WO2015032723A1

WO2015032723A1 - Provision and uses of wettability modifiers

Info

Publication number: WO2015032723A1
Application number: PCT/EP2014/068532
Authority: WO
Inventors: Kristian Mogensen; Martin BENNETZEN
Original assignee: Maersk Olie Og Gas A/S
Priority date: 2013-09-04
Filing date: 2014-09-01
Publication date: 2015-03-12
Also published as: GB201315743D0; DK201570245A1

Abstract

The present invention relates to an improved method of oil recovery using aptamers which are designed to adhere or bind to subterranean surfaces, such as hydrocarbon-bearing rock surfaces. There is also provided a method of obtaining suitable aptamers for use in a method of hydrocarbon, such as oil recovery.

Description

Provision and uses of wettability modifiers

Field of the invention

The present invention relates to an improved method of oil recovery using aptamers which are designed to adhere or bind to subterranean surfaces, such as hydrocarbon-bearing rock surfaces. There is aiso provided a method of obtaining suitable aptamers for use in a method of hydrocarbon, such as oii recovery.

Background to the invention

Water flooding as an oil recovery technique has been in use since 1890 when operators in the US realised that water entering the productive reservoir formation was stimulating production, in some cases, water is supplied from an adjacent connected aquifer to push the oil towards the producing wells, in situations where there is no aquifer support, water must be pumped into the reservoir through dedicated injection wel!s. The water phase replaces the oil and gas in the reservoir and thereby serves to maintain pressure. Recovery factors from water flooding vary from 1-2% in heavy oil reservoirs up to 50% with typically vaiues around 30-35%, compared to 5-10% obtained from primary depletion without water flooding. Techniques designed to modify the wettability of rock within the reservoir have been sought. Wettability refers to the tendency of a fluid to preferentially cover the surface of a solid, in this case the reservoir rock, and is often estimated by measuring the contact angle. If a water droplet has a contact angle below 90°, the rock is said to be water-wet. On the other hand, if oil preferentially wets the rock surface, the rock is referred to as oil-wet. Many rocks, however, contain some surfaces that are water- wet and others that are oil-wet; such rocks are called mixed-wet. When a fluid such as water wets the rock surface, it forms a film layer, which maintains hydraulic connectivity throughout the rock. In a mixed-wet system, both oil and water can maintain hydraulic continuity at the same time over a large saturation span. Wettabiiity is thought to be one of the most important factors governing oil recovery (Morrow, N.R., "Wettability and Its Effect on Oil Recovery", JPT (Dec. 1990), 1476- 1484.). A large amount of experimental evidence suggests that oil recovery increases as the contact angle approaches 90°, a situation called neutral wettability, where neither of the fluids tends to wet the rock. Research efforts aimed at altering wettability to improve oil recovery are known.

Current methods for altering reservoir rock wettability from strongly oil-wet or strongly water-wet towards neutral-wet have so far all been water-based. Since wettability reflects the interplay between fluids and rock surface, it is important to realise that methods, which have been shown to work for sandstones may not reveal a similar potential in carbonate rocks and vice versa. Caustic Flooding (Effect of pH): Caustic flooding has been reported to change the wettability as a change in pH wiil affect the surface charge of the rock and hence alters the force balance at the fluid-rock interface (Hirasaki, G.J., Miller, C.A. and Puerto, M., "Recent Advances in Surfactant EOR", SPE 1 15386 (2008). Surfactant Flooding: Several researchers have reported wettability alteration effects with anionic as well as non-ionic surfactants for both sandstone and carbonate rocks. One such study was reported by Mohan et al., (Mohan, K., Gupta, R. and Mohanty, K.K., "Surfactant Flooding for a Maersk Carbonate Reservoir", UT Austin Final Report 2009). The mechanism of action is thought to be removal of adsorbed naphthenic acids through ion-pairing with the surfactant.

Low-Salinity Flooding in Sandstones: Low-salinity effects were reported back in the 1960's but it was the work by Tang and Morrow (Tang, G.Q. and Morrow, N.R., "Salinity, Temperature, Oil Composition, and Oil Recovery by Waterflooding", SPE 36680, 1997), which kick-started research to understand the physical mechanisms behind Sow-salinity effects. BP has demonstrated fieid examples that show improved oil recovery from their lo-sal™ recovery technique.

