, at or adjacent a lower end of a perforated casing positioned in the wellbore such that at least a portion of the mixture is located between the perforated casing and a surrounding rock structure; substantially continuously raising the injection tool towards an upper end of the perforated casing while substantially continuously injecting the mixture into the wellbore; and allowing the mixture to at least partially set.

[0012] The settable fluid may be introduced into the wellbore using any suitable technique. Preferably, however, the settable fluid may be pumped into the wellbore, preferably from above ground level. Any suitable pump may be used to pump the settable fluid into the wellbore.

[0013] In some embodiments, the settable fluid may be introduced to the wellbore through a tubing (a jointed tubing or, more preferably, a coiled tubing) within the well. In some embodiments of the invention, production tubing in the wellbore may be withdrawn or removed from the wellbore to allow the coiled tubing to be inserted into the wellbore and the settable fluid to be introduced.

[0014] In some embodiments of the invention, it may be necessary to prepare the wellbore prior to the introduction of the settable fluid. Any suitable preparation technique may be used, although in a preferred embodiment of the invention, the preparation of the wellbore may involve removing as many solid particles as possible from the wellbore and/or the annular space between the pre-perforated casing and the surrounding rock structure. However, it will be understood that it may not be possible to remove bridges of solid material formed between the perforated casing and the surrounding rock structure.

[0015] The wellbore may be prepared using any suitable technique. For instance, solid particles may be removed from the wellbore using a fluid (such as water, brine, air or nitrogen, foam, or any suitable combination thereof) and then the solid particles may be circulated to the surface. Also, specialised downhole cleaning tools such as the one described in Australian innovation patent no. 2016101412 may be used to enhance the cleaning behind the casing .

[0016] The settable fluid may be provided as a substantially homogenous mixture. Alternatively, the settable fluid may comprise two or more components that must be mixed together to form the settable fluid. The two or more components may be mixed at any suitable location. For instance, the two or more components may be mixed at the surface prior to the introduction of the settable fluid into the wellbore. Alternatively, the mixing of the two or more components may occur during the pumping of the two or more components into the wellbore. In a preferred embodiment of the invention, the settable fluid is introduced into the wellbore in a substantially liquid pumpable form. Once in location between the perforated production casing and the surrounding rock structure, the settable fluid may set to form an at least partially solid material.

[0017] The settable fluid may be of any suitable form. For instance, the settable fluid may at least partially comprise a cementitious material. Alternatively, the settable fluid may comprise a natural polymeric material, a synthetic polymeric material, or a combination of the two (for instance, rubber, polyurethane, polyester, polyvinylchloride, polyethylene or the like, or a combination or cross linked thereof).

[0018] The settable fluid may be provided in the form of a foam, and particularly a pumpable foam which is preferably mixed and pumped from the surface. Alternatively, the settable fluid may be introduced to the wellbore as a liquid and then, optionally, converted to a foam in situ. It is envisaged that the settable fluid may further comprise a foaming agent. Any suitable foaming agent may be used, such as a surfactant and/or a blowing agent. Any suitable surfactant may be used, such as sodium lauryl ether sulfate (SLES), sodium dodecyl sulfate (SDS), ammonium lauryl sulfate (ALS) or the like, or any suitable combination thereof. Similarly, any suitable blowing agent may be used, such as, but not limited to, nitrogen, air, carbon dioxide, or the like, or any suitable combination thereof.

[0019] In some embodiments of the invention, the settable fluid may further comprise one or more of the following components: a curing agent, a gellant (such as guar or guar derivatives, synthetic polymers, cellulose or viscoelastic surfactant), a catalyst, an activator, a crosslinker (such as metallic or borate types), a strengthening agent, or the like. The settable fluid, once set, may be in the form of a foam, a gel, a solid, a semi-solid or the like, or a combination thereof.

