Downhole seal

Abstract

A method for forming a seal in a well, the method comprising providing a cross-linked super absorbent polymer in a carrier fluid 4. The cross-linked super absorbent polymer in the carrier fluid is dep

Description

TECHNICAL FIELD The present invention relates to a method for forming a seal, in particular, the method concerns forming a seal in a well, as well as a well comprising a seal, and a kit of parts for forming the well seal. BACKGROUND In the oil and gas exploration and production industry, boreholes are drilled through rock formations to gain access to hydrocarbon-bearing formations, to allow the hydrocarbons to be recovered to surface. When boreholes are drilled, for example, for rock and soil sampling, water, oil, gas, or geothermal developments, many different rock formations are encountered. These rock formations each may have different physical characteristics, such as high-permeability. Various operations are performed in the drilled bore, such as for completion of the bore, ready for production. Similarly, once production has commenced, other operations can be performed in the bore. For example, remedial operations may be undertaken, such as to accommodate changes in production fluids or downhole equipment. For example, where production occurs, remedial action may be taken to mitigate flowing water in the production fluid to surface. Accordingly, intervention apparatus may be deployed downhole for such operations. To enable many intervention operations, the bore may be plugged, especially downhole of the location of the intervention operation. A double plug may be employed to mitigate a failure of a single plug. Furthermore, it may be desired to seal the well to form zones, such that an operation may be carried out in one zone, without interfering with operations in another zone. Particularly after production from the bore has been completed, or where the bore has become no longer viable, the bore may be plugged and abandoned. The plugging and abandonment of bores is generally performed to mitigate against the unintended transport of fluid, for example, a liquid, gas or a combination of a liquid and gas, through the bore. For example, where the bore has accessed a pressurised subterranean hydrocarbon reserve, the redundant post-production bore is plugged and abandoned to prevent an undesired release of oil or gas into the surrounding environment, such as an adjacent seabed. It is important to ensure the integrity of the seal or plug which has been formed. The subject matter of at least some examples of the present disclosure may be directed to overcoming, or at least reducing the effects of, one or more of the problems of the prior art, such as may be described above. SUMMARY According to a first aspect of the invention, there is provided a method. The method according to the first aspect of the invention may be for forming a seal in a well. The method may comprise providing a cross-linked super absorbent polymer in a carrier fluid. The method may comprise deploying the cross-linked super absorbent polymer in the carrier fluid to a predetermined location in the well. The method may include the cross-linked super absorbent polymer swelling as a hydrogel to a first expansion volume to form a seal after contact with an activating liquid. Herein, a cross-linked superabsorbent polymer is considered to be a cross-linked homopolymer or copolymer which can absorb liquid, and retain a volume of said liquid equivalent to many times its own volume. For instance, a cross-linked superabsorbent polymer may absorb a volume of liquid up to 400 times the original volume of the cross-linked superabsorbent polymer. Herein, a hydrogel is considered to be a cross-linked superabsorbent polymer which has contacted and, at least partially, absorbed a liquid. The first expansion volume of the cross-linked super absorbent polymer may be of from 2 to 100 times its original volume. Preferably, the first expansion volume of the cross-linked super absorbent polymer is of from 5 to 95 times its original volume, preferably of from 10 to 90 times its original volume, more preferably of from 20 to 80 times its original volume, and even more preferably of from 30 to 70 times its original volume. In some examples, the first expansion mass of the cross-linked super absorbent polymer may be of from 2 to 100 times its original mass, on a dry weight basis. Preferably, the first expansion mass of the cross-linked super absorbent polymer is of from 5 to 95 times its original mass, preferably of from 10 to 90 times its original mass, more preferably of from 20 to 80 times its original mass, and even more preferably of from 30 to 70 times its original mass. Preferably, the first expansion volume is not the maximum volume by which the cross-linked super absorbent polymer can expand. Said another way, at the first expansion volume, the cross-linked super absorbent polymer has the potential for further expansion of volume. In some examples, the first expansion mass is not the maximum mass by which the cross-linked super absorbent polymer can expand. Said another way, at the first expansion mass, the crosslinked super absorbent polymer has the potential for further expansion of mass. In some embodiments, the cross-linked super absorbent polymer swells to the first expansion volume within a period of 2 hours after contact with an activating liquid. Preferably, the crosslinked super absorbent polymer swells to the first expansion volume within 1 hour, and even more preferably, within 30 minutes, after contact with the activating liquid. In examples, the cross-linked super absorbent polymer preferably has a potential expansion volume of up to (and including) 400 times its original volume after contact with the activating liquid. For example, the cross-linked super absorbent polymer may expand in volume from 2 to 400 times its original volume. In examples, the cross-linked super absorbent polymer may have a potential expansion mass of up to (and including) 400 times its original mass after contact with the activating liquid. In some embodiments, the desired amount of swelling of the cross-linked superabsorbent polymer may be achieved by deploying a known volume of the activating liquid, for example, water, after deployment of the cross-linked superabsorbent polymer in the carrier fluid. The volume of activating liquid may be calculated by the skilled person to obtain the desired amount of swelling of the cross-linked superabsorbent polymer. In some examples, the hydrostatic force, for example, hydrostatic pressure, from a column of a fluid, for example, the volume of the activating liquid or another fluid present in the well, above the cross-linked super absorbent polymer in the carrier fluid may limit the expansion of the cross-linked superabsorbent polymer, such that the cross-linked superabsorbent polymer does not expand more than the desired expansion volume, for example, the first expansion volume. In some examples, the cross-linked superabsorbent polymer is deployed as a particulate in the carrier fluid. The area of the particle which is adjacent to the activating liquid may have a greater expansion volume than areas of the particle which are not in direct contact with the activating liquid. In some examples, after swelling to the first expansion volume, the cross-linked super absorbent polymer may undergo further swelling, for example, greater than 2 times its original volume, to 400 times its original volume. After swelling to the first expansion mass, the cross-linked super absorbent polymer has the potential to undergo further swelling, for example, greater than 2 times its original mass, to 400 times its original mass. Preferably, in some examples, after swelling as a hydrogel to the first expansion volume, the cross-linked super absorbent polymer may undergo further swelling to a second expansion volume. The second expansion volume may be of from greater than 2 times the original volume of the cross-linked super absorbent polymer, to 400 times the original volume of the cross-linked super absorbent polymer. For example, the second expansion volume may be preferably of from 10 to 375 times the