AU2021106078A4

AU2021106078A4 - A Method for Reducing Solids Migration into New Wellbores

Info

Publication number: AU2021106078A4
Application number: AU2021106078A
Authority: AU
Inventors: Andrew Harper; Diego Tarre Moser
Original assignee: Australian Coil Services Pty Ltd
Current assignee: Australian Coil Services Pty Ltd
Priority date: 2021-08-20
Filing date: 2021-08-20
Publication date: 2021-10-28
Anticipated expiration: 2029-08-20

Abstract

A method for reducing solids migration into wellbores, the method comprising the steps of: Introducing a breakable gel into the wellbore, such that the breakable gel is located substantially within a perforated casing positioned in the wellbore; Introducing a settable foamed material into the wellbore, such that the settable foamed material is substantially located in an annular space between the perforated casing and a surrounding rock structure; Allowing the settable foamed material to at least partially set; and Removing broken breakable gel from the wellbore, Wherein the settable foamed material is configured to at least partially set before the breakable gel breaks. 2/4 23 12 14 13 20 F15 19 16 17 22 18 FIG 2

Description

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A METHOD FOR MITIGATING SOLIDS MIGRATION INTO NEW 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, in particular, new coal seam gas wellbores including vertical, deviated and horizontal wellbores) with pre-perforated or slotted production casing.

BACKGROUND

[0002] Many coal seam 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). In recent times, these production casings are typically installed with multiple swellable packers carefully located to isolate different sections of the rock structure in which the wellbore is drilled. This leads to a reduction in the migration of solids into the wellbore from these sections of the rock once the well is in production and water begins to flow from coal seams into the wellbore.

[0003] These swellable packers are relatively inexpensive and their use has been shown to provide solids migration mitigation in some coal seam gas fields. However, the conventional method of using swellable packers to reduce solids migration into a wellbore has some important drawbacks.

[0004] For instance, it is well-known that the lithology of every well is unique. Inotherwords, the exact depth of each coal seam varies from well to well, meaning that it is important to precisely identify the exact location of each coal seam top and bottom during the drilling process of the well in order to develop a specific casing design, known as casing tally, incorporating a specific location for each swellable packer. This process is time-consuming and exacting: once the packers are installed in the wellbore and swell when in contact with fluids in the wellbore, the casing can no longer be removed. Thus, errors in locating the packers can have significant ramifications and, in practice, has resulted in the loss of wells.

[0005] Coal seam gas wells often have several coal seams, therefore several packers are typically required as part of the casing tally. Most new well designs include at least fifteen swellable packers which are installed on the casing as the casing is deployed into the wellbore. In general, the larger the number of swellable packers the higher the risk that any of them might end up at the wrong depth, for example across a coal seam, which reduces production from the well.

[0006] In addition, it is often necessary to compromise access to some coal seams in a well in order to achieve a minimum length of interburden isolation. On one hand the larger the section of the rock that is isolated, the lower the risk of excessive solid production, which is a serious and expensive problem. On the other hand, the larger the section of the rock that is isolated, the greater the isolation of coal seams which results in reduced gas production.

[0007] Lastly, it has more recently been identified that many wells that use multiple swellable packers produce excessive solids, resulting in increased operating expenses and suboptimal gas production. Thus, conventional methods using swellable packers are not as effective and or long lasting as initially thought, and therefore an alternative cost-effective method would be very valuable to the industry.

[0008] Some attempts have been made to overcome these problems. For instance, software tools have been developed to automate the casing tally design once the location of coal seams is identified. In addition, there have been improvements in swellable packer material selection in order to ensure sufficient time is provided to run the casing with a safety margin before the packers start to swell.

[0009] Further, strategies have been adopted that attempt to find a satisfactory balance between access to coal seams and isolation of interburden. However, these optimizations inevitably result in a compromise and cannot achieve 100% access to coal seams while providing 100% isolation of interburden. Moreover, in all cases, unremovable obstructions form in the annular space behind the production casing, which makes future intervention, such as solid mitigation remedial treatments, more difficult and expensive.