[0020] In a preferred embodiment of the invention, the specific gravity of the settable fluid may not exceed 2.0. More preferably, the specific gravity of the settable fluid may be between approximately 0.1 and 1 .7. Still more preferably, the specific gravity of the settable fluid may be between approximately 0.3 and 1 .5. Most preferably, the specific gravity of the settable fluid may be between about 0.5 and 1 .2.

[0021] It is also preferred that the settable fluid may have a relatively high viscosity. The settable fluid may have any suitable viscosity, although in a preferred embodiment of the invention, the settable fluid may have a viscosity greater than the viscosity of water (i.e. greater than 0.894 cP). More preferably, the settable fluid may have a viscosity of greater than 100.0 cP. Still more preferably, the settable fluid may have a viscosity of greater than 200.0 cP. Even more preferably, the settable fluid may have a viscosity of greater than 250.0 cP. It will be understood that the viscosity of the settable fluid refers to the viscosity of the liquid form of the settable fluid. Once set, the viscosity of the settable fluid may be much higher than these values.

[0022] As previously stated, a setting additive is added to the settable fluid. Preferably, the setting additive may be added to the settable fluid in the wellbore downhole, so as to accelerate the setting reaction of the settable fluid. Therefore, it is preferred that the injection tool may be provided with a mixing apparatus in which the settable fluid and the setting additive may be mixed prior to the mixture being introduced into the wellbore through the injection tool. The mixing apparatus may be of any suitable form and may comprise, for instance, an agitated chamber (including an impeller, stirrer or the like) or a Venturi device. More preferably, however, the mixing apparatus may comprise a static mixer.

[0023] As previously stated, the injection tool may be provided with the mixing apparatus. Preferably, the mixing apparatus may be in fluid communication with the injection tool. Preferably, the mixing apparatus may be located adjacent, or in close proximity, to the injection tool. In a specific embodiment, the mixing apparatus may be located vertically above the injection tool in the wellbore. In this way, the mixture exiting the mixing apparatus may flow into the injection tool.

[0024] In some embodiments of the invention, a source of the setting additive may be carried in the injection tool (for instance, in a tank, canister or similar container) and may be added to the settable fluid pumped from the surface. In an alternative embodiment of the invention, the setting additive may be pumped from the surface to the injection tool and, more accurately, the mixing apparatus. Preferably, the setting additive is pumped from the surface in a separate conduit to the settable fluid. In one embodiment, the setting additive may be pumped from the surface to the mixing apparatus in the coiled tubing. More preferably, the setting additive may be pumped from the surface to the mixing apparatus in a conduit within the coiled tubing. Still more preferably, the setting additive may be pumped from the surface to the mixing apparatus in a capillary line located within the coiled tubing. In this embodiment, it is envisaged that the setting additive may be introduced to the mixing apparatus with the settable fluid. Preferably, the mixing apparatus forms a substantially homogenous mixture of the setting additive and the settable fluid for introduction to the wellbore.

[0025] The setting additive may be of any suitable form, although in a preferred embodiment of the invention, the setting additive may comprise sodium silicate solution, calcium chloride solution, triethyl aluminum solution, ammonium persulphate solution, triethylenediamine solution, triethanolamine solution, potassium hydroxide solution, piperidine solution, diethanolamine solution, epoxy resin, or any suitable combination thereof. The setting additive may be added in any suitable quantity to the settable fluid, and it will be understood that the quantity of setting additive added to the settable fluid may depend on a number of factors, such as the chemical composition of the settable fluid, the chemical composition of the setting additive, the volume of settable fluid, the desired rate of setting of the settable fluid and so on.

[0026] It is envisaged that the mixture of the settable fluid and the setting additive may set in a shorter time than the settable fluid without the setting additive. In a preferred embodiment, the addition of the setting additive to the settable fluid may reduce the setting time of the settable fluid from hours to minutes, or even seconds. Once set, it is envisaged that the mixture may form a physical barrier between the rock structure and the perforated casing.

