AU2017101559B4 - A method for reducing solids migration into wellbores - Google Patents

Abstract

A method for reducing solids migration into wellbores, the method comprising the steps of: introducing a settable foamed material into a wellbore such that at least a portion of the settable foamed material is located between a perforated production casing and a surrounding rock structure, allowing the settable foamed material to at least partially set, and creating one or more passages in the settable foamed material, the one or more passages being adapted to bring one or more coal seams in the rock structure into fluid communication with the wellbore.

Description

METHOD FOR REDUCING SOLIDS MIGRATION INTO WELLBORES

TECHNICAL FIELD [0001] The present invention relates to a method for reducing solids migration into wellbores. In particular, the present invention relates to a method for reducing solids migration into wellbores (and particularly coal seam gas wellbores including vertical, deviated and horizontal wellbores) with pre-perforated or slotted production casing sections that also serves to reinforce the wellbore.

BACKGROUND ART [0002] Many oil and gas wells are provided with perforated or slotted production casings in lower regions thereof (also known as pre-perforated casings, and these terms will be used interchangeably). These production casings allow for low cost completions due to the fact that cement slurry does not need to be pumped behind the production casing and then the cement and casing subsequently perforated to allow the flow of fluid (e.g. water, gas or oil) from the surrounding rock structure into the wellbore. In addition, the invasion of cement slurry into the surrounding rock structure (and in particular into coal seams) results in a reduced ability to effectively dewater coal seams and produce gas.

[0003] However, while the cost is reduced, these perforated production casings suffer from the drawback that solid particles from the surrounding rock structure may enter the wellbore through the casing holes (pre-perforations). These solid particles may cause blockages in down-hole pumps and production tubing, resulting in the premature failure of completion systems.

[0004] Thus, while pre-perforated production casings may provide a relatively low-cost completion, the experience of many well operators is that the cost of lost production and maintenance due to the excessive migration of solids into the wellbore may outweigh the cost benefit of installing a perforated casing.

[0005] Unfortunately, however, it is not possible to remove a perforated casing from a live well. Thus, there would be an advantage if it were possible to provide a

2017101559 03 Nov 2017 method for reducing solids migration into wellbores (and particularly a wellbore in which a pre-perforated casing is present) that also serves to reinforce the wellbore.

[0006] Some attempts have been made to overcome these problems. For instance, PCT patent application no. PCT/US2014/037474 discloses a method of using an aqueous curable resin to penetrate formation laminae in the rock structure of a wellbore so as to stabilise or strengthen the rock structure and reduce the migration of solids into the wellbore. However, this method suffers from the drawback that the aqueous curable resin is also highly likely to penetrate coal seams in the rock structure, resulting in plugging of or damage to the coal seams and thereby reducing the ability to recover coal seam gas from the coal seams. In fact, it would be anticipated that the aqueous curable resin of PCT/US2014/037474 may preferentially invade or penetrate coal seams due to the relatively high permeability of coal compared to the rock above and below the coal seams.

[0007] In addition, the method of PCT/US2014/037474 suffers from the drawback that, as aqueous curable resin is introduced between the rock structure and a perforated casing in a lower region of the wellbore, a significant amount of the aqueous curable resin may flow inside the casing through the perforations in the perforated casing. This not only causes blockages in the perforated casing (potentially reducing the ability to dewater the coal seams) but also increases the likelihood of downhole tools getting stuck in the wellbore and being unable to be readily withdrawn (when such a tool is used). This in turn results in lost production, as well as the potential to lose the well entirely. Thus, there would be a further advantage if it were possible to provide a method for reducing solids migration into wellbores (and particularly a wellbore in which a pre-perforated casing is present) that doesn’t adversely affect the recovery of coal seam gas from a wellbore.

[0008] 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.

2017101559 03 Nov 2017

SUMMARY OF INVENTION [0009] The present invention is directed to a method for reducing solids migration into wellbores, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0010] With the foregoing in view, the present invention in one form, resides broadly in a method for reducing solids migration into wellbores, the method comprising the steps of: introducing a settable foamed material into a wellbore such that at least a portion of the settable foamed material is located between a perforated production casing and a surrounding rock structure, allowing the settable foamed material to at least partially set, and creating one or more passages in the settable foamed material, the one or more passages being adapted to bring one or more coal seams in the rock structure into fluid communication with the wellbore.