Smart Water in Chalk: Work by Professor Tor Austad's group at University of Stavanger has shown that potential determining ions like calcium and sulphate influence the surface charge of some North Sea chalk samples (Zhang, P. and Austad, T., "Wettability and oil recovery from carbonates: Effects of temperature and potential dwetermining ions", Colloids and Surfaces A: Physicochem. Eng. Aspects 279 (2006), 179-187). An alternative explanation was proposed by Hiorth et a!. (Hiorth, A., Cathles, L.M., Madland, M.V., "The Impact of Pore Water Chemistry on Carbonate Surface Charge and Oil Wettability", Transport in Porous Media 2010), who claimed that rock dissolution was responsible for the improved oil recovery.

Modified Seawater Flooding in Carbonates: Recent publications by Saudi-Aramco and ExxonMobil (Yousef, A.A., A!-Saieh, S., and A!-Jawfi, M. (2011), "New Recovery Method for Carbonate Reservoirs through Tuning the Injection Water Salinity: Smart Waterflooding", SPE 143550 (2011) and Gupta, R. et al., "Enhanced Waterfiood for Middle East Carbonate Cores - Impact of Injection Water Composition", SPE 142668 (201 1) indicate that a modified seawater composition may lead to wettability alteration in certain limestone and dolomite rocks. Exxon concluded that phosphate should be added to the injection water Although a number of techniques have been described, there is a desire to provide an improved method of modifying rock surface wettability. Indeed, there is a need to provide a method which may find application in a variety of situations. Summary of the Invention

The present invention is based on the development of apiamers which are designed to adhere or bind to surfaces such as rock surfaces so as to preferentially reduce the ability of hydrocarbons, such as oil (including crude oil), to wet the surface. By reducing the ability of hydrocarbons/oil to wet a surface, for example a rock surface, oil recovery from a particular hydrocarbon-bearing reservoir or rock formation may be enhanced or made easier. As described in the background to the present invention, it may be desirable to render the surface or rock surface more neutral in terms of its wettability to water and oil. in a first aspect there is provided a method of selectively modifying wettability of a surface or substrate, the method comprising providing one or more aptamer molecules to the surface or substrate, wherein said aptamer molecule(s) is/are designed to adhere to the surface or substrate, in order to modify said wettability of the surface or substrate.

The surface or substrate may comprise a subterranean surface or substrate, for example a rock surface, as may be found in a hydrocarbon-bearing reservoir and/or rock formation. Hereinafter, the term "substrate" shall encompass both the terms "surface" and "substrate".

Desirably, the said aptamer molecu!e(s) are intended to modify the wettability of the (subterranean) substrate to make the substrate closer to neutral wettability to a hydrocarbon, such as oil and/or water, than before the aptamer(s) is added. The skilled reader can easily measure and test for the wettability of a substrate.

Wettability may be measured, for example, according to the Amott-Harvey imbibition test, as described in Amott E. Transactions AI E 216 ( 959): 56 - 162 and/or a US Bureau of Mines (USBM) test (Donaldson et al., (1969) Society of Petroleum Engineers Journal, p13-20). Wettability may also be measured from tracer tests where chromatographic separation of unreactive thiocyanide that do not adhere to a rock surface and sulphate (or another tracer) that bind to water-wet area of a rock is a proxy for water-wetness of the rock system (see Strand, S., Standnes, D.C. and Austad, T., 2006b. New wettability test for chalk based on chromatographic separation of SCN^" and S0₄ ² Journal of Petroleum Science and Engineering, 52: 187-197.

It will be appreciated that generally a large quantity of aptamer molecules may be used and the amount used will depend on the surface area (namely - the total target surface area) of the surfaces of the reservoir and/or rock formation. Furthermore, the actual or most appropriate amount of aptamer will also depend on the chemistry/physical properties of the surface(s) of the reservoir/formation (charge/porosity and the like). For example, sufficient aptamers to provide at least about 1 , 10, 100, 10³, 10⁴, 10⁵, 10⁶, 10⁷, 10^8"15 aptamer molecules per cm²/surface (target surface area) may be used.

