Carbon dioxide viscosification

Abstract

The disclosed technology relates to a method of continuously injecting a viscosified carbon dioxide composition into an injection well for the sequestration of carbon dioxide or production and recovery of hydrocarbons from an underground formation.

Description

Docket No. 4828-01TITLE CARBON DIOXIDE VISCOSIFICATION BACKGROUND OF THE INVENTION

[0001] The disclosed technology relates to a method of continuously injecting a viscosified carbon dioxide composition into an injection well for the sequestration of carbon dioxide or production and recovery of hydrocarbons from an underground formation.

[0002] Carbon dioxide is the most prevalent greenhouse gas. CO2 sequestration is the process of capturing and storing that carbon dioxide. The process begins by pressurizing CO2 to a liquid and then injecting it into porous rock formations in geologic basins. These geologic basins include, for example, active and depleted oil and gas wells, saline aquifers, salt domes, and deep un-mineable coal deposits. Carbon sequestration faces certain issues, including the potential for the compressed supercritical carbon dioxide to escape the formation.

[0003] Sequestering CO2 underground is a complex engineering endeavor and is not without challenges. Continuous supercritical carbon dioxide injection tends to be problematic due to unfavorable mobility, viscous fingering / channeling and early breakthrough of carbon dioxide, especially in the presence of reservoir heterogeneities. Reservoirs modeling of supercritical carbon dioxide shows that the non-uniform distribution of carbon dioxide injection into the formation has negative effect on the overall usage of the storage capacities. Furthermore, unfavorable mobility can cause unwanted leakage of carbon dioxide back to the surface thus defeating the reason for sequestration. The inherent ultralow viscosity of liquified CO2 is its Achilles’ heel. Large scale implantation of carbon dioxide sequestration will be significantly limited without thickening of the supercritical carbon dioxide for more effective mobility control and ultimately higher carbon dioxide storage capacity in formations.

[0004] A significant increase in CO2 viscosity at a reasonable cost could greatly improve the storage capacity and thus reduce the capital cost of implementing CO2 storage. This could potentially propel a broad adoption of the technologyacross governments and commercial organizations in pursuit of a net-zero carbon future.

[0005] Carbon dioxide can also be employed in recovery of hydrocarbons. In the recovery of hydrocarbons, such as oil or natural gas, from subterranean hydrocarbon-bearing formations or reservoirs, it is usually possible to recover only a limited proportion of the original hydrocarbons present in the reservoir by primary recovery methods that utilize the natural formation pressure to produce the hydrocarbons through suitable production wells. A variety of supplementary recovery techniques have therefore evolved to maintaining formation pressure or improve hydrocarbon displacement. For example, “miscible flooding” is a common supplementary recovery technique to improve hydrocarbon displacement.

[0006] Miscible flooding involves introducing fluids into a formation that are miscible with the hydrocarbon to be displaced. One such fluid is carbon dioxide. Carbon dioxide is considered a miscible-type flooding agent because under supercritical conditions, usually high pressure, carbon dioxide acts as a solvent. However, the low viscosity of supercritical carbon dioxide can limit (or affect or diminish) its ability to sweep the oil that is targeted for enhanced recovery to the producing well (or zone). Thus, the use of carbon dioxide is limited without thickening significantly for more effective mobility control and ultimately higher incremental oil recovery.

[0007] The idea of thickening carbon dioxide with polymers is known in the art, from, for example, Heller, J.P. et al., “Direct Thickeners for Mobility of CO2 Floods.” Soc. Pet. Eng. J. 1985, 25, 679-686.

[0008] Further, Zhang, S. et al. (2013). Effects of polymers as direct CO2 thickeners on the mutual interactions between a light crude oil and CO2. J. Polym. Res., 20(61). pp. 1-13 teaches the use of poly(vinyl ethyl ether) (“PVEE”) and a low molecular weight (-900 Mw) poly(l-decene) (“P1D”) as effective thickeners for carbon dioxide.

[0009] It is common to simply pump fluids together into an orifice, which allows turbulent flow to mix the feeds. However, with respect to the mixture of polymersand carbon dioxide, simply pumping the fluids together will not allow the dissolution needed to achieve the viscosities needed in the carbon dioxide downhole. While the idea of thickening carbon dioxide has been discussed, an efficient method for employing thickened CO2 in a single phase fluid downhole is still needed.SUMMARY OF THE INVENTION

[0010] The disclosed technology solves the problem of providing the viscosities needed in a carbon dioxide mixture downhole by providing a method that will allow the proper droplet sizes of the polymer in the carbon dioxide mixture to form, which are critical to attain the dissolution needed to achieve viscosities needed.

[0011] The disclosed technology thus provides a method of injecting thickened fluid carbon dioxide into a subterranean formation. The method includes entraining a polyolefin into carbon dioxide in a mixing device to form a thickened injection blend having droplet sizes no larger than 50 microns. The injection blend can then be transferred from the mixing vessel into at least one injection well accessing the subterranean formation. The method allows for the injection blend to achieve a single phase fluid within the subterranean formation.