[0011] The settable foamed material may be introduced into the wellbore using any suitable technique. Preferably, however, the settable foamed material may be pumped into the wellbore, preferably from above ground level. Any suitable pump may be used to pump the settable foamed material into the wellbore.

[0012] The settable foamed material may be introduced to the wellbore in any suitable location. Preferably, however, the settable foamed material 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 at least partially withdrawn or removed from the wellbore to allow the tubing to be inserted into the wellbore and the settable foamed material to be introduced.

[0013] In some embodiments of the invention, it may be necessary to prepare the wellbore prior to the introduction of the settable foamed material. Any suitable preparation technique may be used, although in a preferred embodiment of the invention, the preparation of the wellbore may involve removing solid particles from the wellbore and/or the annular space between the pre-perforated casing and the

2017101559 03 Nov 2017 surrounding rock structure. In this way, an unimpeded flowpath for the settable foamed material may be formed.

[0014] The wellbore may be prepared using any suitable technique. For instance, solid particles may be removed from the flowpath using a fluid (such as water, brine, air or nitrogen, or any suitable combination thereof) and then the solid particles may be pumped 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.

[0015] The settable foamed material may be provided as a substantially homogenous mixture. Alternatively, the settable foamed material may comprise two or more components that must be mixed together to form the settable foamed material. 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 foamed material 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 foamed material is introduced into the wellbore in a substantially liquid pumpable foam form. Once in location between the perforated production casing and the surrounding rock structure, the settable foamed material may set to form an at least partially solid material.

[0016] The settable foamed material may be of any suitable form. For instance, the settable foamed material may at least partially comprise a cementitious material. Alternatively, the settable foamed material 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). In some embodiments of the invention, the settable foamed material may also include a settable resin, such as, but not limited to, an epoxy resin.

[0017] It will be understood that the settable foamed material may be provided in the form of a foam, and particularly a liquid foam which is preferably mixed and pumped from the surface. Alternatively, the settable foamed material may be

2017101559 03 Nov 2017 introduced to the wellbore as a liquid and then converted to a foam in situ. It is envisaged that the settable foamed material 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.

[0018] In some embodiments of the invention, the settable foamed material 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 foamed material, once set, may be in the form of a foam, a gel, a solid, a semi-solid or the like, or a combination thereof.

[0019] In a preferred embodiment of the invention, the specific gravity of the settable foamed material may not exceed 2.0. More preferably, the specific gravity of the settable foamed material may be between approximately 0.1 and 1.0. Still more preferably, the specific gravity of the settable foamed material may be between approximately 0.3 and 0.9. Most preferably, the specific gravity of the settable foamed material may be between about 0.5 and 0.8.

[0020] It is also preferred that the settable foamed material may have a relatively high viscosity. The settable foamed material may have any suitable viscosity, although in a preferred embodiment of the invention, the settable foamed material may have a viscosity greater than the viscosity of water (i.e. greater than 0.894 cP). More preferably, the settable foamed material may have a viscosity of greater than 100.0 cP. Still more preferably, the settable foamed material may have a viscosity of greater than 200.0 cP. Even more preferably, the settable foamed material may have a viscosity of greater than 250.0 cP. It will be understood that the viscosity of the settable foamed material refers to the viscosity of the liquid form of the settable foamed material. Once set, the viscosity of the settable foamed material may be much higher than these values.

2017101559 03 Nov 2017 [0021] In a preferred embodiment of the invention, the foam quality of the settable foamed material at downhole conditions may not exceed 70%. More preferably, the foam quality of the settable foamed material may be between 20% and 50%. Most preferably, the foam quality of the settable foamed material may be between 30% and 40%.

[0022] As previously stated, the settable foamed material is introduced into the wellbore such that at least a portion of the settable foamed material is located between a perforated production casing and a surrounding rock structure. More preferably, a substantial portion ofthe settable foamed material may be located between the perforated production casing and a surrounding rock structure. Still more preferably, substantially all of the settable foamed material may be located between the perforated production casing and a surrounding rock structure.