[0010] Australian patent no. 2017101559 discloses an alternative method to swellable packers to find a satisfactory balance between access to coal seams and isolation of interburden. However, while effective, this method results in the placement of settable foam inside the production casing in addition to the annular space behind the production casing. The volume of settable foam placed inside the casing is effectively waste product which is removed and disposed after the settable foam has set. This product is placed inside the casing in order to ensure that the settable foam injected into the annular space remains in the annular space until it sets and does not flow back inside the casing through the existing perforations.

[0011] In addition, the method of Australian patent no. 2017101559 is used to enable dynamic placement of the settable foam by pulling and pumping across the entire perforated section, which is required when wells have annular obstructions such as bridges formed by wellbore solids or swellable packers. However, leaving settable foam inside the casing increases the cost of the treatment and can make this method cost prohibitive on new coal seam gas wells. As a result, this method is unlikely to be adopted as an alternative to swellable packers in new wells.

[0012] 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

[0013] Embodiments of the present invention provide a method for reducing solids migration into coal seam gas wellbores and, in particular, new coal seam gas 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.

[0014] 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 breakable gel into the wellbore, such that the breakable gel is located substantially within a perforated casing positioned in the wellbore; Introducing a settable foamed material into the wellbore, such that the settable foamed material is substantially located in an annular space between the perforated casing and a surrounding rock structure; Allowing the settable foamed material to at least partially set; and Removing broken breakable gel from the wellbore, Wherein the settable foamed material is configured to at least partially set before the breakable gel breaks.

[0015] Preferably, the wellbore is a new wellbore. It will be understood that the term "new wellbore" is intended to refer to wellbores that have been newly-drilled and/or are yet to be put into production. While it is envisaged that the method of the present invention could be used in wellbores that have been in production, the method of the present application may not be suitable for wellbores in which bridges of solid material have, over time, formed in the annular space between the rock structure and the perforated casing.

[0016] The breakable gel may be introduced into the wellbore using any suitable technique. Preferably, however, the breakable gel may be pumped into the wellbore. Preferably, a source of the breakable gel may be located above ground level, and the breakable gel may be pumped into the wellbore from above ground level, and in particular from the source of the breakable gel.

[0017] The breakable gel may be introduced into the wellbore at any suitable location.

Preferably, however, the breakable gel may be introduced into the wellbore at a location at or adjacent the lower end of the wellbore. Preferably, the breakable gel may be pumped into the wellbore through a conduit. Any suitable conduit may be used, such as tubing. The tubing may comprise coiled tubing orjointed tubing.

[0018] The tubing may be of any suitable form, and it is envisaged that the tubing may comprise standard jointed tubing. In addition, standard casing hardware may be installed on the casing within the wellbore. In some embodiments, the same tubing used by a completion rig may be used as the tubing in the method of the present invention. The tubing may be of any suitable diameter, although it is preferred that the tubing may be between 1in (2.54cm) and 5in (12.7cm) in diameter. More preferably, the tubing may be between 2in (5.08cm) and 3.5in (8.89cm) in diameter. It will be understood, however, that the size of tubing used in the present invention may vary depending on the nature of the specific well in which the method is to be used.

[0019] It is envisaged that, prior to the introduction of the breakable gel, the wellbore may contain a fluid. Any suitable fluid may be contained within the wellbore, although in some embodiments of the invention the fluid may comprise brine. Preferably, as the breakable gel is introduced into the wellbore, the fluid may be displaced by the breakable gel.