The aptamer molecules may be identical or different so as to take account of different surfaces which may be present in a particular formation. For example, in a particular subterranean location or formation, many different substrate surfaces, each with different chemistries and/or physical features, may be found. As such the present invention may provide for a population or pool of aptamer molecules designed to bind to such a variety of surfaces. Thus, unlike other known applications in which extremely specific aptamers are developed and employed, the present invention may use a population or pool of aptamers of differing affinities to a particular substrate surface. Thus, a population of aptamers which are able to bind slightly differing surfaces may be provided. Such a population of aptamers may allow for minor or natural alterations in the surface of the substrate which may be observed and different aptamers within the population of aptamers will be capable of binding to different locations on the surface of the substrate. In contrast, a single very specific aptamer may not be capable of binding a surface displaying a variety of surface chemistries and/or physical attributes.

The terms "substrate" and/or "surface" as used herein may encompass any type or form of inorganic and/or organic surface. In use a particular hydrocarbon reservoir may be formed and/or comprise a variety of rock surfaces/structures and/or types/compositions of rock e.g. sandstone and carbonate. Thus, the aptamers of the present invention may be selected on the basis of a particular hydrocarbon reservoir from where it is desirable to be able to recover particular hydrocarbon(s), such as oil. One or more samples of rock from the reservoir may be obtained in order to identify said one or more aptamers which are designed to adhere to the rock surfaces found within the particular reservoir. Alternatively, representative aptamers may be obtained which bind to a particular type of rock. Such representative aptamers may be used if a reservoir is known to be formed from a particular rock type. In this manner previously obtained aptamers may be kept for use in reservoir rock formations of a similar rock type. For example, panels of aptamers may be developed based on types of different rock surfaces and stored for use at a later date. Once the type of rock in a particular reservoir is known, the skilled person may select a desired aptamer, or population of aptamers from a panel of aptamers previously made.

In use, the aptamers of the present invention are intended to adhere and/or bind to the surface of rock found within a hydrocarbon bearing reservoir in preference to the hydrocarbon itself. In this manner, the aptamers are intended to modify the ability of the hydrocarbon, such as oil, to wet the surface of the rock. Without wishing to be bound by theory, it is expected that if a rock surface becomes more neutral in terms of its ability to be water wet and/or oil wet, there is a reduction in the degree of oil binding to the rock surface and more hydrocarbon (e.g. oii) may be available for recovery from the reservoir. The one or more aptamers of the present invention may be added/provided to the reservoir by way of a fluid comprising said one or more aptamers. Said one or more aptamers may be allowed to adhere/bind to the rock surface for a period of time, before any hydrocarbon recovery is commenced. Alternatively, said one or more aptamers may be provided to the reservoir by a displacement substance which is intended to facilitate recovery of any hydrocarbon from the reservoir.

Thus, in a further aspect the present invention provides a fluid or displacement substance, such as a flood fluid for use in hydrocarbon, such as (crude) oil recovery, the displacement substance comprising one of more aptamers designed to specifically bind to a (subterranean) surface, such as a rock surface. Typically the fluid or displacement substance is water, brine or sea-water. The fluid or displacement fluid may comprise other components such as extraction aptamers (these being aptamers with an affinity for hydrocarbons which are bound, adsorbed and/or otherwise associated to or with a surface), polymers, alcohols, basic (alkali) agents, acidic agents, ge!s, including water swel!able gels_f cross-linker molecules, surfactants, materials/compounds for fracking, salts and/or dissolved gases. By way of example, a fluid or displacement substance may comprise an aptamer based hydrocarbon extraction moiety designed to affect the release of hydrocarbons which are bound, adsorbed or otherwise associated to or with a surface (for example a rock surface as might be present in a subterranean formation or hydrocarbon-bearing rock formation). The aptamer(s) may be provided from at least one we!lbore, e.g. injection weilbore. The methods of the present invention may comprise recovering oil from at least one weilbore, e.g. production weilbore. Desirably the wettability of the surface is modified through application of said one or more aptamers, so as to make the surface more, neutral hydrocarbon (e.g. oil) wettabie. In this manner, the recovery factor of said hydrocarbons may be increased. Typically the aptamer(s) of the present invention, which are designed to bind to a substrate surface, such as rock, may also be chosen so as to not be able to bind with any degree of high affinity to the hydrocarbon, such as oil which is desired to be extracted. Thus, when identifying an aptamer or aptamers for use in a method of this invention, potential aptamer(s) may be screened for binding against a sample of hydrocarbon from a target hydrocarbon-containing reservoir or rock formation. in a further aspect there is provided a method of producing one of more aptamers for use in enhancing hydrocarbon (such as (crude) oil) recovery from a hydrocarbon- bearing reservoir or rock formation (for example a subterranean formation), the method comprising providing a mixture of aptamers to a surface of a substrate representative of substrate found within the hydrocarbon-bearing reservoir, or rock formation (for example subterranean formation), each aptamer comprising a region of randomised nucleotides; and isolating said aptamer(s) which bind to the surface of the substrate.