[0023] In some embodiments of the invention, the settable foamed material may, when set, adhere or bond to the perforated casing and/or the surrounding rock structure. Alternatively, the settable foamed material 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 and relatively high viscosity of the settable foamed material, invasion or penetration ofthe settable foamed material into the rock structure may be substantially precluded. In addition, and for the same reason, flow ofthe settable foamed material into the wellbore through perforations in the pre-perforated casing may be substantially precluded. It is also envisaged that, due to the combination of the relatively low density and relatively high viscosity of the settable foamed material, the settable foamed material will flow in the annular cavity rather than either penetrate the rock structure or pass through the perforations in the pre-perforated casing. This is particularly the case when the wellbore contains water having a greater density than the settable foamed material. It is envisaged that, due to the differences in density, the flow of water from the wellbore into the annular cavity between the perforated casing and the rock structure is more likely to occur than the flow of settable foamed material through the perforated casing into the wellbore. Preferably, the settable foamed material will flow in a direction that is substantially parallel to the casing.

2017101559 03 Nov 2017 [0024] Preferably, the settable foamed material has a number of advantageous properties. For instance, as previously stated, the viscosity of the settable foamed material is preferably relatively high. This relatively high viscosity may assist in substantially eliminating the entry of the settable foamed material into coal seams and matrix rock in the surrounding rock structure. Similarly, by providing a settable foamed material with a specific gravity lower than that of water (if required), the settable foamed material may be adapted to float on any water entering the wellbore from a coal seam so as not to restrict or impede the flow of fluid from the coal seam into the wellbore. Furthermore, the relatively low specific gravity of the settable foamed material enables placement with a reduced hydrostatic pressure which further reduces or even eliminates the risk of invasion of the settable foamed material into the coal seams.

[0025] In addition, it is preferred that the settable foamed material is of sufficient mechanical strength and durability so as to remain in use for extended periods of time. Further, it is preferred that the settable foamed material may reduce or mitigate the migration of solid particles from the surrounding rock structure into the wellbore. The settable foamed material 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 foamed material and enter the wellbore through the perforated casing. Another advantage may be that the presence of the settable foamed material 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 settable foamed material 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.

[0026] In a preferred embodiment of the invention, the settable foamed material may be directed into a cavity or gap between the perforated production casing and a surrounding rock structure. In some embodiments of the invention, the settable

2017101559 03 Nov 2017 foamed material may be directed into the annular cavity between the rock structure and the production casing. The direction of the settable foamed material may be achieved using any suitable technique. For instance, the settable foamed material may simply exit the tubing and flow into the cavity between the perforated casing and the rock structure. More preferably, however, the settable foamed material may be directed through one or more conduits between the tubing and the perforated casing. Any suitable conduit may be provided (such as one or more pipes, a downhole tool or the like) through which the settable foamed material may flow. In a preferred embodiment of the invention, the one or more conduits direct the settable foamed material radially outwardly from the tubing towards the perforated casing.

[0027] It is envisaged that the one or more conduits may include an open end distal to the tubing through which the settable foamed material exits the one or more conduits. The open end of the one or more conduits may preferable be located in abutment with, or close proximity to, an inner surface of the perforated casing such that settable foamed material exiting the one or more conduits passes through the perforations in the perforated casing and enters the cavity or gap between the perforated casing and the surrounding rock structure. In some embodiments of the invention, one or more apertures may be created in the perforated casing through which the settable foamed material enters the cavity. The one or more apertures may be new apertures in the perforated casing or may be perforations that are expanded in size to improve the flow of settable foamed material therefore. The apertures may be formed using any suitable technique. For instance, the apertures may be formed in the perforated casing prior to the perforated casing being inserted into the wellbore, or the one or more apertures may be formed in situ using a cutting tool, jet nozzle (such as an abrasion jet nozzle) and/or high velocity fluid or the like. More commonly, perforation charges may be used to form the apertures.

[0028] Preferably, the settable foamed material enters the cavity between the perforated casing and the rock structure in a lower region of the wellbore (i.e. at or adjacent the bottom of the wellbore). Preferably, as further settable foamed material enters the cavity between the perforated casing and the rock structure, the level of the settable foamed material in the cavity rises, and it is envisaged that little or no settable foamed material will enter the wellbore from the cavity through the

2017101559 03 Nov 2017 perforated casing. In a preferred embodiment of the invention sufficient settable foamed material may be introduced into the cavity until substantially the entire rock structure (or, more specifically, substantially the entire exposed face of the rock structure) between the bottom of the wellbore and the cemented well casing (i.e. the cemented casing of the upper wellbore with an external casing packer) is covered by the settable foamed material. In other embodiments, the settable foamed material may extend only a portion of the distance between the bottom of the wellbore and the cemented well casing.