[0012] In an embodiment, the method of the mixing and injection of the injection blend is a continuous process, that is, without temporary residency in a mixing, hold, or storage vessel prior to injection. Likewise, the method allows for carbon dioxide and the polyolefin thickener to be continuously injected into the mixer with continuous mixing.

[0013] The method can include venturi mixers (also known as venturi fluid jets) as the mixing device. The method can also include a static mixer as the mixing device.

[0014] The technology also provides a method of improving hydrocarbon recovery from a subterranean formation. The method includes mixing and injecting an injection blend as taught herein, followed by extracting production fluids from the subterranean formation.

[0015] The technology also includes a method of storing thickened carbon dioxide in a subterranean formation by preparing the injection blend as taught herein followed by injecting the injection blend into a subterranean formation.DETAILED DESCRIPTION OF THE INVENTION

[0016] Various preferred features and embodiments will be described below by way of non-limiting illustration.

[0017] An aspect of the disclosed technology is a method of increasing the viscosity of carbon dioxide by mixing with a polyolefin and continuously injecting into an injection well.

[0018] The carbon dioxide composition described herein can be employed to sequester carbon dioxide in an underground formation, as well as to recover hydrocarbons from an underground hydrocarbon containing formation.

[0019] Hydrocarbons can be recovered from an underground hydrocarbon containing formation or reservoir by injecting a carbon dioxide into the reservoir through an injection well and recovering hydrocarbon containing fluids from a production well which is at a horizontal distance or offset from the injection well. In practice, more than one injection well and more than one production well may be used and these may be arranged in a number of different patterns suitable for solvent drive operations of this kind. For simplicity, however, the present invention is described below with reference only to a single injection well and a single production well.

[0020] Thus, the method of recovering hydrocarbons from an underground hydrocarbon containing formation can involve at least some, if not all, of the following steps, not necessarily in the following order:• determining the temperature and pressure of the hydrocarbon formation;• optionally, screening for a suitable branched polyolefin thickener by, for example, either determining the solubility of the at least one polyolefin thickener at the temperature and pressure encountered in the formation,which may be done, for example, by performing the sapphire rocking cell test, or determining the MMP of the hydrocarbons present;• selecting the at least one polyolefin thickener;• injecting into the hydrocarbon formation a carbon dioxide composition containing carbon dioxide and the at least one branched polyolefin thickener; and optionally,• recovering released hydrocarbons from the hydrocarbon containing formation.

[0021] Thickening of CO2 is desirable to improve mobility control and improve hydrocarbon production rates. The methods provided include thickening CO2 by mixing the CO2 with a polyolefin thickener. It has been found to the surprise of the inventors that maintaining droplet size of the polyolefin thickener of no larger than 50 microns ensures timely dissolution of the polyolefin thickener. The methods include mixing carbon dioxide with a polyolefin in a mixing device to form an injection blend having thickener droplet sizes no larger than 50 microns. In some instances the injection blend can have thickener droplet sizes of no larger than 45 microns, or no larger than 40 microns, or even no larger than 35 microns. Practically speaking, the injection blend can have thickener droplet sizes of from sub-microns to 50 microns, 1 micron to 50 microns, 10 microns to 50 microns, or from 15 microns to 50 microns, or even 20 microns to 50 microns, or even 25 microns to 50 microns.

[0022] It is desirable for the injection blend to remain a single phase fluid within the subterranean formation. Injection wells can be miles deep into the earth’s surface. Transferring the injection blend into the subterranean formation can take minutes, tens of minutes or hours. Thus, the mixing device can be placed inline with the injection well to ensure continuous mixing of CO2 and polyolefin thickener and transfer of the injection blend into the injection well. In the same way, the CO2 and polyolefin can be continuously injected into the mixing vessel. Such a continuous process, without temporary residency in a mixing, hold, or storage vessel prior to injection, not only saves time ensuring the stability of the injectionblend into the subterranean formation, but reduces equipment and process time needed for the method.

[0023] The mixing device can be a venturi or static mixer, either of which is capable of achieving droplet sizes of no larger than 50 microns.

[0024] Venturi mixers (known as venturi fluid jets) are known in the art and are versatile devices that utilize the Venturi effect to achieve efficient and precise fluid mixing. Venturi mixers offer benefits such as accurate mixing ratios, energy efficiency, and suitability for a wide range of applications.

[0025] They operate based on the principle of the Venturi effect, which involves the creation of a pressure difference in a fluid flow. These mixers consist of a constricted section called the Venturi tube, which narrows down the flow area, and a diffuser section that expands the flow area back to its original size.

[0026] When fluid flows through a venturi mixer, the narrowing of the flow area causes an increase in the fluid's velocity, resulting in a decrease in pressure. This pressure drop creates a suction effect, drawing in a secondary fluid or additive through an inlet port. The secondary fluid is then mixed with the primary fluid as they pass through the Venturi tube.

[0027] A carbon dioxide stream can be pumped through the Venturi tube at a rate to maintain a change in pressure across a Venturi inlet port of the venturi mixer of less than 1500 psig, or less than 1000 psig, or less than 750 psig, or less than 500 psig, or less than 250 psig, but greater than 50 psig. The polyolefin thickener stream can be fluidly connected to the Venturi inlet port such that the change in pressure draws in the polyolefin thickener through the inlet port causing the carbon dioxide stream to mix with the polyolefin thickener.