original volume of the cross-linked super absorbent polymer, preferably of from 30 to 350 times the original volume of the cross-linked super absorbent polymer, more preferably of from 50 to 300 times the original volume of the cross-linked super absorbent polymer, even more preferably of from 100 to 250 times the original volume of the cross-linked super absorbent polymer and even more preferably still of from 150 to 200 times the original volume of the cross-linked super absorbent polymer. In some examples, the cross-linked super absorbent polymer swells as a hydrogel to a second expansion mass, after swelling to the first expansion mass. The second expansion mass may be of from greater than 2 times the original mass of the cross-linked super absorbent polymer to 400 times the original mass of the cross-linked super absorbent polymer. For example, the second expansion mass may be of from 10 to 375 times the original mass of the cross-linked super absorbent polymer, preferably of from 30 to 350 times the original mass of the cross-linked super absorbent polymer, more preferably of from 50 to 300 times the original mass of the cross-linked super absorbent polymer, even more preferably of from 100 to 250 times the original mass of the cross-linked super absorbent polymer and even more preferably still of from 150 to 200 times the original mass of the cross-linked super absorbent polymer. Advantageously, the cross-linked super-absorbent polymer may therefore have “self-healing” properties. In the present context, for example, if the seal formed after the cross-linked super absorbent polymer which has swelled to the first expansion volume is, for instance, damaged during operations performed in the well, for example, cracked, then the super-absorbent polymer swells as a hydrogel to a second expansion volume after contact with an activating liquid. The activating liquid may be the same or different to the activating liquid which first contacted the cross-linked superabsorbent polymer. The second expansion volume is preferably greater than 2 times the original volume of the cross-linked super absorbent polymer, to 400 times the original volume of the cross-linked super absorbent polymer. Therefore, any damage to the seal may be repaired, at least through the super-absorbent polymer further expanding to fill any damaged areas of the seal. The further expansion of the cross-linked super absorbent polymer may be in response to an activating liquid, such as water. The activating liquid may be introduced to the well from an external source, e.g. from the surface, or the activating liquid may already be or become present in the well, such as from the formation, aquifer or underground reservoir. Therefore advantageously, the seal does not need to be wholly replaced if damaged. Advantageously, well integrity can be maintained, for example, quicker, at least in part due to the self-healing properties of the seal. In examples, the cross-linked super absorbent polymer may be deployed in the location of an apparatus in the well. For example, the cross-linked super absorbent polymer in the carrier fluid may be deployed next to, or on, or in, an apparatus. The apparatus may comprise one or more of: a sleeve, patch, tubular, tubular member or cylindrical body. The apparatus may be a liner or casing string. The cross-linked super absorbent polymer may be in an inactive or non-activated state when deployed next to or on, or in, an apparatus. The cross-linked super absorbent polymer may be deployed in an annulus. The cross-linked super absorbent polymer may swell as a hydrogel to a first expansion volume to form an annular seal on contact with an activating liquid. The cross-linked super absorbent polymer may be deployed in a tubular. The cross-linked super absorbent polymer may swell as a hydrogel to a first expansion volume to form a tubular seal on contact with an activating liquid. The cross-linked super absorbent polymer may be applied so as to have regions or zones having different expanded volumes. The cross-linked super absorbent polymer may be deployed along the entire length of an apparatus, e.g. a cylindrical body. The cross-linked super absorbent polymer may be deployed along only a portion of the length of the apparatus. The cross-linked super absorbent polymer may be applied as a plurality of elements, e.g. describing regions or layers having different cross-linked super absorbent polymers, coatings or particles. In embodiments, the viscoelasticity of the seal preferably increases as the swelling of the crosslinked super absorbent polymer increases. Viscoelasticity is a property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. For example, viscous materials resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. In the present context, in examples wherein the swelling of the cross-linked super absorbent polymer increases e.g. to a first expansion volume, the viscoelasticity of the super absorbent polymer also increases. For example, when the cross-linked super absorbent polymer is unswollen, or at least partially unswollen (e.g. the cross-linked super absorbent polymer has not yet reached the first expansion volume), then the cross-linked super absorbent polymer has rubber elasticity properties and behaves as an elastomer. As the cross-linked super absorbent polymer swells as a hydrogel to the first expansion volume, the properties of the cross-linked super absorbent polymer becomes increasingly viscoelastic. At the first expansion volume of the cross-linked super absorbent polymer, the cross-linked super absorbent polymer has viscoelastic properties. Advantageously, as the viscoelasticity properties of the cross-linked super absorbent polymer increases, the seal becomes increasingly flexible and thus the seal can flex and move without splitting or cracking. Advantageously, in examples, the seal according to the present invention is flexible, for example, at least in part due to the viscoelastic properties of the seal. Flexible in the present context means that the seal can deform or is deformable when a stress is applied to the seal, and the seal can return to its original shape once the applied stress is removed. Therefore advantageously, the seal can absorb energy from external stimulus and return to its original shape when the stimulus is removed. For example, the stimulus is an applied stress. Therefore, preferably, the seal provides a damping effect. Therefore, in examples, the seal formed using the method of the present invention, is beneficial over prior art solid-state seals, which are rigid. The seal preferably has a Young’s Modulus of from 0.01 to 10 GPa. Preferably, the Young’s Modulus is of from 0.02 to 9 GPa, preferably of from 0.05 to 8 GPa, and even more preferably of from 1 to 6 GPa. The Young’s modulus may be derived or defined from a stress-strain curve obtained using a tensile test method (e.g. ASTM E111-17). In embodiments, preferably the seal does not comprise a cement, a resin or a metal alloy. In examples, the cross-linked super absorbent polymer preferably comprises a cross-linked polyacrylate super absorbent polymer, or a cross-linked polyacrylamide super absorbent polymer. In some examples, the cross-linked super absorbent polymer comprises a cross-linked polyacrylate super absorbent polymer and a cross-linked polyacrylamide super absorbent polymer. In some examples, the cross-linked super absorbent polymer is a salt of a cross-linked polyacrylate super absorbent polymer, or a salt of a cross-linked polyacrylamide super absorbent polymer. In examples, the cross-linked super absorbent polymer is preferably selected from a cross-linked sodium polyacrylate super absorbent polymer, a cross-linked potassium polyacrylate super absorbent polymer, a cross-linked sodium polyacrylamide super absorbent polymer and a cross-linked potassium polyacrylamide super absorbent polymer. In some examples, the crosslinked superabsorbent polymer may be a homopolymer or a copolymer. For example, the crosslinked superabsorbent polymer may be a copolymer of acrylamide and acrylate. In examples, the cross-linked super absorbent polymer is preferably provided as a particulate in the carrier fluid. For