[0029] It is envisaged that, as the level of settable foamed material in the cavity between the perforated casing and the rock structure rises, water in the cavity is displaced into the wellbore through the perforated cavity, from where it is pumped to the surface.

[0030] The settable foamed material may be allowed to at least partially set for any suitable period of time, and it will be understood that the period of time for which the settable foamed material is permitted to set may vary depending on a number of factors, including the exact composition of the settable foamed material, the temperature in the wellbore, the pressure in the wellbore and the like, or a combination thereof. Once set, it is envisaged that the settable foamed material may form a physical barrier between the rock structure and the perforated casing.

[0031] As previously stated, once the settable foamed material has at least partially set (to form the substantially or at least partially impermeable barrier) one or more passages are created in the settable foamed material, the one or more passages being adapted to bring one or more coal seams in the rock structure into fluid communication with the wellbore. The one or more passages may be located at any suitable point in the settable foamed material. For instance, the one or more passages may be located at regular (or irregular intervals) along the height of the barrier formed by the settable foamed material and/or around the inner circumference of the barrier. In this way fluids (such as water, gas and the like) may flow through the one or more passages and into the wellbore, from where the fluids are pumped to the surface. The one or more passages may be of identical size (diameter, length etc.) or may be of different sizes.

2017101559 03 Nov 2017 [0032] More preferably, however, the one or more passages are selectively formed at particular locations within the barrier. Preferably, the one or more passages may be formed at locations corresponding to the location of one or more coal seams within the rock structure. Thus, in this embodiment, it is envisaged that portions of the settable foamed material located between coal seams in the rock structure and the perforated casing may be removed so that the one or more passages created by the removal ofthe settable foamed material extend directly from the coal seams to the wellbore. In this way, the coal seams may be reconnected with the wellbore.

[0033] Conversely, it is preferred that substantially all of the settable foamed material located between bands or regions of matrix rock in the rock structure (i.e. the portion ofthe rock structure that is not the coal seams) and the perforated casing remain in place and are not removed to create the one or more passages. Thus, most preferably, the one or more passages only extend between coal seams in the rock structure and the perforated casing.

[0034] It is envisaged that the location of the one or more coal seams in the rock structure may be known. For instance, drill core samples may assist in identifying the location of the coal seams in the rock structure. In this way, settable foamed material may be selectively removed from the barrier to reconnect the coal seams to the wellbore without substantial exposure of the other non-coal rock sections. Any other suitable technique of locating coal seams depths within the rock structure may also be used, and such techniques will typically be conventional and require no further discussion.

[0035] The one or more passages may be formed in the settable foamed material using any suitable technique. For instance, the one or more passages may be drilled into, or dug out of, the settable foamed material. More preferably, however, the removal of the settable foamed material from the barrier to form the one or more passages may be achieved by passing a fluid through one or more outlets and impacting the fluid on the barrier. In some embodiments of the invention, the fluid may contain solid particles adapted to abrade the settable foamed material. Any suitable outlets may be used, although in a preferred embodiment ofthe invention,

2017101559 03 Nov 2017 the one or more outlets may be in the form of nozzles. In a more specific embodiment of the invention, the one or more nozzles may be in the form of jetting nozzles, or rotating jetting nozzles. In this embodiment of the invention it is envisaged that fluid passing through the one or more jetting nozzles exits the nozzles at a relatively high velocity. When the high velocity fluid impacts on the barrier of settable foamed material, the high velocity fluid may remove settable foamed material from the barrier to create a passage in the settable foamed material by any suitable process, such as, but not limited to, abrasion, erosion, high pressure cutting, perforation charges, skin fracturing treatment, nitrogen treatment or the like, or any suitable combination thereof.

[0036] Any suitable fluid may be used to create the one or more passages in the settable foamed material. For instance, the fluid may comprise one or more liquids, one or more gases, solids (e.g. frac sand) or a combination thereof. In some embodiments of the invention, the fluid may be water, air or an inert gas such as nitrogen. In some embodiments of the invention, a solvent or breaker may be added to the fluid. In this embodiment of the invention, the formation of the one or more passages in the settable foamed material may be through a combination of mechanical action (through the impact of the fluid on the settable foamed material) and chemical action (through the action of the solvent or breaker).