[0028] Static mixers are also known in the art and are efficient and reliable devices used for fluid mixing without the need for any moving parts. Static mixers provide consistent and uniform mixing, have a compact design, and can handle a wide range of fluid types. They are designed to achieve thorough mixing by utilizing the flow characteristics of the fluids passing through them. Static mixersconsist of a series of stationary elements or baffles that create turbulence and promote mixing.

[0029] When fluids flow through a static mixer, they are divided and redirected by the baffles, causing them to repeatedly split, mix, and recombine. This process creates a chaotic flow pattern that enhances the intermingling of the fluids, resulting in efficient mixing.

[0030] Static mixers are structures that mix fluids such as gases or liquids, together with no moving parts. The static mixer comprises a fluid inlet port for introducing a first fluid along a centerline axis of a hollow body; a second fluid inlet port for introducing a second fluid along the centerline axis of the hollow body downstream of the first fluid inlet port; and a fluid outlet downstream of the first and second fluid inlet ports and a restrictor structure in the hollow body.

[0031] Static mixers can result in less pressure drop across the device and consequently the downstream mixture of carbon dioxide and polyolefin thickener retain the high pressures of the upstream, incoming carbon dioxide.

[0032] To assist dissolution of the polyolefin thickener in the CO2 stream, the mixing device, the CO2 and / or polyolefin thickener supply lines, or both can include heating elements to maintain the polyolefin thickener at a temperature of at least 20 °C at the moment of initial contact with the carbon dioxide stream. In some embodiments, the temperature of the polyolefin thickener can be maintained at a temperature of at least 40 °C at the moment of initial contact with the carbon dioxide stream.

[0033] The methods can include extracting production fluids from the subterranean formation through at least one production well accessing and in fluid communication with the same subterranean formation as the injection well. Production fluids means an extracted stream containing a volume of hydrocarbons displaced from the formation, but also includes a portion of the inj ection blend mixed with the volume of the extracted hydrocarbons.

[0034] In such a method, the injection blend can be extracted from the resulting production fluids through a separation module.

[0035] The methods can also include storing thickened carbon dioxide in the subterranean formation. In such a case, the subterranean formation is not a hydrocarbon producing well and is not accessed by a production well. Instead the injection blend is transferred through an injection well accessing the subterranean formation and the injection blend is injected therein.COMPOSITIONS

[0036] The compositions herein will include carbon dioxide. Carbon dioxide consists of two oxygen atoms covalently bonded to a single carbon atom. Carbon dioxide can exist as a solid, liquid, gas, or, at temperatures above its critical point, as a supercritical fluid. Supercritical fluids are those that exhibit properties of both liquids, such as the ability to dissolve other substances, and of gases, such as the ability to effuse through solids. As a supercritical fluid, carbon dioxide has the ability to mix homogeneously, or in other words is miscible with, hydrocarbons, such as crude oil, and can therefore improve the recovery of such hydrocarbons.

[0037] The carbon dioxide composition will also include at least one branched polyolefin thickener, which may be a polymer or oligomer. Polyolefins are well known in the art. In one embodiment, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 2 to 24 carbon atoms.

[0038] As used herein, the term “olefin” refers to an unsaturated hydrocarbon compound having a hydrocarbon chain containing at least one carbon-to-carbon double bond in the structure thereof, wherein the carbon-to-carbon double bond does not constitute a part of an aromatic ring. The olefin may be straight-chain, branched-chain or cyclic. “Olefin” is intended to embrace all structural isomeric forms of olefins, unless it is specified to mean a single isomer or the context clearly indicates otherwise.

[0039] By derivable or derived it is meant the polyolefin is polymerized or oligomerized from the starting polymerizable olefin monomers having the noted number of carbon atoms or mixtures thereof. In embodiments, the polyolefinemployed in the carbon dioxide composition may be derivable (or derived) from olefins with 3 to 24 carbon atoms. In some embodiments, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 4 to 24 carbon atoms. In further embodiments, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 5 to 20 carbon atoms. In still further embodiments, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 6 to 18 carbon atoms. In still further embodiments, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 8 to 14 carbon atoms. In alternate embodiments, the polyolefin employed in the carbon dioxide composition may be derivable (or derived) from olefins with 8 to 12 carbon atoms. PAO base stocks useful for the present subject matter may include a homopolymer made from a single alpha-olefin monomer containing 12 or more carbon atoms or a copolymer made from a combination of two or more alpha-olefin monomers where at least 5 mol percent of one of the monomers has 12 or more carbon atoms.

[0040] As used herein, the term “carbon backbone” of a polyolefin is defined as the straight carbon chain therein having the largest number of carbon atoms.

[0041] As used herein, the term “branching group” with respect to a polyolefin refers to any group other than hydrogen attached to the carbon backbone of the polyolefin, other than those attached to the carbon atoms at the very ends of the carbon backbone.