example, the method may comprise deploying the cross-linked super absorbent polymer in particulate form, e.g. comprising of or consisting of cross-linked super absorbent polymer particles. The particulate form may comprise one or more of: granules, pellets, powder / s. The particulate form may comprise an indeterminate or non-defined arrangement or relationship between crosslinked super absorbent polymer particles of the cross-linked super absorbent polymer, at least prior to deployment. The method may comprise not pre-forming the cross-linked super absorbent polymer prior to deployment to a predetermined location in the well. The method may comprise entraining the cross-linked super absorbent polymer particles in a carrier fluid, such as for some deployment methods, such as fluidised application or spraying. Preferably the average particle size of the cross-linked super absorbent polymer in the carrier fluid is of from 50 to 6000 microns. Preferably, the particle size of the cross-linked super absorbent polymer is of from 60 to 4000 microns, preferably of from 70 to 2000 microns, preferably of from 80 to 1000 microns, preferably of from 90 to 800 microns, more preferably still of from 100 to 400 microns. The average particle size of the cross-linked super absorbent polymer is measured according to ASTM-E 11. The cross-linked super absorbent polymer may be deployed in one or more band(s), layer(s) or zone(s), e.g. annular bands. The one or more bands, layers or zones may be arranged along the length, or a portion of the length, of an apparatus. Each band, layer or zone may have a different cross-linked super absorbent polymer. Each band, layer or zone may be configured to swell or expand to different degrees when activated. The bands, layers or zones may be spaced from one another by regions, or further bands, layers or zones free of cross-linked super absorbent polymer. In examples, the carrier fluid preferably comprises a non-water based fluid. Preferably, the carrier fluid comprises a glycol, glycerol, an oil-based fluid, or weighted liquid silicone. Examples of a glycol include monoethylene glycol, propylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol. Examples of an oil-based fluid include diesel, mineral oil and vegetable oil. Preferably, the carrier fluid is a glycol. Preferably, the carrier fluid is monoethylene glycol. Advantageously, it has been found that the combination of the cross-linked super absorbent polymer and a non-water based fluid as carrier fluid provides a seal which has an improved homogeneity compared to prior art seals which do not deploy a cross-linked super absorbent in a non-water based carrier fluid. Without wishing to be bound by theory, it is thought that using a non-water based fluid, for example, a glycol (e.g. monoethylene glycol), can act as a cross-linking agent which can strengthen the bonding between the super-absorbent polymer and the activating agent, such as a water-based fluid. Advantageously, the seal which has improved homogeneity may have improved sealing properties, for example, improved impermeability e.g. to water in the well. In some embodiments, the cross-linked super absorbent polymer is a crosslinked copolymer of acrylamide and potassium acrylate. In some embodiments, the carrier fluid is monoethylene glycol. For example, the cross-linked superabsorbent polymer may be polyacrylamide super absorbent polymer and the carrier fluid is a glycol. For example, the crosslinked super absorbent polymer may be a cross-linked copolymer of acrylamide and potassium acrylate and the carrier fluid may be monoethylene glycol. In embodiments, the activating liquid preferably comprises a water-based liquid. Preferably, the water-based liquid is water, a brine or a mixture thereof. Preferably, the cross-linked super absorbent polymer in the carrier fluid is introduced into the well using a pump, or an intervention tool such as a hose or a bailer tool. In some examples, the cross-linked superabsorbent polymer in the carrier fluid is introduced into the well over a period of 4 hours. Preferably, the cross-linked superabsorbent polymer in the carrier fluid is introduced into the well over a period of 3 hours, more preferably over a period of 2 hours and even more preferably over a period of 1 hour. In some examples, the amount of cross-linked super absorbent polymer deployed in the carrier fluid in a single well is preferably up to 5000 kg. Preferably, the amount of cross-linked super absorbent polymer deployed in the carrier fluid in a single well is preferably up to 1000 kg, more preferably up to 500 kg, and even more preferably up to 250 kg. Typically, in some examples, the amount of cross-linked super absorbent polymer deployed in the carrier fluid in a single well is of from 100 to 250 kg. Preferably, the method further comprises a spacer fluid being introduced into the well before and / or after introduction of the cross-linked superabsorbent polymer in the carrier fluid. In examples, the spacer fluid preferably comprises a non-water based fluid, such as monoethylene glycol, propylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, glycerol,an oil-based fluid or weighted liquid silicone. Thus advantageously, the spacer fluid may wash through any cross-linked super absorbent polymer in carrier fluid that has been deployed but not reached the intended predetermined location. Various tests may be employed to determine that the cross-linked super absorbent polymer in the carrier fluid is in place in the predetermined location in the well. For example, fluid measurements may be conducted to determine the deployment distance of the cross-linked super absorbent polymer in the carrier fluid. Separately, or in addition to fluid measurements, the crosslinked super absorbent polymer may be tagged with a wireline / slickline. In some examples, the cross-linked super absorbent polymer in the carrier fluid may be deployed to a predetermined location in the well with a dart spacer, which advantageously may enable better spacing and tagging. The dart spacer may be designed to disintegrate in time, or swell to provide a seal, or comprises a swellable component which can swell to provide a seal. In embodiments, preferably the activating liquid which contacts the cross-linked super absorbent polymer is already present in the well. For example, the cross-linked super absorbent polymer in the carrier fluid is deployed to a pre-determined location in a well, where it reacts with an activating liquid, e.g. water, that is already present in the well. Therefore the activating liquid, e.g. water, which is already present in the well can be effectively contained by the seal, and mitigated from flowing, or further flowing, to undesired locations in the well. In some embodiments, the activating liquid, for example, water, may be injected into the well prior to deploying the cross-linked super absorbent polymer in the carrier fluid to the predetermined location in the well. The activating liquid may be injected immediately before deploying the crosslinked super absorbent polymer in the carrier fluid. In other embodiments, the activating liquid may be injected into the well and after a period of time (for example, 30 minutes, 1 hour, 2 hours or longer than 2 hours), the cross-linked super absorbent polymer is deployed in the carrier fluid to the predetermined location. In other embodiments, the liquid is introduced into the well after deployment of the cross-linked super absorbent polymer in the carrier fluid to the predetermined location. Therefore advantageously, the cross-linked super absorbent polymer can be accurately deployed to a predetermined location in the well before the well seal is formed. Advantageously, the well seal formed using the method of the present invention stops, at least partially, permeability between zones separated by the seal in a well. Preferably, the well seal formed is an impermeable well seal which separates said zones in a well. For instance, the well seal stops, at least partially, permeability to fluids, such as a gas, a liquid (e.g. water or oil) or a combination of a gas and a liquid, between zones separated by the well. Therefore the