[0027] By reducing the setting time of the mixture in the annular space between the perforated casing the rock structure, the loss of mixture into coal seams in the rock structure (which results in impaired gas production) may be reduced or eliminated. As additional benefits, this may also improve treatment efficiencies in the form of faster operations and reduce the quantity of mixture required to substantially fill the annular space between the perforated casing and the rock structure.

[0028] As previously stated, the mixture is introduced into the wellbore such that at least a portion of the mixture is located between a perforated production casing and a surrounding rock structure. More preferably, a substantial portion of the mixture may be located between the perforated production casing and a surrounding rock structure.

[0029] As previously stated, the injection tool is, in use, substantially continuously raised towards an upper end of the perforated casing while the mixture is substantially continuously injected into the wellbore. The rate at which the injection tool is raised may depend on a number of factors, such as, but not limited to, the exact chemistry of the mixture, the rate at which the mixture sets, the volume of the annular space between the perforated casing and the rock structure and so on. It is envisaged, however, that the injection tool may be raised at a rate of between 1 m/min and 35 m/min. More preferably, the injection tool may be raised at a rate of between 5 m/min and 25 m/min, although it will be understood that the rate may vary depending on the specific circumstances encountered.

[0030] Although the introduction of the mixture into the wellbore has been described as being substantially continuous, it is envisaged that there may be occasions when the introduction of the mixture is momentarily paused. For instance, in embodiments of the invention in which the location of the bridges of solid material is known (e.g. blank casing sections isolated in the annular cavity with swellable packers or identified sections where wellbore solids have created extensive accumulation of compacted wellbore solids), the introduction of the mixture may be momentarily paused as the injection tool passes the location of the bridges of solid material. This may be done to reduce the amount of mixture that is unable to pass through the perforated casing into the annular cavity due to the presence of the bridges of solid material. In these embodiments of the invention, it is envisaged that as the injection tool rises above the bridges of solid material, the introduction of mixture into the wellbore may recommence.

[0031] Notwithstanding the above, it will be understood that there may be some advantage to some of the mixture being located both within the casing and between the casing and the rock structure. In this situation, having the same material located within the casing and between the casing and the rock structure may reduce or eliminate cross flow or contamination below the injection tool as the mixture is introduced to the wellbore and while the mixture is yet to set.

[0032] Preferably, the injection tool introduces the mixture of the settable fluid and the setting additive into the wellbore at an angle to the orientation of the wellbore. The mixture of the settable fluid and the setting additive may be introduced into the wellbore at any suitable angle to the orientation of the wellbore. In a preferred embodiment of the invention, the injection tool may introduce the mixture of the settable fluid and the setting additive into the wellbore at an angle of between 30° and 150° to the orientation of the wellbore. More preferably, the injection tool may introduce the mixture of the settable fluid and the setting additive into the wellbore at an angle of between 70° and 110° to the orientation of the wellbore. Most preferably, the injection tool may introduce the mixture of the settable fluid and the setting additive into the wellbore at an angle of approximately 90° to the orientation of the wellbore.

[0033] In a preferred embodiment of the invention, the injection tool may include a base member in a lower region thereof. Any suitable base member may be provided, although it will be understood that the purpose of the base member may be to reduce or preclude the ability of the mixture from being introduced into the wellbore at a point below the injecting tool.