[0020] The breakable gel may be provided in any suitable form. For instance, the breakable gel may be supplied in the form of a liquid or foam. The breakable gel may be provided as a substantially homogenous mixture. Alternatively, the breakable gel material may comprise two or more components that are mixed together to form a gel, and particularly a gel having relatively high viscosity. 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 breakable gel 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 breakable gel may be introduced into the wellbore in a pumpable form, such as in a substantially liquid form. Once in location between the tubing and the inner surface of the perforated casing, it is envisaged that the breakable gel will be substantially retained within the perforated casing due to its relatively high viscosity. Thus, it is preferred that the breakable gel will be substantially precluded from flowing through the perforations in the perforated casing and into the annular space between the perforated casing and the surrounding rock structure. In this way, the ability of the settable foamed material to flow into the annular space will be substantially unimpeded. In addition, by retaining the breakable gel within the perforated casing, the broken breakable gel may be readily removed from the wellbore.

[0021] The breakable gel may be of any suitable form. For instance, the breakable gel may comprise a cross-linked polymer and a chemical breaker. In a preferred embodiment of the invention, the breakable gel may comprise a polymer, a crosslinker and a chemical breaker. The polymer may be of any suitable form, although in preferred embodiments of the invention the polymer may comprise one or more of guar gum, xantham gum, carboxymethyl hydroxypropyl guar gum (CMHPG), hydroxypropyl guar gum (HPG), or a cellulose (such as hydroxyethyl cellulose or carboxymethyl cellulose).

[0022] Any suitable crosslinker may be used in the breakable gel. In some embodiments of the invention, the crosslinker may comprise one or more of zirconium, boron or titanium.

[0023] The chemical breaker may be of any suitable form. In some embodiments of the invention, the chemical breaker may comprise an oxidiser (such as, but not limited to, ammonium persulfate, sodium sulfate, calcium peroxide and magnesium peroxide) or an enzyme (such as, but not limited to, mannanase, hemicellulase, amylase and pectinase).

[0024] It will be understood that the term "break" or "breaking" of the breakable gel refers to the degradation of the gel by a chemical breaker to a low-viscosity and watery solution.

[0025] As previously stated, the settable foamed material is configured to at least partially set before the breakable gel breaks. The breakable gel may be configured to break after any suitable period of time. In a preferred embodiment of the invention, however, the breaking time of the breakable gel (i.e. the time between the introduction of the breakable gel to the wellbore and the breaking of the breakable gel) may not exceed 12 hours. More preferably, the breaking time of the breakable gel may be between approximately 0.5 and 6 hours from the introduction of the breakable gel into the wellbore. Still more preferably, the breaking time of the breakable gel may be between approximately 1 and 3 hours from the introduction of the breakable gel into the wellbore.

[0026] In a preferred embodiment of the invention, the specific gravity of the breakable gel may not exceed 1.2. More preferably, the specific gravity of the breakable gel may be between approximately 0.8 and 1.1. Still more preferably, the specific gravity of the breakable gel may be between approximately 0.9 and 1.05.

[0027] As previously stated, it is preferred that the breakable gel has a relatively high viscosity. The breakable gel may be of any suitable viscosity, although in a preferred embodiment of the invention, the breakable gel may have a viscosity greater than the viscosity of water (i.e. greater than 0.894 cP). More preferably, the breakable gel may have a viscosity of greater than 100.0 cP. Still more preferably, the breakable gel may have a viscosity of greater than 2,000.0 cP. Even more preferably, the breakable gel may have a viscosity of greater than 2,500.0 cP. It will be understood that the viscosity of the breakable gel refers to the viscosity of the cross-linked form of the breakable gel. Once the gel breaks, it is envisaged that the viscosity of the broken gel may be significantly lower than the viscosity of the breakable gel prior to breaking.

[0028] The breakable gel may extend to any suitable depth within the wellbore. In some embodiments of the invention, the breakable gel may be allowed to extend to at or adjacent the bottom of the wellbore. In other embodiments of the invention, a barrier member may be located in the wellbore. In these embodiments, the location of the barrier member may define the lowest point within the wellbore to which the breakable gel may extend.

[0029] The barrier member may be of any suitable form. In a preferred embodiment of the invention, the barrier member may comprise a retainer (such as a cement retainer) or a stab-in float positioned within the perforated casing. It is envisaged that the breakable gel is substantially precluded from flowing around and/or below the barrier member.