seal provides an effective barrier to prevent uncontrolled flow of a fluid, such as water, oil or a gas, in the well. In at least some examples, the seal formed using the method of the present invention is, at least partially, resistant to pressures up to 10,000 psi, preferably up to 7,250 psi and even more preferably up to 5000 psi. In at least some examples, the cross-linked superabsorbent polymer can swell to a first expansion volume to form a seal after contact with an activating liquid, wherein the activating liquid comprises a brine having a concentration of 35 ppt. For example, the brine has a salt, for example, NaCI, concentration of 35 ppt (35000 mg / L). Preferably, the first expansion volume of the crosslinked superabsorbent polymer is up to 100 times its original volume after contact with an activating liquid, wherein the activating liquid comprises a brine having a concentration of 35 ppt. For example, the brine has a NaCI concentration of 35 ppt (35000 mg / L). The average salinity of seawater may be considered to comprise NaCI at 35ppt (35000 mg / L). Advantageously, the cross-linked superabsorbent polymer is able to expand in high salinity environments. A high salinity environment may be considered to be a brine comprising 85 ppt NaCI (85000 mg / L). In some examples, the cross-linked superabsorbent polymer may swell to a first expansion volume which is up to 20 times its original volume after contact with an activating liquid, wherein the activating liquid comprises a brine having a concentration of 85 ppt. For example, the brine has a NaCI concentration of 85 ppt (85000 mg / L). In some examples, the swelling capacity of the cross-linked superabsorbent polymer is resistant to changes in pH. For example, the swelling capacity of the cross-linked superabsorbent polymer is not affected by changes in pH between pH 5 to pH 9 at 25 degrees Celsius. According to a second aspect of the present invention, there is provided a well comprising a seal formed according to the first aspect of the invention. According to a yet further aspect of the invention, there is provided a kit for forming a well seal according to the first aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an example flow chart of an example method; Figure 2 is a schematic cross-sectional side view of a portion of an apparatus according to an example; Figure 3 is a schematic cross-sectional side view of a portion of an apparatus according to another example; Figure 4 is a detail schematic view of a portion of an apparatus according to another example; Figure 5 is a schematic sectional side view of a portion of a well in accordance with an 20 example; Figure 5a shows a detail of the well of Figure 5 in a first configuration; Figure 5b shows the detail of the well of Figure 5 in a second configuration; Figure 6 is a schematic cross-sectional side view of a portion of an apparatus according to another example; Figure 7 is a sectional side view of a portion of a well in accordance with an example of the present invention; Figure 8 is a sectional side view of a portion of a well in accordance with another example of the present invention; Figure 9 is a cross-sectional axial view of the portion of the well of Figure 8; Figure 10 is a cross-sectional axial view of a portion of a well according to a further example; Figure 11a is a cross-sectional axial view of a portion of a well in accordance with a further example; Figure 11 b is a detail view of Figure 11a; Figures 12a and 12b depict an annular seal and a tubular seal according to examples of the present invention; Figure 13 depicts a graph which shows the effect of increasing salinity on the absorption properties of the cross-linked superabsorbent polymer; Figure 14 depicts a graph which shows the effect of increasing pH on the absorption properties of the cross-linked superabsorbent polymer; and Figure 15 depicts a graph which shows the effect of increasing temperature on the absorption properties of the cross-linked superabsorbent polymer. DETAILED DESCRIPTION Referring first to Figure 1, there is shown an example flow chart of a method 2 according to the present disclosure. The method 2 comprises a first step 4 of providing a cross-linked super absorbent polymer in a carrier fluid. For example, the carrier fluid is a non-water based fluid such as monoethylene glycol, propylene glycol, dipropylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, glycerol, an oil-based fluid or weighted liquid silicone. The second step 6 in the method involves deploying the cross-linked super absorbent polymer in the carrier fluid to a predetermined position. The predetermined position is preferably a position in the well where a seal is intended to be formed, such as an annulus, for forming an annular seal. The cross-linked super absorbent polymer in the carrier fluid may be deployed using, for example, a pump or an intervention tool such as a hose or bailer tool. The method 2 includes a further step 8 of activating the cross-linked super absorbent polymer by contacting it with an activating liquid. In at least some examples, the activating liquid is a water-based fluid. The liquid may already be present in the well, or it may be introduced into the well before or once the cross-linked super absorbent polymer is in place. After contact with the activating liquid, the super absorbent polymer then swells to a first expansion volume, such that a seal 9 is provided. For example, the crosslinked super absorbent polymer may swell to close a gap or a space in the well, so as to create a seal. Additionally, or alternatively, the seal may be provided in addition to another seal already present in the well. For example, the cross-linked super absorbent polymer may be provided as an enhancement or a back-up for another seal. Referring now to Figure 2, there is shown a schematic cross-sectional side view of a portion of an apparatus 10. Here, the apparatus 10 comprises a cylindrical body 14, the cylindrical body 14 having a central axis 16. In at least some examples, the apparatus 10 is a sleeve or a tubular in an oil / gas wellbore. The cylindrical body 14 describes a central passage 140, e.g. for conveying fluid. The cross-linked super absorbent polymer 12 is deployed in the well in a carrier fluid along the entire axial length of the cylindrical body 14, in particular, to an outer surface 141 thereof. The cross-linked super absorbent polymer 12 is shown in an inactive or non-activated state in this example, e.g. the cross-linked super absorbent polymer has not yet come into contact with an activating liquid. The cross-linked super absorbent polymer 12 may be provided, e.g. mixed, in the carrier fluid at surface, prior to deployment to a predetermined location in the well. The cross-linked super absorbent polymer 12 may be activated once in contact with, in the presence of or in response to an activating liquid, once deployed. In the present example, once activated, the cross-linked super absorbent polymer 12 will expand radially, away from the central axis 16 cylindrical body 14. It will be appreciated that in the case of apparatus 10, the cross-linked super absorbent polymer 12 is intended to seal, when activated, against a body or member located externally of the apparatus 10. The super absorbent polymer 12 absorbs the activating liquid on contact, such that the super absorbent polymer 12 expands against the externally located body or member. The cross-linked super absorbent polymer 12 is intended to seal, when activated, against a body or member having a larger diameter than that of apparatus 10, e.g., the formation, a wellbore or casing string. Referring now to Figure 3, there is shown a schematic cross-sectional side view of a portion of another apparatus 110 according to another example. The apparatus 110 is generally similar to that 10 shown in Figure 2, with like features denoted by like reference numerals, incremented by 100. Accordingly, the apparatus 110 comprises a cylindrical body 114 having a central axis 116. Here, in contrast to Figure 2, the cross-linked super absorbent polymer 112 in the carrier fluid 112 is deployed and positioned on a portion of the axial length of the