[0034] The injecting tool may comprise a nozzle, hose, conduit or the like, or any suitable combination thereof. In a preferred embodiment, the mixture may be introduced into the wellbore between an upper member and a lower member. The upper member and the lower member may be of any suitable form, and may be the same as one another, or may be different to one another. In some embodiments, the upper member and the lower member may comprise packers, drag blocks or the like. In one embodiment, the upper member and the lower member may both comprise packers. In an alternative embodiment, the upper member may comprise a packer, while the lower member may comprise a drag block. In embodiments of the invention in which the upper member or the lower member comprise packers, the packers are deactivated in the run in hole position. Preferably, the upper member and the lower member are associated with an outer surface of the coiled tubing. The upper member and the lower member may be associated with the outer surface of the coiled tubing using any suitable technique. For instance, the upper member and the lower member may be configured for connection to the outer surface of the coiled tubing. In some embodiments of the invention, the upper member and/or the lower member may be substantially annular members. In this embodiment of the invention, it is envisaged that the substantially annular members may be substantially ring-shaped with a central aperture. In these embodiments it is envisaged that the coiled tubing may pass through the central aperture and the upper member and/or lower member may be retained on the coiled tubing. In other embodiments, the upper member and the lower member may be associated with the outer surface of the coiled tubing by being located adjacent thereto. Preferably, the upper member and the lower member extend at least partway between the coiled tubing and an inner surface of the wellbore, or an inner surface of a casing (such as, but not limited to, a perforated casing, solid casing (i.e., a casing with no perforations in at least a portion thereof) or the like) located within the wellbore.

[0035] In embodiments of the invention in which the upper member and the lower member extend at least partway between the coiled tubing and an inner surface of the wellbore or an inner surface of a perforated casing located within the wellbore, it is envisaged that the upper member and the lower member may define a passageway along which the mixture flows at least partway between the coiled tubing and the perforated casing. By extending at least partway between the coiled tubing and the perforated casing, the upper member and the lower member may provide at least partially isolate the portion of the wellbore between the upper member and the lower member from the wellbore. In addition, the upper member and the lower member may create a barrier to reduce or eliminate the ability of the mixture to enter any part of the wellbore other than the part of the wellbore isolated between the upper member and the lower member.

[0036] In a preferred embodiment, at least the upper member may be expanded so that the outer periphery of the upper member may be located in close proximity to, or abutment with, the inner surface of the casing. In this way, it is envisaged that the flow of the mixture upwardly in the wellbore between the upper member and the casing may be substantially precluded. Thus, the upper member may substantially isolate the region of the wellbore into which the mixture is introduced through the injection tool. In this embodiment, it is envisaged that the mixture may exit the injection tool at or adjacent the inner surface of the casing. The upper member may be expanded using any suitable technique. Preferably, however, the upper member may be inflatable. Any suitable inflating fluid may be used to inflate the upper member. Preferably, however, the inflating fluid may be water, brine or setting additive

[0037] It is envisaged that the inflating fluid may be pumped from the surface to the upper and lower members. The inflating fluid may be pumped through any suitable conduit, although in a preferred embodiment of the invention the inflating fluid may be pumped to the upper and lower members using a capillary line. In some embodiments of the invention, the capillary line may be the same capillary line through which the setting additive is introduced to the mixing apparatus.

[0038] In another embodiment, it is envisaged that the upper member may be expanded so that the outer periphery of the upper member may be located in close proximity to, or abutment with, the inner surface of the casing. However, the lower member may be provided in such a manner that a gap is provided between the outer periphery of the lower member and the inner surface of the casing. In this way, a portion of the mixture of the settable fluid and the setting additive may flow between the lower member and the casing and enter the wellbore inside the casing at a point below the injection tool. By allowing at least some of the mixture to enter the wellbore within the casing, cross-flow and/or contamination between the interior of the casing and the annular cavity between the casing and the rock structure may be reduced or eliminated. In addition, providing a gap between the outer periphery of the lower member and the inner surface of the casing allows the mixture to continue to flow if the mixture does not flow through the perforations in the casing, or if there are not perforations in a section of the casing.

[0039] Preferably, the injection tool may be ran initially to or adjacent the bottom of the well. Specifically, the injection tool may be ran in the run in hole (RIH) position to or adjacent the bottom of the well. In this way, the upper and/or lower members may be retracted and there exists a flow path between the injection tool and casing, which enables the injection tool to be easily pushed down into the wellbore with low or no risk of hanging on casing collars or other casing obstructions and with minimum snubbing force. By using an injection tool designed to retract the members when running in hole, smaller size coiled tubing pipe may be used with minimum risk of pipe buckling failure. Another advantage may be that the chemical treatments can be performed more cost-efficiently as there is a reduced snubbing force requirement. In one specific example, using a small to medium size coiled tubing unit set up to run smaller size pipe (for example, 2 in or 5.08 cm) with a medium capacity injector (for instance 60,000 lb or 27.22t snubbing capacity) in a 7 in (17.78 cm) casing may become an effective option.