[0030] Preferably, the barrier member is positioned within the perforated casing such that one or more perforations in the perforated casing are located below the barrier member. More preferably, a plurality of perforations is located below the barrier member. In a specific embodiment, the barrier member may be located above one casing join or pup join. In this embodiment, between approximately 1 and 2 metres of perforations may be located below the barrier member. More preferably, the barrier member may be located a sufficient distance below the design depth for a progressive cavity pump (PCP) so that it does not have to be milled out after the placement of the settable foam.

[0031] Preferably the perforated casing may be installed with sufficient centralizers to achieve at least 70% stand-off in order to enhance adequate flow of the settable foamed material in the annular space. More preferably, the casing stand-off may be greater than 80%.

[0032] 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.

[0033] 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.

[0034] 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 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.

[0035] 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.

[0036] 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 1.2. Still more preferably, the specific gravity of the settable foamed material may be between approximately 0.3 and 1.0. Most preferably, the specific gravity of the settable foamed material may be between about 0.5 and 0.9.

[0037] 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.

[0038] 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 10% and 60%. Most preferably, the foam quality of the settable foamed material may be between 15% and 40%.

[0039] As previously stated, the settable foamed material is introduced into the wellbore such that the settable foamed material is located substantially between perforated casing and a surrounding rock structure. 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 of the settable foamed material into the rock structure may be substantially precluded. In addition, and for the same reason along with the presence of the breakable gel within the perforated casing, flow of the settable foamed material into the wellbore through perforations in the 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 perforated casing. This is particularly the case when the wellbore contains breakable gel of relatively high viscosity and having a similar or greater density than the settable foamed material. Preferably, the settable foamed material will flow in a direction that is substantially parallel to the casing.

[0040] In some embodiments, a setting additive may be added to the settable foamed material so as to accelerate the setting reaction of the settable foamed material. 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.

[0041] In some embodiments of the invention, the settable foamed material may be introduced into the wellbore through the same tubing used to introduce the breakable gel. In this embodiment, the outlet of the tubing may be lowered to a point below the breakable gel in order to introduce the settable foamed material into the wellbore. In other embodiments, however, the tubing may be provided with a stinger at the outlet end thereof. It will be understood that the stinger may be used to "sting" into the barrier member (or other suitable casing hardware) with one or more check valves that preclude the flow of fluid into and out of the tubing unless the check valve is opened by the stinger.

[0042] In this embodiment, the stinger may be engaged (or "stung") into the barrier member (in the form of a cement retainer or stab-in float) in order to open the check valve and allow the flow of settable material into the wellbore. It is envisaged that the settable foamed material may flow below the barrier member, through the perforations in the perforated casing below the barrier member and into the annular space between the perforated casing and the surrounding rock structure.

[0043] In some embodiments, a packer may be associated with the tubing. Preferably, the packer may be located towards an upper end of the perforated casing. In embodiments of the invention in which a barrier member is present, it is envisaged that the barrier member and the packer may be spaced apart from one another by at least a portion of the length of the perforated casing. The packer may be of any suitable form, although in some embodiments of the invention, the packer may comprise a rotation-set packer.

[0044] Preferably, a locating member may be positioned in the wellbore to assist in locating the packer in the desired region within the wellbore. The locating member may be of any suitable form, although it is envisaged that the locating member may be configured to provide a marker for the desired location of the packer as it is run-in to the wellbore.

[0045] In a preferred embodiment of the invention, the locating member may comprise a pup joint, and in particular a blank pup joint. Preferably, the blank pup joint is located below an external casing packer (ECP), and is spaced apart from the ECP by one or more perforated pup joints.

[0046] It is envisaged that the packer may be set by rotation so that an outer periphery of the packer is in abutment with, or close proximity to, an inner surface of the pup joint.