Typically the aptamers are in the form of an oligonucleotide comprising a central region comprising the randomised nucleotides and 3' and/or 5' regions of known sequence. Typically the region of randomised nucleotides is from 8 to 250 nucleotides in length. The 3' and/or 5' regions of known sequence can facilitate isolation and/or amplification of the aptamers, as will be described in more detail. The above method of identifying aptamers is generally based on the Systematic Evolution of Ligands by Exponential enrichment method, termed SELEX, as described in WO 91 19813 which is herein specifically incorporated by reference. The SELEX method involves selection from a mixture of candidate oligonucleotides and generally step-wise iterations of binding, separation and amplification, using the same genera! selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of nucleic acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with a target, which in the case of the present invention is typically a rock surface, under conditions favourable for binding; separating unbound nucleic acids from those nucleic acids which have bound specifically to target molecules; dissociating the nucleic acid-target complexes; often amplifying the nucleic acids dissociated from the nucleic acid-target complexes to yield a ligand-enriched mixture of nucleic acids; and then reiterating the steps of binding, separating, dissociating and amplifying through as many cycles as desired to yield highly specific, high affinity nucleic acid ligands to the target molecule.

The basic SELEX method described in W09119813 has been Improved and modified over the years and Dua et al (Recent Patents on DNA & Gene sequence 2008, 2, 172-186) provide a review of relevant patents relating to SELEX. Each of the identified patent/patent applications identified in the above paper by Dua et al, are incorporated herein by way of reference.

Certain terms used to describe the invention herein are defined as follows:

"SELEX" methodology refers to a method of selecting nucleic acid aptamers which interact with a substrate/surface in a desirable manner, for example binding to the substrate/surface, with amplification/isolation of those selected nucleic acids as described in detail above and in the SELEX Patent Applications and papers referred to above, iterative cycling of the selection/amplification steps allows selection of one or a small number of nucleic acids which interact most strongly with the substrate/surface from a pool which contains a very large number of nucleic acids. Cycling of the selection/amplification procedure is continued until a selected goal is achieved. Although cycling/amplification is often employed, Hoon et al (Biotechniques, 2011 , 51:413-416), to which the skilled reader is directed and the contents of which are herby incorporated, describe a method of selecting aptamers by high-throughput sequencing and informatics analysis, which only requires one round of positive selection followed by high-throughput DNA sequencing and informatics analysis in order to select high-affinity aptamers. Thus, cycling and/or amplification is seen as optional, although desired in many cases.