[0042] Often the polymerizable olefin monomers comprise one or more of propylene, isobutene, 1-butene, isoprene, 1,3 -butadiene, or mixtures thereof.

[0043] An example of a useful polyolefin is polyisobutylene.

[0044] Polyolefins also include poly-a-olefins derivable (or derived) from a-ole-fins. As used herein, the term “alpha-olefin” refer to an olefin having a terminal carbon-to-carbon double bond ((R1R2) — C=CH2) in the structure thereof. As used herein, “polyalpha-olefin(s)” (“PAO(s)”) includes any oligomer(s) and polymers) of one or more alpha-olefin monomer(s). PAOs are oligomeric orpolymeric molecules produced from the polymerization reactions of alpha-olefin monomer molecules in the presence of a catalyst system, optionally further hydrogenated to remove residual carbon-carbon double bonds therein. Thus, the PAO can be a dimer, a trimer, a tetramer, or any other oligomer or polymer comprising two or more structure units derived from one or more alpha-olefin monomers). The PAO molecule can be highly regio-regular, such that the bulk material exhibits an isotacticity, or a syndiotacticity when measured by 13C NMR. The PAO molecule can be highly regio-irregular, such that the bulk material is substantially atactic when measured by 13C NMR. A PAO material made by using a metallocene-based catalyst system is typically called a metallocene-PAO (“mPAO”), and a PAO material made by using traditional non-metallocene-based catalysts (e.g., Lewis acids, supported chromium oxide, and the like) is typically called a conventional PAO (“cP AO”). The poly-a-olefins used herein may be mPAOs. The poly-a-olefins used herein may also be cP AOs.

[0045] The a-olefins may be linear or branched or mixtures thereof. Examples include mono-olefins such as propylene, 1-butene, isobutene, 1-pentene, 1-hex-ene, 1 -heptene, 1 -octene, 1 -nonene, 1 -decene, etc. Other examples of a-olefins include 1 -decene, 1 -undecene, 1 -dodecene, 1 -tridecene, 1 -tetradecene, 1-penta-decene, 1 -hexadecene, 1 -heptadecene 1 -octadecene, and mixtures thereof. Other examples of a-olefins include 1 -nonadecene, 1-eicosene, 1-heneicosene, 1-do-cosene, 1 -tricosene, 1 -tetracosene in yet another embodiment. Preferred LAO feeds are 1 -hexene, 1 -octene, 1 -decene, 1 -dodecene, 1 -tetradecene, 1 -hexadecene and 1 -octadecene. Examples of preferred a-olefin mixtures as monomers for making the poly-a-olefins include, but are not limited to: C6 / C8; C6 / C10; C6 / C12; C6 / C14; C6 / C16; C6 / C8 / C10; C6 / C8 / C12; C6 / C8 / C14; C6 / C8 / C16; C8 / C10; C8 / C12; C8 / C14; C8 / C16; C8 / C10 / C12; C8 / C10 / C14; C8 / C10 / C16; C10 / C12; Cl 0 / C 14; Cl 0 / C 16; Cl 0 / C 12 / C 14; Cl 0 / C 12 / C 16; and the like. An example of a useful a-olefin is 1 -decene.

[0046] The polyolefin may also be a copolymer of at least two different olefins, also known as an olefin copolymer (OCP). These copolymers are preferably copolymers of a-olefins having from 2 to about 28 carbon atoms, preferablycopolymers of ethylene and at least one a-olefin having from 3 to about 28 carbon atoms, typically of the formula CH2=CHR1 wherein R1 is a straight chain or branched chain alkyl radical comprising 1 to 22 carbon atoms. Preferably R1 in the above formula can be an alkyl of from 1 to 8 carbon atoms, and more preferably can be an alkyl of from 1 to 2 carbon atoms.

[0047] The composition may be substantially free of ethylene and polymers thereof. The composition may be completely free of ethylene and polymers thereof. By substantially free, it is meant that the composition contains less than 0.01wt% of the given material. In some embodiments, substantially free means less than 0.005 wt.% of the given material. Substantially free can also mean less than 1000 ppm of the given material. In some embodiments, substantially free means less than 500 ppm of the given material. Substantially free can also mean less than 250 ppm of the given material. In some embodiments, substantially free means less than 100 ppm of the given material. Substantially free can also mean less than 50 ppm of the given material. In some embodiments, substantially free means less than 30 ppm of the given material. Substantially free can also mean less than 10 ppm, or less than 5 ppm, or even less than 1 ppm of the given material. The composition may be substantially free of propylene and polymers thereof. The composition may be completely free of propylene and polymers thereof.

[0048] The polyolefin thickeners prepared from the aforementioned olefin monomers can have a number average molecular weight of from 140 to 5000. The polyolefin thickeners prepared from the aforementioned olefin monomers can also have a number average molecular weight of from 200 to 4750. The polyolefin thickeners prepared from the aforementioned olefin monomers can also have a number average molecular weight of from 250 to 4500. The polyolefin thickeners prepared from the aforementioned olefin monomers can also have a number average molecular weight of from 500 to 4500. The polyolefin thickeners prepared from the aforementioned olefin monomers can also have a number average molecular weight of from 750 to 4000 as measured by gel permeation chromatography with a polystyrene standard.