[0047] It is envisaged that, as the breakable gel enter the wellbore, the packer may be set by rotation such that an outer periphery of the packer is brought into abut, or close proximity to, an inner surface of the perforated casing. In this way, the packer may form a barrier to the flow of breakable gel upwardly within the wellbore. The packer may be set prior to the introduction of breakable gel into the wellbore, although more preferably the packer may be set after the breakable gel has been introduced to the wellbore.

[0048] Preferably, once the breakable gel begins to break, the packer may be unset such that broken breakable gel may flow upwardly in the wellbore past the packer to be removed from the wellbore.

[0049] It is envisaged that, as breakable gel is removed from the wellbore, it may be replaced by completion fluid in order to place the well in readiness for the installation of production tubing and the commencement of production.

[0050] It is envisaged that, prior to the use of the method of the present invention, it may be necessary to prepare the wellbore prior to the introduction of the breakable gel. Any suitable preparation technique may be used, although in a preferred embodiment of the invention the preparation of the wellbore may involve milling or otherwise pushing down a stage tool plug which installed above the top of perforations of the perforated casing during the drilling process. This technique is conventional, and no further discussion is required.

[0051] Once at least partially set, it is envisaged that one or more passages may be formed in the settable foamed material 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 commenced. 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 preferred embodiments, the one or more passages may be formed naturally in the settable foamed material by the flow of water, gas and/or solids out of the producing rock (i.e. coal seams). Thus, it is envisaged that substantially all of the coal seams in the rock structure may be reconnected to the wellbore.

[0052] The present invention provides numerous advantages over the prior art. Firstly, in comparison to prior art methods (such as that described in Australian patent no. 2017101559) settable foamed material is substantially precluded from being located within the perforated casing within the wellbore, and instead the settable foamed material is substantially confined to the annular space between the perforated casing and the surrounding rock structure. As the settable foamed material is typically an expensive product, reducing the amount of this product used represents a significant cost saving, while avoiding the need to remove waste settable foamed material from within the wellbore reduces the time taken to put the well into production.

[0053] In addition, the present invention provides an improved balance between access to coal seams and isolation of interburden in new wells in comparison to existing methods. This is achieved by reducing the isolation of coal seams by design and/or by the incorrect location of packers (which results in lower production from the well) and improving the isolation of interburden, which results in the reduction of solids material from the surrounding rock structure migrating into the wellbore. Not only does this improve well production, but it reduces equipment and maintenance problems caused by the migration of solids into the wellbore.

[0054] 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.

[0055] 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

[0056] 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:

[0057] Figures 1 to 3 illustrate steps in a method for reducing solids migration into wellbores according to an embodiment of the present invention.

[0058] Figure 4 illustrates a well in which the method of the present invention has been used.

DETAILED DESCRIPTION

[0059] Figure 1 illustrates a first step in a method for reducing solids migration into wellbores according to an embodiment of the present invention. In this Figure, a new well 10 has been prepared and is initially full of a brine solution 11 (in particular a KCI brine solution).

[0060] Tubing 12 is run into the wellbore 13, and the lower end of the tubing 12 includes a stinger 21. A rotation-set packer 14 is associated with the tubing 12, and the tubing 12 is run into the wellbore 13 until the packer 14 is located substantially level with a blank pup joint 15 that forms part of the perforated casing 16.

[0061] Once in position, breakable gel 17 is introduced into the wellbore 13 through the tubing 12. The breakable gel is precluded from passing below the barrier member 18 in the form of a cement retainer or stab-in float. Due to the relatively high viscosity and density of the breakable gel 17, the breakable gel 17 is substantially precluded from passing through the perforations in the perforated casing 16 and entering the annular space 19 between the perforated casing 16 and the surrounding rock structure 20. Sufficient breakable gel 17 is introduced to the wellbore 13 to substantially fill the wellbore 13 between the barrier member 18 and the packer 14.

[0062] Figure 2 illustrates a first step in a method for reducing solids migration into wellbores according to an embodiment of the present invention. In this Figure, the packer 14 is set by rotation so that the periphery of the packer 14 is brought into abutment with, or close proximity to, the inner surface of the pup joint 15. The tubing 12 is run into the wellbore 13 further until the stinger (obscured) engages with (or "stings") the barrier member 18. In particular, the stinger stings a check valve (not shown) in the barrier member, thereby actuating the flow of settable foamed material 22 out of the tubing 12 and through the barrier member 18.