"Nucleic acid" means DNA, RNA, single-stranded or doubie-stranded, and any chemical modifications thereof. Modifications include, but are not limited to, those which provide other chemical groups that incorporate additional charge, poiarizability, hydrogen bonding, electrostatic interaction, and fluxionality to the individual nucleic acid bases or to the nucleic acid as a whole. Such modifications include, but are not limited to, modified bases such as 2'-position sugar modifications, 5-position pyrimidine modifications, 8-position purine modifications, modifications at cytosine exocyclic amines, substitution of 5-bromo-uracii; backbone modifications, methylations, unusual base-pairing combinations such as the isobases isocytidine and isoguanidine and the like. Modifications can also include 3^! and 5' modifications such as capping. Modifications that occur after each round of amplification are also compatible with this invention. Post-amplification modifications can be reversibly or irreversibly added after each round of amplification. Virtually any modification of the nucleic acid is contemplated by this invention. "Mixture of aptamers" is a mixture of nuc!eic acids of differing, randomized sequence. The source of a "mixture of aptamers" can be from naturally-occurring nucleic acids or fragments thereof, chemically synthesized nucleic acids, enzymatically synthesized nucleic acids or nucleic acids made by a combination of the foregoing techniques. In a preferred embodiment, each nucleic acid has fixed sequences surrounding a randomized region to facilitate the amplification process. The length of the randomized section of the nucleic acid is generally between 8 and 250 nucleotides, preferably between 8 and 60 nucleotides.

Said "one or more aptamer(s)" is a nucleic acid mo!ecule(s) which has/have been isolated from the mixture of aptamers and which binds to the desired surface.

"Separating" means any process for separating bound nucleic acid aptamers from the remainder of the unbound aptamers. Separating can be accomplished by various methods known in the art. Filter binding, affinity chromatography, liquid-liquid partitioning, filtration, gel shift, equilibrium partitioning density gradient centrifugation are ail examples of suitable separating methods.

"Amplifying" means any process or combination of process steps that increases the amount or number of copies of a molecule or class of molecules. In preferred embodiments, amplification occurs after members of the aptamer mixture have been separated, and it is the surface bound nucleic acid aptamers that are amplified. For example, amplifying RNA molecules can be carried out by a sequence of three reactions: making cDNA copies of selected RNAs, using the polymerase chain reaction to increase the copy number of each cDNA, and transcribing the cDNA copies to obtain RNA molecules having the same sequences as the selected RNAs. Any reaction or combination of reactions known in the art can be used as appropriate, including direct DNA replication, direct RNA amplification and the like, as will be recognized by those skilled in the art. The amplification method should result in the proportions of the amplified mixture being essentially representative of the proportions of different sequences in the mixture prior to amplification. It is known that many modifications to nucleic acids are compatible with enzymatic amplification. Modifications that are not compatible with amplification can be made after each round of amplification, if necessary.

"Randomized" is a term used to describe a segment of a nucleic acid having, in principle, any possible sequence over a given length. Randomized sequences will be of various lengths, as desired, ranging from about eight to more than one hundred nucleotides. The chemical or enzymatic reactions by which random sequence segments are made may not yield mathematically random sequences due to unknown biases or nucleotide preferences that may exist. The term "randomized" is used instead of "random" to reflect the possibility of such deviations from non-ideality. In the techniques presently known, for example sequential chemical synthesis, large deviations are not known to occur. For short segments of 20 nucleotides or less, any minor bias that might exist would have negligible consequences. The longer the sequences of a single synthesis, the greater the effect of any bias.

A bias may be deliberately introduced into a randomized sequence, for example, by altering the molar ratios of precursor nucleoside (or deoxynucleoside) triphosphates in the synthesis reaction or the ratio of phosphoramidites in the chemical synthesis. A deliberate bias may be desired, for example, to affect secondary structure, to introduce bias toward molecules known to have facilitating activity, to introduce certain structural characteristics, or based on preliminary results.

In a basic form, the SELEX process for use in accordance with the present invention may be defined by the following series of steps: 1) A candidate mixture of aptamers of differing sequence is prepared. The candidate mixture generally includes regions of fixed sequences (i.e., each of the members of the candidate mixture contains the same sequences in the same location) and regions of randomized sequences. The fixed sequence regions are selected either: (a) to assist in the amplification steps described below, (b) to mimic a sequence known to bind to the target, and/or (c) to enhance the concentration of a given structural arrangement of the nucleic acids in the candidate mixture. The randomized sequences can be totally randomized (i.e., the probability of finding a base at any position being one in four) or only partially randomized (e.g., the probability of finding a base at any location can be selected at any level between 0 and 100 percent).