apparatus 110. Furthermore, rather than being located on an external surface 114a, the cross-linked super absorbent polymer 112 here is positioned internally of apparatus 110, on an inner wall 114b of the cylindrical body 114. It should be appreciated, however, that the cross-linked super absorbent polymer need not be positioned on the inner wall 114b of apparatus 110, and may be deployed internally of apparatus 110, for example, as a layer, or otherwise, in the cylindrical body 114. As is the case in Figure 2, the cross-linked super absorbent polymer 112 is shown in an inactive or non-activated state in this example. It will be appreciated that in this example, the apparatus 110 is a sleeve or a tubular for deployment downhole in an oil / gas wellbore. The cross-linked super absorbent polymer 112 may be provided in the carrier fluid at surface e.g. mixed, prior to deployment in the well. It should be appreciated, however, that in other examples, the cross-linked super absorbent polymer 112 is provided in the carrier fluid downhole. For example, the carrier fluid and the cross-linked superabsorbent polymer 112 may be provided separately and then mixed downhole, prior to being deployed to the predetermined position in the well. The cross-linked super absorbent polymer 112 may be activated after contact with, once in contact with, in the presence of or in response to an activating liquid, once deployed. In the present example, once activated, the cross-linked super absorbent polymer 112 will expand radially, away from the cylindrical body 114 towards the central axis 116. It will be appreciated in the case of apparatus 110, contrary to the arrangement of Figure 2, the cross-linked super absorbent polymer 112 is intended to seal, when activated, against a body or member located internally of the apparatus 110. The cross-linked super absorbent polymer 112 is intended to seal, when activated, against a body or member having a smaller diameter than that of apparatus 110, e.g. a casing string or liner. It should be understood that the cross-linked super absorbent polymer 112 need not seal against a body or member located internally of the apparatus 110, but may be intended to form a seal across the entire width of cylindrical body 114 e.g. from the inner wall 114b to the opposing inner wall 114b. Referring now to Figure 4, there is shown a schematic cross-sectional sideview of a portion of an apparatus according to another example. In this example, the cross-linked super absorbent polymer 212 is deployed and positioned as a layer under a primary seal 218. It should be appreciated, however, that cross-linked superabsorbent polymer 212 does not need to take the configuration as a layer. The cross-linked superabsorbent polymer is configured to act as a “backup” seal, or a “secondary” seal for the primary seal 218. For instance, in some examples, should any damage to primary seal 218 occur, for example, a crack occurring in primary seal 218 leading to unwanted fluid communication between zones in the well, then the cross-linked super absorbent polymer 212 is configured to activate on contact with that fluid, such that the back-up seal forms when damage occurs to the primary seal 218. In other examples, the cross-linked superabsorbent polymer is deployed on top of to the primary seal 218 and is activated by contacting with an activating liquid, prior to any damage to seal 218 occurring (e.g. an intentional seal is formed by manually contacting the cross-linked superabsorbent polymer with an activating liquid), thus the cross-linked superabsorbent polymer 212 forms a back-up seal as a precaution to any damage occurring. The primary seal 218 may be formed of a cross-linked superabsorbent polymer, or another material, such as cement. Figure 5 is a schematic sectional side view of a portion of a well in accordance with an example. The apparatus 310 here comprises a casing or tubing patch, such as for bridging a damaged section of tubing or casing. The damaged section of tubing or casing may have been removed (as shown in Figure 5), such as by milling, drilling or the like, leaving an upper tubing or casing section 320 and a lower tubing or casing section 322 separated by an open section 324. Alternatively, the damaged section of tubing or casing may remain in situ, with the patch 310 being located radially internally thereof. Here, the apparatus 310 comprises a cylindrical body 314. The cylindrical body 314 has a central axis 316 that coincides with the central axes of upper and lower sections 320, 322. In the present example, the cylindrical body 314 has a smaller diameter than the upper and lower sections 320, 322. A portion at either end of the cylindrical body 314 overlaps with a respective upper and lower section 320, 322 by a distance L. Together, the cylindrical body 314, upper section 320 and lower section 322 define a conduit C for conveying fluid. The cylindrical body 314 describes a similar arrangement to that of Figure 2. The cross-linked super absorbent polymer 312 is deployed in a carrier fluid along the entire axial length of the cylindrical body 314, in particular, to an outer surface 314a thereof. The cross-linked super absorbent polymer 312 is in an inactive or non-activated state in this example. As shown in greater detail in Figure 5a, the well of Figure 5 is in a first configuration, with the cross-linked super absorbent polymer 312 in an inactive or non-activated state. In this state, a gap or space G is described between the outer surface 314a of the cylindrical body 314, and an internal surface 320a of the upper section 320. It will be appreciated that in the present example, the gap or space G is annular, and present between the cylindrical body 314 and both the upper section 320 and lower section 322. In the present configuration, with the cross-linked super absorbent polymer 312 in an inactive or non-activated state, the conduit C is not fluid tight, and therefore is not suitable for conveying fluid. Once the cylindrical body 314 is correctly positioned, the cross-linked super absorbent polymer 312 can be activated to as to seal against an internal surface of each of the upper section 320 and lower section 322. The seal is configured to provide a fluid tight conduit C. An activating liquid is brought into contact with the cross-linked super absorbent polymer 312, which subsequently expands away from the central axis 316 towards the internal surface of each of the upper section 320 and lower section 322. It should be appreciated that the activating liquid, e.g. water, may already present in the well. Figure 5b shows the detail of the well of Figure 5 in a second configuration, with the cross-linked super absorbent polymer 312 in an activated state. For instance, the cross-linked super absorbent polymer 312 has been activated after contact with the activating liquid and the cross-linked super absorbent polymer 312 has expanded to a first expansion volume. In this case, the cross-linked super absorbent polymer 312 is in contact with, and seals against the internal surface 320a of the upper section 320. It will be appreciated that the cross-linked super absorbent polymer 312 will also contact, and seal against, the internal surface of the lower section 322, in a similar manner. Referring now to Figure 6, there is shown a schematic cross-sectional sideview of a portion of an apparatus 410 according to another example. The apparatus 410 is generally similar to apparatus 310 shown in Figure 5, with like features denoted by like reference numerals, incremented by 100. Accordingly, the apparatus 410 comprises a cylindrical body 414 having a central axis 416 and defining a substrate for a cross-linked super absorbent polymer 412. The apparatus 410 shown in Figure 6 comprises a screwthread portion 418, configured to engage a cooperating screwthread portion 422 of a tubular member 420. The tubular member 420 may be a tubing, casing or liner section, e.g. having a threaded free-end. The apparatus 410 may comprise a corresponding tubing, casing or liner section. The apparatus 410 and tubular member 420 are connected via the cooperating screwthread portions 418, 422. In examples, the apparatus 410 may