[0049] In certain embodiments, there is provided a 1,000 to 10,000 Dalton molecular weight PAO polymer, such as a polydodecene polymer, as measured by gel permeation chromatography with a polystyrene standard. The PAO polymer, such as copolymers of octene and decene, may have a number average molecular weight of from 1,250 to 5,000 Dalton, as measured by gel permeation chromatography with a polystyrene standard. The PAO polymer, such as copolymers of octene and decene, may have a number average molecular weight of from 1,500 to 4,500 Daltons, as measured by gel permeation chromatography with a polystyrene standard. The PAO polymer, such as polydecene, may have a number average molecular weight of from 2,000 to 4,250 Daltons, as measured by gel permeation chromatography with a polystyrene standard. The PAO polymer, such as copolymers of octene and dodecene, may have a number average molecular weight of from 2,500 to 4,000 Daltons, as measured by gel permeation chromatography with a polystyrene standard

[0050] Some of the polyolefins will include branching by virtue of the structure of the polymer. For example, the polymerization of a-olefins along the a bond results in the tails of these monomers branching along the polymerized a olefin. The longer the a-olefin, the longer the branches off of the resultant polyolefin. Polydecene for example can result in a polymer having branches of 8 carbon atoms.

[0051] A typical, hydrogenated PAO molecule can be represented by the following formula I:Iwhere R1, R2, R3, each of R4and R5, R6, and R7, the same or different at each occurrence, independently represents a hydrogen or a substituted or unsubstitutedhydrocarbyl (preferably an alkyl) group, and n is an non-negative integer corresponding to the degree of polymerization.

[0052] Thus, where n=0, formula I represents a dimer produced from the reaction of two monomer molecules after a single addition reaction between two carboncarbon double bonds.

[0053] Where n=m, m being a positive integer, formula I represents a molecule produced from the reactions of m+2 monomer molecules after m+1 steps of addition reactions between two carbon-carbon double bonds.

[0054] Thus, where n=l, formula I represents atrimer produced from the reactions of three monomer molecules after two steps of addition reactions between two carbon-carbon double bonds.

[0055] Assuming a straight carbon chain starting from R1 and ending with R7 has the largest number of carbon atoms among all straight carbon chains existing in formula I, then the straight carbon chain starting from R1 and ending with R7 having the largest number of carbon atoms constitutes the carbon backbone of the poly-a-olefin molecule formula I. R2, R3, each of R4 and R5, and R6, which can be substituted or unsubstituted hydrocarbyls (preferably alkyl) groups, are branching groups (if not hydrogen).

[0056] If only alpha-olefin monomers are used in the polymerization process, and no isomerization of the monomers and oligomers ever occurs in the reaction system during polymerization, about half of Rl, R2, R3, all R4 and R5, R6, and R7 would be hydrogen, and one of Rl, R2, R6, and R7 would be a methyl, and about half of groups Rl, R2, R3, all R4 and R5, R6, and R7 would be hydrocarbyl groups introduced from the alpha-olefin monomer molecules. In a specific example of such case, assuming R2 is methyl, R3, all R5, and R6 are hydrogen, and Rl, all R4, and R7 have 8 carbon atoms in the longest carbon chains contained therein, and n=8, then the carbon backbone of the formula I PAO molecule would comprise 35 carbon atoms, and the average branching group length of the branching groups (R2, all of R4) would be 7.22 (i.e., (l+8*8) / 9). This PAO molecule, which can be produced by polymerizing 1 -decene using certain metallocenecatalyst systems described in greater detail below, can be represented by formula II below:

[0057] Depending on the polymerization catalyst system used, however, different degrees of isomerization of the monomers and / or oligomers can occur in the reaction system during the polymerization process, resulting in different degrees of substitution on the carbon backbone. In a specific example of such case, assuming R2, R3, all R5 are methyls, and R6 is hydrogen, R1 has 8 carbon atoms in the longest straight carbon chain contained therein, and all R4 and R7 have 7 carbon atoms in the longest straight carbon chain contained therein, and n=8, then the carbon backbone of the formula I PAO molecule would comprise 34 carbon atoms, and the average branching group length of the branching groups (R2, all R4, and R5) would be 3.67 (i.e., (l+l+7*8+l*8) / 18). This PAO molecule, which may be produced by polymerizing 1 -decene using certain non-metallocene catalyst systems described in greater detail below, can be represented by the following formula III:

[0058] PAO base stocks useful for the present invention may be a homopolymer made from a single alpha-olefin monomer or a copolymer made from a combination of two or more alpha-olefin monomers.

[0059] The branching groups on the PAO molecules can be straight chain alkyls having at least 6 carbon atoms. The branching groups on the PAO molecules can be straight chain alkyls having at least 8 carbon atoms.

[0060] In one embodiment, there is provided a 1000 to 5000 molecular weight branched PAO polymer, such as a polydecene, polyoctene, or polydodecene polymer. The branched PAO polymer, such as polydecene, polyoctene, or polydodecene, can also have a number average molecular weight of from 1250 to 4750. The branched PAO polymer, such as polydecene, can also have a number average molecular weight of from 1500 to 4500. The branched PAO polymer, such as polydecene, can have a number average molecular weight of from 2000 to 4250. The branched PAO polymer, such as polydecene, can also have a number average molecular weight of from 2500 to 4000 as measured by gel permeation chromatography with a polystyrene standard.