2) The candidate mixture is contacted with a sample of substrate under conditions favourable for binding between the target and members of the candidate mixture. Typically the sample substrate may be a surface of a rock sample obtained from a hydrocarbon, such as an oil reservoir where it may be desired to apply an enhanced oil recovery technique. Under these circumstances, the interaction between the target and the nucleic acids of the candidate mixture can be considered as forming nucleic acid-target pairs between the target and those nucleic acids having the strongest affinity for the target.

3) The nucleic acids with the highest affinity for the target are separated from those nucleic acids with lesser affinity to the target. Because only an extremely small number of sequences (and possibly only one molecule of nucleic acid) corresponding to the highest affinity nucleic acids exist in the candidate mixture, it is generally desirable to set the separating criteria so that a significant amount of the nucleic acids in the candidate mixture (approximately 5-50%) are retained during separation. 4) Those nucleic acids seiected during separation as having the relatively higher affinity to the target are then amplified to create a new candidate mixture that is enriched in nucieic acids having a relatively higher affinity for the target.

5) By repeating the separation and amplifying steps above, the newfy formed candidate mixture contains fewer and fewer unique sequences, and the average degree of affinity of the nucleic acids to the target will generally increase. Taken to its extreme, the SELEX process will yield a candidate mixture containing one or a small number of unique nucleic acids representing those nucleic acids from the original candidate mixture having the highest affinity to the target molecule. The SELEX process when used in accordance with the present invention thus provides high affinity aptamers for binding to a subterranean substrate, such as a rock surface.

The present application is concerned with identifying nucieic acid aptamers to subterranean substrates, typically rock surfaces. The aptamers so identified can be used in enhanced oil recovery techniques known in the art. The aptamers of the present invention may be identified generally according to the above method known as the SELEX process.

According to a further aspect there is provided a method of recovering hydrocarbons from a formation, comprising: providing one or more aptamer moiecuie(s) to the formation, wherein said aptamer molecuie(s) is/are designed to adhere to a rock surface within the formation, in order to modify said hydrocarbon wettability of the rock surface; and providing a displacement substance in the formation to displace hydrocarbons from the formation. The method may comprise injecting a flood fluid, such as water, in the formation, to displace hydrocarbons from the formation.

The method may comprise injecting the flood fluid from at least one first wellbore, e.g. injection wellbore.

The method may comprise recovering hydrocarbons from at least one first wellbore, e.g. production wellbore.

The features described in relation to any other aspect or the invention, can apply in respect of the method according to a second aspect of the present invention, and are therefore not repeated here for brevity.

Detailed Description

The present invention will now be further described with reference to the following figure which shows:

Figure 1 shows in schematic form the SEVVEX process as described by the present inventors. SEWEX stands for systemic evolution of wettabi!ity-modifiers by exponential enrichment and is based on the known SELEX process.

Briefly and with reference to Figure 1 , a DNA library ( 0) comprising 100s - 1000s of different oligonucleotides comprising different randomised sequences is provided. The large oligonucleotide library (10) comprises randomly generated sequences of fixed length flanked by constant 5' and 3' ends that serve as primer binding sites.This DNA library ( 0) is provided in brine and contacted with a sample of rock (12) in order that oligonucleotides (14) within the DNA library (10) are able to bind to the surface (16) of the sample of rock (12). The rock sample may be taken directly from a particular hydrocarbon bearing reservoir, or couid be a samp!e of rock which is representative of rock within hydrocarbon bearing reservoirs. Some of the o!igonuc!eotides (18) within the library (10) do not bind to the surface (16) of the sample of rock (12) and can be washed away using brine, for example.