[0037] Any suitable solvent or breaker may be used. For instance, the solvent may be a non-polar solvent, a polar aprotic solvent, a polar protic solvent, or a combination thereof, and the breaker may be an oxidizer, enzyme or acid type or a combination thereof. In some embodiments of the invention, the solvent or breaker may comprise one or more of the following substances: pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, 1,4-dioxane, chloroform, diethyl ether, dichloromethane, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide, persulfate, perborate, hypochlorite, sodium hypochlorite, nitromethane, propylene carbonate, formic acid, hydrochloric acid, synthetic acids, n-butanol, isopropanol, n-propanol, ethanol, methanol, acetic acid or glycols. The solvent or breaker may be provided in any suitable quantity and in any suitable concentration, and it will be understood that the selection of the solvent or breaker, the quantity of the solvent or breaker used, the concentration of the solvent

2017101559 03 Nov 2017 or breaker and so on, will be dependent on a number of factors, such as the composition of the settable foamed material, the temperature in the wellbore, the pressure in the wellbore and the like.

[0038] The one or more nozzles may be provided at any suitable location. In a preferred embodiment of the invention, however, the one or more nozzles may be provided on the tubing. In a preferred embodiment of the invention, at least one of the one or more nozzles may be rotating nozzles.

[0039] It is envisaged that, once the one or more passages are formed in the settable foamed material, the passages may extend between the coal seams in the rock structure and the perforated casing, thereby reconnecting the coal seams and the wellbore. In this way, the coal seams may be brought into fluid communication with the wellbore. This fluid communication allows for the dewatering of the coal seams as water may flow out of the coal seams, through the one or more passages and into the wellbore, from where it may be pumped to the surface. It is envisaged that, once the coal seams are at least partially dewatered, coal seam gas may flow from the coal seams through the one or more passages and into the wellbore, from where it may be recovered to the surface.

[0040] By selectively removing settable foamed material located between the coal seams and the perforated casing (and leaving in place settable foamed material located between matrix rock in the rock structure and the perforated casing), the matrix rock may remain substantially unexposed (i.e. the barrier formed by the settable foamed material may substantially cover the matrix rock. In this way, the movement or migration of solid particles from the matrix rock through the perforated casing into the wellbore may be reduced or mitigated. This may be in part due to water from the coal seams being unable to erode the matrix rock, or by preventing interaction between clay-rich sections of the rock with fresh water produced from coal seams, which often results in severe swelling and solids migration into the wellbore.

[0041] In turn, reducing or eliminating the migration of solid particles into the wellbore reduces the prevalence of restrictions or blockages in the production tubing or and downhole pump caused by solid particles being pumped out of the well

2017101559 03 Nov 2017 through the production tubing. This results in improved well production and reduced well downtime and maintenance costs.

[0042] In addition, the composition of the settable foamed material is selected to as to provide the settable foamed material with properties (such as a relatively high viscosity and relatively low density) that substantially eliminates the ability of the settable foamed material to enter (or invade) the coal seams (or indeed, the matrix rock surrounding the coal seams). It will be understood that the invasion of a settable material into the coal seams (which are typically more permeable than the surrounding matrix rock and therefore more susceptible to invasion by a settable material) will reducing the ability of water and gas to flow out of the coal seams and into the wellbore. This results in reduced or even halted well production.

[0043] 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.

[0044] 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 [0045] 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 the Invention in anyway. The Detailed Description will make reference to a number of drawings as follows:

[0046] Figure 1 illustrates a wellbore according to the prior art.

[0047] Figure 2 illustrates a wellbore according to a step in the method of the present invention.

[0048] Figure 3 illustrates a wellbore according to a step in the method of the present invention.

2017101559 03 Nov 2017 [0049] Figure 4 illustrates a wellbore according to a step in the method of the present invention.

[0050] Figure 5 illustrates a wellbore according to a step in the method of the present invention.

[0051] Figure 6 illustrates a wellbore according to a step in the method of the present invention.

DESCRIPTION OF EMBODIMENTS [0052] Figure 1 illustrates a wellbore 10 according to the prior art. 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. The wellbore of Figure 1 is shown when in production.