[0040] As previously stated, the mixture is introduced into the wellbore such that at least a portion of the mixture is located between a perforated production casing and a surrounding rock structure. More preferably, a substantial portion of the settable fluid may be located between the perforated production casing and a surrounding rock structure.

[0041] In some embodiments of the invention, the mixture may, when set, adhere or bond to the perforated casing and/or the surrounding rock structure. Alternatively, the mixture may, when set, simply abut the perforated casing and/or the surrounding rock structure. It is envisaged that, due to the combination of the relatively low density/relatively high viscosity of the mixture and a rapid setting time, invasion or penetration of the mixture into the rock structure (i.e. coal seams) may be substantially precluded, even in relatively depleted wells.

[0042] It is preferred that the mixture is of sufficient mechanical strength and durability so as to remain in use for extended periods of time. Further, it is preferred that the mixture may reduce or mitigate the migration of solid particles from the surrounding rock structure into the wellbore. The mixture may do this by providing a physical barrier between the rock structure and the perforated casing and/or by reinforcing the rock structure such that solid particles do not become detached or separated from the rock structure and/or by having relatively low permeability such that solid particles from the rock structure are physically unable to pass through the settable fluid and enter the wellbore through the perforated casing. Another advantage may be that the presence of the mixture may prevent or reduce the instance of erosion of the rock structure (and therefore migration of solid particles into the wellbore) through exposure of the rock structure to water flowing from the coal seams. Furthermore, the presence of the mixture may reduce or minimise interaction between clay-rich rock in the rock structure and water from coal seams. This interaction can result in swelling of clay in the rock structure and migration of clay particles into the wellbore.

[0043] It is envisaged that, as the level of the mixture in the annular cavity between the perforated casing and the rock structure rises, water in the cavity is displaced into the wellbore through the perforated casing, from where it continues to be displaced to the surface.

[0044] When the injection tool is raised to a point at or adjacent an upper end of the perforated casing, the introduction of the mixture into the wellbore may cease.

[0045] It is envisaged that a portion of the mixture introduced to the wellbore may not pass through the perforated casing and may instead remain in the wellbore inside the perforated casing. In this embodiment of the invention, it is envisaged that at least a portion of the mixture within the perforated casing may be removed. The mixture within the perforated casing may be removed using any suitable technique, such as drilling, milling, jetting or the like.

[0046] Once set, it is envisaged that one or more passages may be formed in the mixture in order to reconnect at least a portion of the producing rock (i.e. coal seams) to the wellbore. In this way, well production may be resumed. The one or more passages may be formed using any suitable technique. For instance, the one or more passages may be drilled or bored, or may be formed using an abrasive material, solvent, jetting water or the like. In other embodiments, the one or more passages may be formed naturally in the mixture by the flow of water, gas and/or solids out of the producing rock (i.e. coal seams).

[0047] In a second aspect, the invention resides broadly in an apparatus for introducing a mixture of a settable fluid and a setting additive into a wellbore, the apparatus comprising a mixing apparatus in fluid communication with a coiled tubing and a capillary line located within the coiled tubing, the mixing apparatus configured to be in fluid communication with an injection tool located substantially below the mixing apparatus in use, and wherein the injection tool further comprises an upper member and a lower member, the upper member and the lower member being configured to extend at least partway between the coiled tubing and an inner surface of the wellbore or an inner surface of a casing located within the wellbore, and the injection tool is configured to introduce the mixture into the wellbore along a passageway formed between the upper member and the lower member.

[0048] It is envisaged that the apparatus may be used to introduce the mixture to a single location within a wellbore. Alternatively, the apparatus may be raised within the wellbore during use so that the mixture may be introduced to multiple locations within the wellbore.