The bound oligonucleotides (14) are then e!uted from the surface (16) of the sample of rock (12) using, for example, urea, trifluoroacetic acid (TFA) or trifluoroethanol (TFE), in order to provide isolated oligonucleotides (20). The isolated oligonucleotides (20) are then enriched by an amplification technique, such as PCR, in order to make more copies of the isolated oligonucleotides (20) and provide an enriched sample of oligonucleotides (22). Thereafter the enriched oligonucleotides (22) are allowed to bind the surface ( 6) of the sample of rock (12), but the conditions employed for washing the surface (16) of the sample of rock (12) are more stringent that the previous round, so that some of the enriched oligonucleotides (22) will not be capable of binding to the surface (16) of the sample of rock (12) when the more stringent conditions are employed. The more stringent conditions may be provided by adding e.g. (but not limited to) urea, TFA or TFE to the brine used for washing the surface (16) of the sample of rock (12). Thereafter the bound oligonucleotides can be eiuted, amplified and enriched as before.

The cycle of binding to the surface (16) of the sample of rock (12) followed by elution, amplification and enrichment, can be repeated a number of times, such as 5 - 15 times, but at each successive round of binding, the conditions for binding are made more stringent, by increasing the concentration of the urea, TFA or TFE. Eventually after the successive rounds of binding, eluting, amplification and enrichment, a small number or even a single highly specific o!igonucleotide(s) (24) is/are obtained which will be capable of binding to the surface (16) of the sample of rock (12) with very high affinity. Such oligonucleotide(s) may then be made in large quantities by, for example, cloning the oligonucieotide(s) into a vector, such as a plasmid and expressing the cioned oligonucieotide(s) and purifying the oiigonucleotide(s) using recombinant techniques well known to the skilled reader - see for example Sambrook and Russell (2001). o!ecuiar Cloning: A laboratory manual (3^rd ed.) CSHL press.

An exemplary method for generation of DNA aptamer(s) with high adsorption affinity to reservoir rock is described be!ow: 1. A thin surface (S) is prepared from rock sample from an oil reservoir is collected. No washing should be done. Size: approx. 1cm².

2. S is placed in brine

3. Measure absorbance at 260nm (at 260nm DNA absorbs) (A^. Molecular extinction coefficient of single-stranded or double-stranded DNA is 0.027(pg/ml)^"1 cm^"1 and 0.02(Mg/m!)^"1 cm ¹ respectively.

4. Add a library of DNA molecules with 0(10.000) different sequences.

Equilibrate. Measure absorbance, A₂.

5. Remove supernatant. Measure absorbance in supernatant, A_supernatant.

6. Wash S with brine, 3-4 times. Measure absorbances, A_brin! ..3.

7. E!ute remaining DNA from S using e.g. urea, TFA (trifluoro acetic acid) or TFE (trifluoroethanol) - other interaction disruptors can be used.

8. Desalt the eluate and concentrate sample using e.g. CentriCon-spinfilters or gel electrophoresis.

9. Amplify DNA using PCR (polymerase chain reaction).

10. Measure absorbance after amplification

(Absorbances are used to estimate DNA kept at surface and washed away in each step respectively using Lambert Beer law. Repeat steps (1-10) 10 - 15 times using increasing concentrations of urea, TFE, TFA or surfactant added to the brine for the wash in step 6. In this way DNA with continuously increasing binding affinity to the rock surface is enriched heavily. Note than in step 4 the eluate from the previous round is used instead of the initial huge DNA library.

After final round DNA aptamerfs) with extremely strong affinity to the rock is generated due to the very hard selection pressure in the successive washing steps.

Optionally aptamer(s) with a desired affinity for the substrate, e.g. rock surface may be contacted with the hydrocarbon e.g. oil, to be extracted in order to ensure that the aptamer(s) do not substantially bind to the hydrocarbon.

This final pool is amplified by PCR and the nucleotide sequence can be determined by DNA sequencing (e.g. Sanger or Solexa techniques). Large scale production of the high affinity aptamer(s) can be carried out using recombinant techniques as described above.

As mentioned previously, in certain embodiments it may not be necessary to have only a single aptamer and a pool of aptamers may be provided which show affinity to the surface (16) of the sample of rock (12).