comprise or be a valve or control mechanism, configured to be connected with a tubular member 420. The cross-linked super absorbent polymer 412 can be deployed in the carrier fluid to the location of at least one of the components of the screwthread 418, 422 (e.g. male and / or female) prior to mating of the screwthreads 418, 422. Additionally, or alternatively, the cross-linked super absorbent polymer 412 may be deployed in the carrier fluid to the location of at least one of the components of the screwthread 418, 422 during and / or after the mating or engaging of the screwthreads 418, 422. The cross-linked super absorbent polymer 412 may be deployed by injecting into the region between the screwthreads 418, 422 using a pump. In the present example, the cross-linked super absorbent polymer 412 is depicted as being located between the screwthreads 418, 422, and in contact with each. The cross-linked super absorbent polymer 412 is in an inactive or non-activated state when deployed to one or each of screwthreads 418, 422. The cross-linked super absorbent polymer 412, in the present example, may be activated either prior to, or after, locating the apparatus 410 and tubular member 420, e.g. in a downhole location. The cross-linked super absorbent polymer 412 may be activated by applying an activating liquid thereto. Once activated, the cross-linked super absorbent polymer 412 will expand between the screwthreads 418, 422, e.g. so as to provide a fluid tight seal at the connection between the apparatus 410 and tubular member 420. Referring now to Figure 7 the cross-linked superabsorbent polymer is shown in an activated form. For example, the cross-linked superabsorbent polymer 512 has swelled to a first expansion volume and is at least partially swollen. Figure 7 shows a portion of a well 501 in accordance with an example. Here, the well 501 comprises an apparatus 510 having a cylindrical body 514 and a central passage 640. In at least the present example, the apparatus 510 is a tubular. A crosslinked superabsorbent polymer 512 in a carrier fluid is deployed along the central passage 640 to a predetermined location. The cross-linked superabsorbent polymer 512 may be deployed using a method which would be familiar to the skilled person, such as a pump. It will be appreciated that in this example, the cross-linked superabsorbent polymer 512 has been applied remotely by pumping from an uphole location, such as from a wellhead 516, without tool-string apparatus. Once the cross-linked superabsorbent polymer 512 is deployed at the predetermined location in the tubular, it is brought into contact with an activating liquid, for example, water. It should be appreciated that the activating liquid may already be present in the well, for example, inherently present in the well, or it may be injected into the well, for example, before and / or after the crosslinked superabsorbent polymer 512 is deployed. Once contacted with the activating liquid, the cross-linked superabsorbent polymer 512 swells as a hydrogel to a first expansion volume to form a seal 532. In the present example, the seal 532 thus forms a tubular seal. Figure 7 therefore depicts the cross-linked superabsorbent polymer 512 in an activated form e.g. after the crosslinked superabsorbent polymer has come into contact with an activating liquid and is, at least partially, swollen. The cross-linked superabsorbent polymer has the potential for further expansion in the present example, for example, the cross-linked superabsorbent polymer may swell to a second expansion volume, after having swelled to the first expansion volume. For instance, in the present example should the tubular seal become damaged, e.g. cracked, then the cross-linked superabsorbent polymer may swell to a second expansion volume on contact with an activating liquid, to fill the crack and thus repair the seal. Turning now to Figure 8, the method comprises sealing-in the bore 630, such as to prevent passage of fluid / s into and / or out of the bore, as shown here plugging the bore. Here, the seal 632 comprises a cross-linked super absorbent polymer which has swelled to a first expansion volume on contact with a first activating liquid. The method therefore comprised at least partially swelling the cross-linked super absorbent polymer to a first expansion volume, such as to form the seal 632. Accordingly, the seal 632 formed by the cross-linked super absorbent polymer comprises a cross-linked super absorbent polymer that is capable of subsequent swelling to a second expansion volume after the initial seal 632 has been formed. The seal 632 may be considered to comprise an adaptive seal, such as capable of adapting to changes such as environmental or conditional changes over time. For example, the seal 632 may be configured to swell in the presence of a second activating liquid (which may be the same or different to the first activating liquid) such as water, so as to re-seal or increase sealing in contact with the activating liquid, such as may be associated with changes in the bore subsequent to sealing. By way of example, where downhole pressure changes, such as in a reservoir (not shown) associated with the bore, establish a fluid path; or where cracks or changes in an associated formation 618 or section of sealed bore, the seal 632 may adapt to re-seal such as by swelling (e.g. to fill the crack / s I block the fluid path). Here, the cross-linked super absorbent polymer comprises an abandonment cross-linked super absorbent polymer, at least after or upon completion of the sealing with the cross-linked super absorbent polymer to abandon the well 601. As shown here, the method comprised applying the cross-linked super absorbent polymer after the completion of the bore. The method comprised applying the cross-linked super absorbent polymer after termination of production from the production bore. In other examples, the method comprises applying the cross-linked super absorbent polymer before completion of the bore, such as to seal, and optionally abandon, a partial bore. For example, other methods comprise applying the cross-linked super absorbent polymer to one or more of: a blind bore; a test bore; a sidebranch bore; a deviated bore; a main bore; an abandoned bore; an exploration bore; a collapsed bore; a damaged bore. It will be appreciated that the cross-linked super absorbent polymer as applied in Figure 8 provides a seal 632 with an inner seal portion 632a within the central passage 740 of the bore 630 and also an outer seal portion 632b in an annular passage 626 of the bore 630 defined between an apparatus 10, for example, casing, and the formation 618 (the borewall). Accordingly all fluid flow along the bore is blocked by the seal 632, as shown in Figure 9, which is a cross-sectional portion of the well 610 of Figure 8, where the bore comprises the seal. Referring to Figure 10, there is shown a further example cross-sectional portion of a well 701. The well 701 shown in Figure 10 is generally similar to that shown in Figure 9, with similar features referenced by similar reference numerals incremented by 100, not all of which are repeatedly recited in this passage of the description for brevity. Accordingly, the well 701 comprises a bore 730 and a swellable seal 732. It will be appreciated that the swellable seal 732 of Figure 10 has been formed using a similar method to that for Figure 8 and 9, as described above. However, rather than providing an outer seal portion 732b in an annular passage between the casing 710 and the formation 718, the outer portion 732b here is provided in an annular passage 726 between the casing 710 and an outer casing 736. It will be appreciated that the outer casing 736 was installed prior to the inner casing 710 and sealed to the formation 718 with a cementing operation in the outer annulus 745 prior to production. Referring now to Figures 11a and 11b, there is shown a further example portion of a well 801. The well 801 comprises a bore 830 and a seal 832. It will be appreciated that the seal 832 of Figures 11a and 11b has been formed by a method according to the present invention. For example, a cross-linked superabsorbent polymer has been deployed