[0061] The polyolefins may also be functionalized with substituents to add branching along the polyolefin backbone. For example, the polyolefin may be functionalized with at least one of an aromatic hydrocarbyl group, aliphatic hy-drocarbyl group, cyclic hydrocarbyl group, and mixtures thereof, so that the branching of the branched polyolefin thickener includes at least one of an aromatic hydrocarbyl group, aliphatic hydrocarbyl group, cyclic hydrocarbyl group, and mixtures thereof.

[0062] In some embodiments, the polyolefin may be functionalized with at least one aromatic hydrocarbyl group so that the branching of the branched polyolefin thickener includes at least one aromatic hydrocarbyl group. The aromatic hydrocarbyl group may be, for example, a hydroxyl containing aromatic group, such as, for example, a phenol group, an amine containing aromatic group, such as, for example, aniline, and mixtures thereof. Other aromatic groups can include, for example, phenylmethylene; o-heptyl-phenylmethylene; and p-heptylphenyl-methylene; aniline and alkyl anilines; indole and alkyl indoles; quinoline and alkyl quinoline; isoquinoline and alkyl isoquinoline; pyrazine and alkyl pyrazine; quinoxaline and alkyl quinoxaline; acridine and alkyl acridine; pyrimidine and alkyl pyrimidine; quinazoline and alkyl quinazoline. The aromatic group can alsobe a polyaromatic group, such as, for example, naphthalene, naphthol or other homologues of phenol with fused aromatic rings, naphthylamine or other homologues of aniline.

[0063] In an embodiment, the aromatic group is a hydroxyl containing aromatic group. In an embodiment, the aromatic group is a phenol group. In an embodiment, the aromatic group is an amine containing aromatic group. In an embodiment, the aromatic group is a hydroxyl and amine containing aromatic group. In an embodiment, the aromatic group is a 2-((dimethylamino)methyl)phenol group.

[0064] In some embodiments, the polyolefin may be functionalized with at least one aliphatic hydrocarbyl group, so that the branching of the branched polyolefin thickener includes at least one aliphatic hydrocarbyl group.

[0065] In some embodiments, the polyolefin may be functionalized with at least one cyclic hydrocarbyl group, so that the branching of the branched polyolefin thickener includes at least one cyclic hydrocarbyl group. An example cyclic group includes, for example, cyclohexylmethylene. Other cyclic groups can include heterocyclic groups, such as, for example, pyridines and alkyl pyridines, pyrrole and alkyl pyrroles, piperidine and alkyl piperidines, pyrrolidine and alkyl pyrrolidines, imidazole and alkyl imidazole. Other cyclic groups can include, in particular, vinyl-pyridine and / or vinyl-imidazole, as well as styrene.

[0066] In some embodiments, the polyolefin can be a polyisobutylene polymer with a number average molecular weight from 140 to 5000. The polyisobutylene polymer can also have a number average molecular weight of from 200 to 4500. The polyisobutylene polymer can also have a number average molecular weight of from 250 to 4000. The polyisobutylene polymer can have a number average molecular weight of from 300 to 3500. The polyisobutylene polymer can have a number average molecular weight of from 350 to 3000. The polyisobutylene polymer can also have a number average molecular weight of from 400 to 2500 as measured by gel permeation chromatography with a polystyrene standard.

[0067] The carbon dioxide composition can include the branched polyolefin thickener at from 0.01 to 5 wt.% based on the weight of the composition. Thecarbon dioxide composition may include the branched polyolefin thickener at from 0.05 to 4.5 wt.% based on the weight of the composition. The carbon dioxide composition may also include the branched polyolefin thickener at from 0.1 to 4 wt.% based on the weight of the composition. The carbon dioxide composition could also include the branched polyolefin thickener at from 0.5 to 3.5 wt.% based on the weight of the composition.

[0068] One purpose of the polyolefin thickener is to increase the viscosity of liquid and supercritical carbon dioxide. The absolute viscosity of supercritical carbon dioxide will vary depending on the temperature and pressure at which the viscosity is measured, but has been seen to be about 0.07 cP at 2000 psi and 0.09 cP at 2900 psi as measured by viscometer. The polyolefin thickener can be dosed into liquid and / or supercritical carbon dioxide to increase the viscosity of the composition relative to the starting carbon dioxide viscosity, up to the point at which the carbon dioxide becomes un-flowable.