The aptamers or pool of aptamers of the present invention, are intended to modify the ability of a hydrocarbon, such as oil, to wet the surface of rock within a particular hydrocarbon bearing reservoir. Typically, the aptamer or pool of aptamers, are intended to decrease the ability of the hydrocarbon to wet the rock surface and/or increase the ability of a flood fluid, such as water to wet the rock surface. In this manner less hydrocarbon, such as oi!, is expected to adhere to the surface of the rock, enhancing the ability of the hydrocarbon to be extracted. For example, the aptamer or pool of aptamers may be added to the hydrocarbon bearing reservoir, typically by adding a fluid such as water. The aptamer(s) are allowed a period of time to adhere to the rock surfaces within the reservoir in order to displace any bound hydrocarbon from the rock surface. Thereafter a water flood or chemical flood can be injected into the reservoir in order to allow the displaced hydrocarbon such as oil to be recovered, typically by way of a production well.

Claims

1. A method of selectively modifying wettability of a substrate in a subterranean formation, the method comprising providing one or more aptamer molecules to the substrate, wherein said aptamer molecule(s) is/are designed to adhere to the substrate, in order to modify said wettability of the substrate.

2. The method according to claim 1 wherein said aptamer molecule(s) is/are intended to modify the wettability of the substrate to make the substrate closer to neutral wettability to a hydrocarbon, such as oil and/or water, than before the aptamer(s) is/are added.

3. The method according to either of claims 1 or 2 wherein at least 1, 10, 100, 10³, 10⁴, 10⁵, 10⁶, 10⁷, 10^8"16 aptamer molecules is/are provided to each cm² substrate.

4. The method according to any proceeding claim, wherein said aptamer molecules are identical.

5. The method according to any of claims 1 - 3 wherein a pool of different aptamer molecules is provided so as to take account of different surfaces which may be present in the particular formation.

6. The method according to any preceding claim wherein said aptamer molecule(s) do not substantially bind or have low binding affinity for hydrocarbon within the formation.

7. The method according to any preceding claim, wherein said aptamer molecule(s) are provided to the formation by way of a fluid.

8. The method according to any preceding claim, wherein said aptamer molecule(s) are provided to the formation by way of one or more injection well(s) which extend within the formation.

9. The method according to any preceding ciaim further comprising recovering said hydrocarbon, such as oil, from the formation.

10. A displacement substance, such as a flood fluid for use in hydrocarbon, such as oil recovery, the displacement substance comprising one of more aptamers designed to specifically bind to a subterranean surface, such as a rock surface.

11. The displacement substance according to claim 10, wherein the displacement substance is water or sea-water, which may comprise other components such as salts or dissolved gases.

12. A method of producing one of more aptamers for use in enhancing hydrocarbon (such as oil) recovery from a subterranean formation, the method comprising providing a mixture of aptamers to a surface of a substrate representative of substrate found within the subterranean formation, each aptamer comprising a region of randomised nucleotides; and isolating said aptamer(s) which bind to the surface of the substrate. 3. The method according to claim 12, wherein the substrate has been obtained from one or more locations of the formation.

14. The method according to claims 12 or 13 wherein said aptamers are selected to have a high affinity for the substrate.

15. The method according to claims 12 - 14, wherein said aptamers are selected to have a low affinity for said hydrocarbon.

16. A method of recovering hydrocarbons from a formation, comprising: providing one or more aptamer molecule(s) to the formation, wherein said aptamer mo!ecuie(s) is/are designed to adhere to a rock surface within the formation, in order to modify said hydrocarbon wettability of the rock surface; and providing a displacement substance in the formation to displace hydrocarbons from the formation, 7. The method according to claim 16 further comprising injecting a flood fluid, such as water, in the formation, to displace hydrocarbons from the formation.

18. The method according to either of claims 16 or 17 further comprising injecting the flood fluid from at least one first wellbore, e.g. injection we!ibore.

19. The method according to any of claims 16 - 8 further comprising recovering hydrocarbons from at least one first wellbore, e.g. production wellbore. 20. A method or displacement substance according to any preceding claim wherein the aptamers is/are in the form of an oligonucleotide comprising a central region comprising randomised nucleotides and 3' and/or 5' regions of known sequence.

21. A meihod or dispiacement substance according to ciaim 20 wherein the region of randomised nucleotides is from 8 to 250, such as 10 - 100, 15 - 50, 20 - 40 nucleotides in length.