to a predetermined location in a well, and the cross-linked super absorbent polymer swells to a first expansion volume on contact with an activating liquid. In the present example, the method comprises providing the seal 832 formed by an at least partially swollen cross-linked superabsorbent polymer, that is further swellable to a second expansion volume, so as to be self-sealing (e.g. to fill cracks, microannuli, or voids). The method comprises the provision of swellable particles sized and proportioned so as to penetrate cracks and microannuli or other potential flowpaths for fluid, such as hydrocarbon fluid. The method here comprises providing a range of sizes of swellable. In the present example, the method comprises the provision of particles with at least a portion with an average particle size of 100 to 400 microns, which can penetrate and expand in microfractures. It should be appreciated, however, that other particle sizes of the cross-linked super absorbent polymer are possible. For instance, in other examples, the average particle size of the cross-linked super absorbent polymer is 1000 to 6000 microns. In this example, the cross-linked super absorbent polymer can therefore be used in fracture sealing operations. As can be seen from the detailed view of Figure 11b, the cross-linked super absorbent polymer particles are configured to penetrate into such small recesses that may otherwise provide a leakpath. For example, an unwanted leakpath may be provided by a crack 862 formed in the seal 832. The cross-linked super absorbent polymer can “self-heal” as the cross-linked superabsorbent polymer has the potential to expand to at least a second expansion volume. For example, on contact with an activating liquid, such as the first activating liquid or another liquid in the well, the cross-linked super absorbent polymer can swell into the volume of crack 862, which is otherwise suitable for hydrocarbon fluid passage or migration. Thus, integrity of the seal is maintained. Turning now to Figures 12a and 12b, there is depicted seals formed according to the method of the present invention. Figure 12a depicts an annular seal 950. For instance, in this example, a cross-linked super absorbent polymer was provided in a carrier fluid and deployed on an external surface of a tubular, for example, between the tubular and formation. The cross-linked super absorbent polymer was activated using an activating liquid, in this case water. It should be appreciated that the activating liquid, for example, water, can come from an external source e.g. manually introduced into the well, or may already be present in the well. The cross-linked super absorbent polymer swelled to a first expansion volume on contact with the water. An annular seal 950 was thus formed. The annular seal is impermeable to water and therefore provides an effective barrier against unwanted fluid communication in an annulus. Figure 12b depicts an example of a seal formed according to the present invention. In this example, the cross-linked super absorbent polymer was provided in a carrier fluid, for example, monoethylene glycol, and deployed to a predetermined location in the well. The cross-linked super absorbent polymer in the carrier fluid was deployed in a tubular present in the well. On contact with an activating liquid, for instance, water, the super-absorbent polymer expanded to a first expansion volume. The first expansion volume was such that the expanded / activated crosslinked super absorbent polymer expanded against the internal surface of the tubular. Thus a tubular seal 952 was formed. EXAMPLES Example 1 - Effect of salinity on absorbency A sample of the cross-linked superabsorbent polymer, in the present example, poly(acrylamide-co-acrylic acid) potassium salt was weighed and then placed in a known volume of brine which had a concentration of NaCI at 35 ppt (35000 mg / L). The known volume of water was greater than 400 times the volume of polymer to ensure maximum swelling could occur. The absorbency of the cross-linked superabsorbent polymer was evaluated by measuring the weight of the cross-linked super absorbent polymer over a defined time period, and comparing it with the starting weight of the cross-linked super absorbent polymer. It was found that when the cross-linked superabsorbent polymer was placed in the 35 ppt NaCI brine solution, the cross-linked superabsorbent polymer readily swelled to around 50 times its initial weight. To assess the effect of increasing salinity on the absorption properties of the cross-linked super absorbent polymer, a further sample of the cross-linked superabsorbent polymer was weighed and then placed in a known volume of brine which had a concentration of NaCI at 85 ppt (85000 mg / L). It was found that when the cross-linked superabsorbent polymer was placed in the brine solution having a higher concentration of NaCI, the cross-linked superabsorbent polymer had a reduced ability to swell compared to when it was placed in a lower salinity solution. However, the cross-linked superabsorbent polymer was still able to swell up to around 10 times its initial starting weight. These results are shown in the graph in Figure 13. Therefore, it was found that the cross-linked superabsorbent polymer was able to swell up to 50 times its initial weight when dissolved in a brine solution correspondent to the salt concentration that would be expected in seawater. Accordingly, the cross-linked superabsorbent polymer is suitable for use as a seal in environments where seawater may be present. Example 2 - Effect of pH changes on absorbency Three samples of a cross-linked superabsorbent polymer (poly(acrylamide-co-acrylic acid) potassium salt) were prepared and each was placed in a known volume of water. The known volume of water was greater than 400 times the volume of polymer to ensure maximum swelling could occur. The pH of the water in which each sample was placed was increased over a period of 2 hours, 4 hours and 6 hours respectively. It can be seen from the results in Fig. 14 that the swelling capacity of the cross-linked superabsorbent polymer shows no significant reduction between pH5 to pH9 at 25 degrees Celsius. Beyond this range, the swelling ratio reduces, but even at pH 12, the cross-linked superabsorbent polymer can still swell up to 200 times its original mass. Example 3 - Effect of temperature on absorbency The absorbency of the cross-linked superabsorbent polymer (poly(acrylamide-co-acrylic acid) potassium salt) was evaluated at elevated temperatures. The results of this testing is shown in Figure 15. Three samples of the cross-linked superabsorbent polymer of known mass (10 g) were placed in a known volume of water (4500 ml). The temperature of each sample was increased over 2, 4 and 6 hours, respectively, and a weight measurement was taken at 25, 50, 75 and 100 degrees Celsius for each sample. The maximum swelling achieved by the samples of the cross-linked superabsorbent polymer was evaluated by determining the change in mass of the samples at the elevated temperatures. Over time and at increased temperatures, the samples free expansion reduced to around 200 times the size and weight. Therefore it can be concluded from these results that the swelling capacity of the cross-linked superabsorbent polymer is not significantly affected by elevated temperatures, such as those temperatures which may be experienced downhole. It will be appreciated that any of the aforementioned devices may have other functions in addition to the mentioned functions, and that these functions may be performed by the same device. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present disclosure may consist of any such individual feature or combination of features. It should be understood that the embodiments described herein are merely exemplary and that various modifications may be made thereto without departing from the scope of the disclosure. For example, it will be appreciated that although shown here as a bore with a vertical orientation, other bores may have other orientations. For example, other example bores may have at least non-vertical portions, such as deviated or horizontal sections or bores.