[0069] In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition by at least 100% which can also be referenced as 2 times or “2x;” meaning the absolute viscosity of the carbon dioxide composition is 100% greater than the absolute viscosity of the carbon dioxide on its own. For example, if the absolute viscosity of the carbon dioxide is 0.05, a relative viscosity 100% greater would be 0.05 + (0.05)* 100% = 0.01, or 2 times. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition from about 100% to about the point at which the composition does not flow freely, such as about 1 or 2 or 3 or 3.5 or 4 or 4.5 or 5 orders of magnitude and more. As used herein “order of magnitude” means approximately a factor of 10. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition by at least 150% which can also be referenced as 2.5 times or “2.5x.” In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition from about 150% to about the point at which the composition does not flow freely, such as about 1or 2 or 3 or 3.5 or 4 or 4.5 or 5 orders of magnitude and more. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition by at least 200%, which can also be referenced as 3 times or “3x.” In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition from about 200% to about the point at which the composition does not flow freely, such as about 1 or 2 or 3 or 3.5 or 4 or 4.5 or 5 orders of magnitude and more. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition by at least 250% or 3.5x. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition from about 250% to about the point at which the composition does not flow freely, such as about 1 or 2 or 3 or 3.5 or 4 or 4.5 or 5 orders of magnitude and more. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition by at least 300% or 4x. In some embodiments, the polyolefin thickener can be dosed into the carbon dioxide composition to increase the relative viscosity of the composition from about 300% to about the point at which the composition does not flow freely, such as about 1 or 2 or 3 or 3.5 or 4 or 4.5 or 5 orders of magnitude and more.

[0070] Given the temperatures and pressures involved with obtaining supercritical carbon dioxide, measurements of absolute viscosity are difficult and may provide slightly differing results from well to well. However, when comparing viscosity between two samples out of the same well by the same measurement method (i.e., viscosity of supercritical carbon dioxide to viscosity of carbon dioxide composition containing the polyolefin thickener) the relative viscosity trends should be the same or similar between methods. Thus, the relative viscosity numbers herein may be arrived at by measure of the absolute viscosity of the comparable samples by any reasonable test method. One method may be to employ a viscometer.

[0071] One useful measure to screen polyolefin thickeners can be to check the solubility of the polymer in liquid or supercritical carbon dioxide. In general, themore soluble a second substance is in a first substance, the more available the second substance is to act on the first substance. While complete solubility is desired, a partially soluble polymer can also provide viscosity improvements. The solubility of the polymer can be measured by methods known in the art, such as, for example, by visual inspection or cloud point. In an embodiment, the solubility of the polyolefin thickener in the carbon dioxide composition can be measured by a sapphire rocking cell test. The sapphire rocking cell test employs an apparatus having two rocking cells generally of about 20 mL volume, each equipped with a stainless steel ball to aid agitation. Each cell is charged with a designated volume of the chosen branched polyolefin thickener and injected with carbon dioxide to a desired carbon dioxide pressure. The cells are then submerged in a constant temperature water bath. The cells are rocked in the water bath from a 45 ° angle to a -45 ° angle at a pre-determined rocking frequency, for example, 15 times / min. The water bath is brought to the desired temperature and the sapphire cells are observed for solubility of the polyolefin thickener in the carbon dioxide. If the polyolefin is completely soluble in the carbon dioxide at the given pressure and temperature, the mixture will appear homogeneous. Otherwise, separate phases will be observed in the cells.

[0072] In some cases, the pressure actually necessitated for solubility of the hydrocarbons in the carbon dioxide could depend upon the minimum miscibility pressure (MMP) of the hydrocarbons present. The MMP may be found by simple experiment, using samples of the hydrocarbons from the reservoir and the carbon dioxide composition, which anyone skilled in the art would be readily able to perform.

[0073] The amount of each chemical component described is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, that is, on an active chemical basis, unless otherwise indicated. However, unless otherwise indicated, each chemical or composition referred to herein should be interpreted as being a commercial grade material which may contain the isomers, by-products, derivatives, and other such materials which are normally understood to be present in the commercial grade.

[0074] As used herein, the term "hydrocarbyl substituent" or "hydrocarbyl group" is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, it refers to a group having a carbon atom directly attached to the remainder of the molecule and having predominantly hydrocarbon character. Examples of hydrocarbyl groups include:

[0075] hydrocarbon substituents, that is, aliphatic (e.g., alkyl or alkenyl), alicyclic (e.g., cycloalkyl, cycloalkenyl) substituents, and aromatic-, aliphatic-, and alicy-clic-substituted aromatic substituents, as well as cyclic substituents wherein the ring is completed through another portion of the molecule (e.g., two substituents together form a ring);

[0076] substituted hydrocarbon substituents, that is, substituents containing nonhydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbon nature of the substituent (e.g., halo (especially chloro and fluoro), hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, amino, and sulfoxy);

[0077] hetero substituents, that is, substituents which, while having a predominantly hydrocarbon character, in the context of this invention, contain other than carbon in a ring or chain otherwise composed of carbon atoms and encompass substituents as pyridyl, furyl, thienyl and imidazolyl. Heteroatoms include sulfur, oxygen, and nitrogen. In general, no more than two, or no more than one, nonhydrocarbon substituent will be present for every ten carbon atoms in the hydrocarbyl group; alternatively, there may be no non-hydrocarbon substituents in the hydrocarbyl group.