[0049] In a third aspect the invention resides broadly in a method for introducing a chemical treatment into a wellbore;

Introducing an injection tool into the wellbore, the injection tool comprising an upper member and a lower member;

Expanding at least the upper member so that an outer periphery of the upper member extends towards an inner surface of a perforated casing located within the wellbore;

Introducing the chemical treatment into the wellbore between the upper member and the lower member such that at least a portion of the chemical treatment is located between the perforated casing and a surrounding rock structure; and substantially continuously raising the injection tool towards an upper end of the perforated casing while substantially continuously injecting the chemical treatment into the wellbore.

[0050] In a preferred embodiment of the invention, it is envisaged that, as the injection tool is raised towards the upper end of the perforated casing, a portion of the chemical treatment may pass between the lower member and the inner surface of the perforated casing. In this way, a portion of the chemical treatment may be retained in the wellbore within the perforated casing.

[0051] While the present invention has been described largely in terms of its use to reduce the migration of solids from a surrounding rock structure into a wellbore, a skilled addressee would understand that the present invention may be used for a number of other purposes. For instance, the present invention could be used to reduce or eliminate unwanted water production in a well, or to reduce or eliminate unwanted gas production in a well. Further, the present invention could be used to stimulate hydrocarbon production within a well. In addition, the present invention may be used to treat a “loss zone” or “loss circulation section” of a wellbore by pumping fluid or the mixture into one or more specific sections of a wellbore to reduce or eliminate the loss of fluids from these sections. The present invention may also be used to performing cleaning, scale removal and/or washing of the inner wall of a perforated casing in a manner that provides more control (and is therefore more effective) than existing methods. Finally, the present invention may be used for performing applying corrosion prevention, treating corrosion, or coating the inner surface of the perforated casing.

[0052] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0053] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF DRAWINGS

[0054] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0055] Figures 1 to 5 illustrates steps in a method for reducing solids migration into wellbores according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0056] Figure 1 illustrates a first step in a method for reducing solids migration into a wellbore 10 according to an embodiment of the present invention. The wellbore 10 of Figure 1 is a coal seam gas wellbore with a plurality of coal seams 11 surrounded by matrix rock 12 in the rock structure 13 surrounding the wellbore 10.

[0057] In the wellbore 10 of Figure 1 , a region of exposed rock structure 13 exists between an upper cement casing 14 and a lower pre-perforated production casing 15. In production, water from the coal seams 11 (and, once the coal seams are at least partially dewatered, coal seam gas) enters the wellbore 10 through the pre-perforated production casing 15.

[0058] In Figure 1 it may be seen that bridges 16 of solid material have formed between the rock structure 13 and the perforated casing 15. These bridges 16 act to prevent the free flow of fluid vertically within the annular cavity 17 formed between the casing 15 and the rock structure 13.

[0059] In Figure 1 , coiled tubing 18 is introduced into the wellbore 10 and located in the RIH position adjacent the bottom of the wellbore 10. At least the upper member 19 (and optionally the lower member 20) are deactivated to improve the ease with which the coiled tubing 18 is run into the wellbore 10.

[0060] A capillary line 21 is located within the coiled tubing 18, and the setting additive is pumped from the surface therethrough, while the settable fluid is pumped from the surface through the coiled tubing 18.

[0061] Figure 2 illustrates a second step in a method for reducing solids migration into a wellbore 10 according to an embodiment of the present invention. In this Figure, an upper member 19 in the form of a packer is inflated (for instance using the setting additive pumped through the capillary line 21) so that the outer periphery of the upper member 19 extends to a point at or adjacent an inner surface of the perforated casing 15. In this way, the mixture introduced into the wellbore 10 through the injection tool 25 may exit the injection tool 25 between the upper member 19 and the lower member 20 at or adjacent to the perforated casing 15. The lower member 20 (which may be a packer, drag block or the like) may also be inflated or expanded.