Claims

1. A method for forming a seal in a well, the method comprising:- providing a cross-linked super absorbent polymer in a carrier fluid,- deploying the cross-linked super absorbent polymer in the carrier fluid to a predetermined location in the well, wherein the cross-linked super absorbent polymer swells as a hydrogel to a first expansion volume to form a seal after contact with an activating liquid.

2. A method as claimed in claim 1, wherein the first expansion volume of the cross-linked super absorbent polymer is of from 2 to 100 times its original volume.

3. A method as claimed in any preceding claim, wherein the cross-linked super absorbent polymer has a potential expansion volume of up to 400 times its original volume on contact with the liquid.

4. A method as claimed in any preceding claim, wherein the viscoelasticity of the seal increases as the swelling of the cross-linked super absorbent polymer increases.

5. A method as claimed in any preceding claim, wherein the seal provides a damping effect.

6. A method as claimed in any preceding claim, wherein the seal has a Young's Modulus offrom 0.01 to 10 GPa.

7. A method as claimed in any preceding claim, wherein the seal does not comprise a cement, a resin or a metal alloy.

8. A method as claimed in any preceding claim, wherein the cross-linked super absorbent polymer comprises a cross-linked polyacrylate super absorbent polymer, or a cross-linked polyacrylamide super absorbent polymer.

9. A method as claimed in claim 8, wherein the cross-linked super absorbent polymer comprises a cross-linked sodium polyacrylate super absorbent polymer, a cross-linked potassium polyacrylate super absorbent polymer, a cross-linked sodium polyacrylamide super absorbent polymer, or a cross-linked potassium polyacrylamide super absorbent polymer.

10. A method as claimed any preceding claim, wherein the cross-linked super absorbent polymer is provided as a particulate in the carrier fluid.

11. A method as claimed in claim 10, wherein the average particle size of the cross-linked super absorbent polymer in the carrier fluid is of from 50 to 6000 microns.

12. A method as claimed in any preceding claim, wherein the carrier fluid comprises a nonwater based fluid.

13. A method as claimed in any preceding claim, wherein the carrier fluid comprises monoethylene glycol, propylene glycol, dipropylene glycol or triethylene glycol, polyethylene glycol, glycerol or an oil based fluid.

14. A method according to any preceding claim, wherein the activating liquid comprises a water-based liquid.

15. A method according to any preceding claim, wherein the water-based liquid is water, a brine or a mixture thereof.

16. A method as claimed in any preceding claim, wherein the cross-linked super absorbent polymer in the carrier fluid is introduced into the well using a pump, or an intervention tool such as a hose or a bailer tool.

17. A method as claimed in any preceding claim, wherein the method further comprises a spacer fluid being introduced into the well before and / or after introduction of the crosslinked super absorbent polymer in the carrier fluid.

18. A method as claimed in claim 17, wherein the spacer fluid is a glycol.

19. A method as claimed in any preceding claim, wherein the activating liquid which contactsthe cross-linked super absorbent polymer is already present in the well.

20. A method as claimed in any one of claims 1 to 19, wherein the activating liquid is introduced into the well after deployment of the cross-linked super absorbent polymer in the carrier fluid to the predetermined location.

21. A well comprising a seal formed according to the method of any preceding claim.

22. A kit for forming a well seal according to any one of claims 1 to 21.

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