[0053] 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 16 from the coal seams 11 (and, once the coal seams are at least partially dewatered, coal seam gas 30) enters the wellbore 10 through the preperforated production casing 15. Water 16 is removed from the wellbore 10 by being pumped to the surface by a down-hole pump 17, while the coal seam gas 30 rises through the wellbore 10 to the surface.

[0054] Water 16 from the coal seams 11 can erode matrix rock 12 or cause clay particles in the matrix rock 12 to swell. Both of these actions result in solid particles 18 from the matrix rock 12 entering the wellbore 10 through the pre-perforated production casing 15. The pump 17 then pumps the solid particles 18 to the surface with the water 16 as a slurry. However, depending on the volume of solids 18 entering the wellbore 10, the production tubing 19 and/or pump 17 through which the water 16 and solids 18 are pumped may become obstructed or blocked, resulting in lost production and maintenance costs.

[0055] In Figure 2, a wellbore 10 according to a step in the method of the present invention is illustrated. In this Figure, production tubing (not shown) has been withdrawn to allow tubing 20 to be introduced into the wellbore 10. The tubing 20

2017101559 03 Nov 2017 includes a downhole tool 22 having conduits that extends radially outwardly from the centre of the wellbore 10 towards the pre-perforated casing 15. In use, the conduits of the downhole tool 22 include outlets 31 that are located adjacent to, or in abutment with, the pre-perforated casing 15.

[0056] In Figure 3, a wellbore 10 according to a step in the method of the present invention is illustrated. In this Figure, settable foamed material 21 in the form of a liquid face and a gas (nitrogen) and also including a foaming agent in the form of a surfactant and other components required to prepare a settable stable foam system for the specific wellbore conditions (such as a gallant, activator and the like) is introduced into the wellbore 10 through the tubing 20. When the settable foamed material 21 reaches the lower end of the tubing 20, the settable foamed material 21 is directed radially outwardly from the tubing 20 towards the pre-perforated casing 15 through the conduits of the downhole tool 22. The settable foamed material 21 exits the downhole tool 22 through outlets 31 and passes through the pre-perforated casing 15 to enter the annular cavity 23 between the pre-perforated casing 15 and the surrounding rock structure 13. Sufficient settable foamed material 21 is introduced into the annular cavity 23 until the settable foamed material 21 substantially covers the entire exposed rock structure 13 between the bottom of the wellbore 10 and the cement casing 14 in an upper section of the wellbore 10.

[0057] The settable foamed material 21 of Figure 3 has a specific gravity of between 0.5 and 0.8 and a relatively high viscosity. The relatively high viscosity and relatively low density of the settable foamed material 21 ensures that the settable foamed material flows substantially upwardly in the annular cavity 23 towards the cement casing 14. The combination of relatively high viscosity and relatively low density of the settable foamed material 21 substantially precludes the settable foamed material 21 from invading or penetrating the rock structure 13 (and in particular the coal seams 11) and from passing into the wellbore 10 through perforations in the pre-perforated casing 15 . Indeed, it is likely that the wellbore 10 may be at least partially filled with water having a greater density than the settable foamed material 21. Thus, the presence of water in the wellbore 21 may substantially preclude the settable foamed material 21 from entering the wellbore 10 through the pre-perforated casing 15.

2017101559 03 Nov 2017 [0058] In Figure 4, a step in the method of the present invention is illustrated. In this Figure, sufficient settable foamed material 21 has been introduced into (and allowed to at least partially set) the annular cavity 23 between the rock structure 13 and the pre-perforated casing 15 so as to form a physical barrier between the rock structure 13 and the pre-perforated casing 15. It will be noted that, while the settable foamed material 21 may abut or adhere to the rock structure 13, the settable foamed material 21 has not penetrated or invaded the rock structure (and particularly the coal seams 11). Thus, the settable foamed material 21 simply forms a physical barrier that separates the rock structure 13 from the wellbore 10. At this point in the method, solid particles from the rock structure 13 are substantially precluded from migrating into the wellbore 10 due to the presence of the settable foamed material 21. However, water and gas from the coal seams 11 are also substantially precluded from entering the wellbore 10.