[0062] It will be note in Figure 2 that, while the upper member 19 extends to a point in close proximity to the perforated casing 15, the lower member 20 extends in such a manner that a gap 26 remains between the outer periphery of the lower member 20 and the inner surface of the casing 15. By leaving this gap 26, the mixture (not shown in this Figure) may flow into the wellbore 10 inside the casing 15 as the injection tool 25 moves upwardly within the wellbore 10.

[0063] By ensuring that a portion of the mixture is located in the wellbore 10 within the casing 15 (best seen in Figures 4 and 5), cross-flow and/or contamination between the wellbore 10 within the casing 15 and the annular cavity 17 between the casing 15 and the rock structure 13 may be reduced or eliminated.

[0064] Figure 3 illustrates a third step in a method for reducing solids migration into a wellbore 10 according to an embodiment of the present invention. In Figure 3, the settable fluid 22 is pumped through the coiled tubing and into a static mixer 23 located at the lower end of the coiled tubing 18. The setting additive is pumped through the capillary line 21 and is mixed with the settable fluid 22 in the static mixer 23 to produce a substantially homogenous mixture.

[0065] The mixture 24 of the settable fluid and the setting additive is then pumped through the injection tool 25 into the annular cavity 17 between the perforated casing 15 and the rock structure 13.

[0066] In Figure 3 it may be seen that the mixture 24 has entered the annular cavity 17 but cannot rise in the annular cavity past the bridges 16 of solid material extending between the rock structure 13 and the perforated casing 15. Thus, for this reason, the injection tool 25 is raised continuously while the mixture 24 is being introduced into the wellbore 10. The injection tool 25 is raised by retracting the coiled tubing 18 from the wellbore 10.

[0067] Figure 4 illustrates a fourth step in a method for reducing solids migration into a wellbore 10 according to an embodiment of the present invention. In Figure 4, the coiled tubing 18 has been retracted from the RIH position shown in Figure 1 , and the injection tool 25 has moved upwardly within the wellbore 10 in the direction of the upper end of the perforated casing 15.

[0068] By continuously raising the injection tool 25 while continuously introducing the mixture 24 into the wellbore 10, it may be seen that the mixture 24 is able to enter the annular cavity 17 between the bridges 16 of solid material. In addition, as the injection tool 25 rises within the wellbore 10, a portion of the mixture 24 remains inside the perforated casing 15 below the injection tool 25. By ensuring that the mixture 24 is located both inside the casing 15 and in the annular cavity 17, the chance of cross-flow or contamination between the annular cavity 15 and the wellbore 10 inside the perforated casing 15 is reduced or eliminated.

[0069] Figure 5 illustrates a final step in a method for reducing solids migration into a wellbore 10 according to an embodiment of the present invention. In this Figure it may be seen that the coiled tubing 18 has been retracted to a point where the injection tool 25 is now above the upper end of the perforated casing 15. At this point, the introduction of the mixture 24 into the wellbore 10 is ceased. [0070] The presence of the setting additive in the mixture 24 significantly reduces the setting time of the mixture 24 in comparison to the setting time of the settable fluid alone. In this way, particularly in relatively depleted wells, the flow of the settable fluid into the coal seams 11 may be reduced or eliminated.

[0071] It is envisaged that the mixture 24 may substantially fill the annular cavity 17. However, it is possible, and may be desired, that some of the mixture 24 may remain inside the perforated casing 15. Mixture 24 that remains within the perforated casing 15 may be removed by milling or the like.

[0072] While not illustrated, it is envisaged that the coal seams 11 may be reconnected to the wellbore through the natural action of water, coal seam gas and/or solid material flowing out of the coal seams 11 and forming passageways through the mixture 24 so that water and coal seam gas may once again flow into the wellbore 10. Alternatively, passageways may be formed through the mixture 24 by perforating, hydraulically jetting, or abrading (and the like) the mixture 24 in order to reconnect the coal seams 11 with the wellbore 10.

[0073] In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0074] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0075] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.