[0078] It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may be different from those that are initially added. For instance, metal ions (of, e.g., a detergent) can migrate to other acidic or anionic sites of other molecules. The products formed thereby, including the products formed upon employing the composition of the present invention in its intended use, may not be susceptible of easy description. Nevertheless, all such modifications and reaction products are includedwithin the scope of the present invention; the present invention encompasses the composition prepared by admixing the components described above.EXAMPLES

[0079] The relationship between thickener droplet size and dissolution time is evaluated using a high-pressure pendant-drop instrument. The chamber is charged with CO2 and allowed to equilibrate at the target temperature and pressure. A CO2 thickener droplet is then formed and introduced into the chamber using a syringe needle of known diameter. The droplet is monitored through the viewing window with a camera, and video is recorded under quiescent conditions until the droplet fully disappears, which is taken as the endpoint for complete dissolution.Table 1: dissolution time of C8 / C12 alpha olefin oligomer in CO2

[0080] Each of the documents referred to above is incorporated herein by reference, including any prior applications, whether or not specifically listed above, from which priority is claimed. The mention of any document is not an admission that such document qualifies as prior art or constitutes the general knowledge of the skilled person in any jurisdiction. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts of materials, reaction conditions, molecular weights, number of carbon atoms, and the like, are to be understood as modified by the word "about." It is to be understood that the upper and lower amount, range, and ratio limits set forth herein may be independently combined. Similarly, the ranges and amounts foreach element of the invention can be used together with ranges or amounts for any of the other elements.

[0081] As used herein, the transitional term “comprising,” which is synonymous with “including,” “containing,” or “characterized by,” is inclusive or open-ended and does not exclude additional, un-recited elements or method steps. However, in each recitation of “comprising” herein, it is intended that the term also encompass, as alternative embodiments, the phrases “consisting essentially of’ and “consisting of,” where “consisting of’ excludes any element or step not specified and “consisting essentially of’ permits the inclusion of additional un-recited ele-ments or steps that do not materially affect the essential or basic and novel characteristics of the composition or method under consideration.

Claims

What is claimed is:

1. A method of injecting thickened fluid carbon dioxide into a subterranean formation, comprising(a) mixing carbon dioxide with a polyolefin thickener in a mixing device to form a thickened injection blend, wherein the polyolefin thickener has droplet sizes no larger than 50 microns;(b) transferring the injection blend from the mixing device into at least one injection well accessing the subterranean formation and injecting the injection blend into the formation, wherein the injection blend is a single phase fluid within the subterranean formation.

2. The method of claim 1, wherein the mixing and injection is a continuous process without temporary residency in a mixing, hold, or storage vessel prior to injection.

3. The method of claim 1, wherein the thickened carbon dioxide is injected in a continuous manner without temporary residency in a mixing, hold, or storage vessel prior to injection.

4. The method of claim 1, wherein the mixing vessel is a venturi mixer.

5. The method of claim 4, where the venturi mixer maintains a change in pressure across a Venturi tube of the venturi mixer of less than 1500 psi.

6. The method of claim 1, wherein the mixing vessel is a static mixer.

7. The method of claim 1, wherein the polyolefin thickener is maintained at a temperature of at least 20 °C at the moment of initial contact with the carbon dioxide stream.

8. The method of claim 1, wherein the polyolefin thickener is maintained at a temperature of at least 40 °C at the moment of initial contact with the carbon dioxide stream.

9. The method of claim 1, wherein the polyolefin is selected from the group consisting of polyalphaolefins having a number average molecular weight (Mn) of 1000 to 5000, or 1250 to 1475, or 1500 to 4500.

10. The method of claim 1, wherein the thickener is present at from 0.01 to 5 wt.% of the composition.

11. The method of claim 1, wherein the polyolefin thickener consists of alphaole- fins containing 4 to 20 carbon atoms, or 8 to 16 carbon atoms, and mixtures thereof.

12. A method of improving hydrocarbon recovery from a subterranean formation, comprising(a) mixing carbon dioxide with a polyolefin thickener in a mixing device to form a thickened injection blend, wherein the polyolefin thickener has droplet sizes no larger than 50 microns;(b) transferring the injection blend from the mixing vessel into at least one injection well accessing the subterranean formation and injecting the injection blend into the formation, wherein the injection blend is a single phase fluid within the subterranean formation,(c) extracting production fluids from the subterranean formation through at least one production well accessing the formation, wherein the production fluids comprise a volume of hydrocarbons displaced from the formation, a portion of the injection blend mixed with a volume of hydrocarbons extracted from the formation and the bulk of the injection blend; and(d) separating the carbon dioxide from a resulting injection blend through a separation module; wherein the at least one injection well and at least one production well are in fluid communication with the subterranean formation; wherein the mixing vessel is in fluid communication with the at least one injection well.

13. A method of storing thickened carbon dioxide in a subterranean formation, comprising(a) mixing carbon dioxide with a polyolefin thickener in a mixing device to form a thickened injection blend, wherein the polyolefin thickener has droplet sizes no larger than 50 microns;(b) transferring the injection blend from the mixing vessel into at least one injection well accessing the subterranean formation that is not a hydrocarbon producing well and injecting the injection blend into the formation, wherein the injection blend is a single phase fluid within the subterranean formation.

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