[0059] In addition, the combination of the relatively low density and the relatively high viscosity of the settable foamed material 21 means that the settable foamed material 21 has been substantially precluded from entering the wellbore 10 through perforations in the pre-perforated casing 15. This allows the downhole tool 22 to be easily withdrawn from the wellbore 10. If the settable foamed material 21 had not been substantially precluded from entering the wellbore 10 through perforations in the pre-perforated casing 15, the settable foamed material 21 would have set on the inner surface of the pre-perforated casing 15 (i.e. within the wellbore 10) and would have physically obstructed the removal of the downhole tool 22 from the wellbore 10. In turn, this would have resulted in lost production or even, in the worst case, the loss of the entire well.

[0060] In Figure 5, a step in the method of the present invention is illustrated. In this Figure, the downhole tool of Figure 4 is replaced by rotating jetting nozzles 24 associated with the tubing 20. In the embodiment of the invention shown in Figure 5, a mixture 32 of water and or a chemical breaker in the form of hydrochloric acid is pumped down the tubing 20 from the surface and is expelled from the jetting nozzles 24 at a relatively high velocity towards the settable foamed material 21. A combination of mechanical and chemical breakage of the settable foamed material

2017101559 03 Nov 2017 by the water and chemical breaker mixture 32 results in the formation of a passage 25 through the settable foamed material 21.

[0061] The passage 25 reconnects a coal seam 11 with the wellbore 10 such that water and coal seam gas from the coal seam 11 can flow through the passage 25 and the pre-perforated casing 15 into the wellbore 10. From here, the water can be pumped to the surface by a pump (not shown) while gas flows upwardly through the wellbore 10.

[0062] It will be noted in Figure 5 that the passage 25 is selectively formed between the coal seam 11 and the pre-perforated casing 15 so that the settable foamed material 21 remains in place as a barrier between the matrix rock 12 above and below the coal seam 11 and the pre-perforated casing 15. By leaving the settable foamed material 21 as a barrier between the matrix rock 12 and the preperforated casing 15, the migration of solid particles from the matrix rock 12 into the wellbore 10 may be mitigated.

[0063] In Figure 6, the wellbore 10 is shown in a state where it is ready for production to commence. A plurality of passages 25 have been formed in the barrier of settable foamed material 21 so that each coal seam 11 within the rock structure 13 is reconnected to the wellbore 10. The passages 25 have been formed selectively between the coal seams 11 and the pre-perforated casing 15 so that the matrix rock 12 is substantially unexposed and remains behind the settable foamed material 21.

It will also be noted that the production tubing 19 has been reintroduced to the wellbore in readiness for the commencement of production.

[0064] Water flows from the coal seams 11 through the passages 25 and into the wellbore 10 from where it is pumped to the surface by pump 17. Once the coal seams 11 are at least partially dewatered, coal seam gas will flow from the coal seams 11 through the passages 25 and into the wellbore 10 from where it flows to the surface.

[0065] As water flows through the passages 25, the ability of the water to come into contact with matrix rock 12 is reduced. Thus, erosion of matrix rock 12 (or

2017101559 03 Nov 2017 swelling of clay particles in the matrix rock 12) is reduced, resulting in fewer solid particles entering the wellbore 10.

[0066] By reducing or mitigating the migration of solid particles into the wellbore 10, the likelihood of obstruction or blockage of the production tubing 19 and/or pump 17 may be reduced, thereby improving well production and reducing maintenance costs due to well shutdowns.

[0067] 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.

[0068] 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.

[0069] 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.

Claims (5)

1. A method for reducing solids migration into wellbores, the method comprising the steps of: introducing a settable foamed material into a wellbore such that at least a portion of the settable foamed material is located between a perforated production casing and a surrounding rock structure, allowing the settable foamed material to at least partially set to form a substantially impermeable barrier, and creating one or more passages in the settable foamed material, the one or more passages being adapted to bring one or more coal seams in the rock structure into fluid communication with the wellbore.

2. A method according to claim 1 wherein the settable foamed material has a specific gravity of between approximately 0.5 and 0.8.

3. A method according to claim 1 or claim 2 wherein the settable foamed material forms a barrier between the perforated casing and the rock structure, and the one or more passages are created by selectively removing settable foamed material from the barrier to reconnect the coal seams to the wellbore without substantial exposure of matrix rock in the rock structure.

4. A method according to any one of the preceding claims wherein the one or more passages are created by passing a fluid through one or more nozzles at a relatively high velocity and impacting the fluid on the settable foamed material.

5. A method according to claim 4 wherein the fluid comprises a liquid and/or a gas, wherein the liquid and/or the gas optionally includes a solvent or breaker.

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