[0063] In Figure 2 it may be seen that the barrier member 18 is positioned in the perforated casing 16 such that perforations are located below the barrier member 18. Thus, as settable foamed material 22 flows through the barrier member 18, it then passes through perforations in the perforated casing 16 below the barrier member 18 and enters the annular space 19 between the perforated casing 16 and the rock structure 20.

[0064] As more settable foamed material 22 is introduced to the wellbore 13, the settable foamed material 22 flows upwardly in the annular space 19 towards the lower end of the cemented casing 23. The presence of the relatively high viscosity breakable gel 17 in the wellbore 13 substantially precludes the settable foamed material 22 from passing back through the perforated casing 16 until it reaches the upper section above the packer 14.

[0065] Figure 3 illustrates a third step in a method for reducing solids migration into wellbores according to an embodiment of the present invention. In this Figure, the settable foamed material 22 has substantially filled the annular space 19 and has at least partially set. At this point, the rock structure 20 is substantially completely isolated, as the settable foamed material 22 covers the entire exposed surface of the rock structure 20. Once sufficient settable foamed material 22 is introduced, the stinger 21 is withdrawn from the barrier member 18 to cease the flow of settable foamed material 22.

[0066] Following the setting of the settable foamed material 22, the breakable gel 17 begins to break due to the action of a chemical breaker within the breakable gel 17. As the breakable gel 17 breaks, the packer 14 is unset so that broken gel may flow upwardly in the wellbore 13 past the packer 14. The broken gel is then removed from the well.

[0067] As the settable foamed material 22 is at least partially set by the time that the breakable gel 17 begins to break, substantially none of the settable foamed material 22 enters the wellbore 13 through the perforated casing 16.

[0068] Figure 4 illustrates a well in which the method of the present invention has been used. In this Figure it may be seen that the settable foamed material 22 is located between the interburden regions 24 of the rock structure 20 and the perforated casing 16. In this way, solids migration from the interburden regions 24 into the wellbore 13 is reduced or eliminated.

[0069] On the other hand, passages 26 have formed through the settable foamed material 22 through which water and gas may enter the wellbore 13 through the perforated casing 16 and be pumped to the surface. These passages 26 may be milled or drilled through the settable foamed material 22 or, more preferably, may be formed by the action of water flowing out of the coal seams 25 within the rock structure 20.

[0070] It may be seen, therefore, that the method of the present invention provides improved isolation of interburden regions 24 within the rock structure 20, while reducing or avoiding the isolation of coal seams 15, which results in reduced production from a well.

[0071] 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.

[0072] 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.

[0073] 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

1. A method for reducing solids migration into wellbores, the method comprising the steps of: Introducing a breakable gel into the wellbore, such that the breakable gel is located substantially within a perforated casing positioned in the wellbore; Introducing a settable foamed material into the wellbore, such that the settable foamed material is substantially located in an annular space between the perforated casing and a surrounding rock structure; Allowing the settable foamed material to at least partially set; and Removing broken breakable gel from the wellbore, Wherein the settable foamed material is configured to at least partially set before the breakable gel breaks.

2. A method for reducing solids migration into wellbores according to claim 1, wherein the settable foamed material has a specific gravity of between about 0.5 and 0.9.

3. A method for reducing solids migration into wellbores according to claim 1 or claim 2, wherein the breakable gel has a specific gravity of between about 0.9 and 1.05.

4. A method for reducing solids migration into wellbores according to any one of the preceding claims, wherein a barrier member is located in the wellbore, and wherein the location of the barrier member defines the lowest point within the wellbore to which the breakable gel may extend.

5. A method for reducing solids migration into wellbores according to any one of the preceding claims, wherein the wellbore is a new wellbore.