Improving the Performance of Supercritical CO2 as an Oil Recovery Solvent

Nearly 5% of the oil produced in the US (about 300,000 barrels of oil each day) is attributable to the injection of supercritical CO2 into subterranean sandstone or carbonate formations. Even though this technology has been used safely and successfully for over 40 years, it is plagued by problems related to the low viscosity of CO2. At reservoir conditions, the viscosity of CO2 is about 0.05 cp, while the viscosity of the crude oil is typically in the 1-10 cp range. As a result, the CO2 tends to finger from the injection well, through the formation, toward the production wells rather than uniformly displacing the oil from the pores of the rock. This results in frustratingly high amounts of CO2 production and recycle, and disappointingly low rates of oil recovery and cumulative amounts of oil production. The ability to alter the mobility of high pressure CO2 flowing through the rock, and to re-direct its flow into oil-bearing layers of rock, is a challenge that is ripe for chemical engineering solutions. In this presentation, a review of techniques for increasing the viscosity of CO2, generating CO2-in-brine foams that lower the mobility of CO2, and using gels to plug up highly permeable watered-out layers of rock that steal CO2 will be presented

The effect of CO2-philic thickeners on gravity drainage mechanism in gas invaded zone

The rate of mass transfer between the fractures and matrix in gas invaded zone can significantly influence on the oil recovery during the forced gravity drainage process. However, in this study, a new approach was suggested to improve the gravity drainage process in gas invaded zone. Poly(fluoroacrylate) (PFA), as a CO2-philic thickener, was injected into the gas invaded zone to illustrate the impact of interfacial mechanisms such as gas diffusion coefficient and interfacial tension (IFT) on oil recovery. Also, the cloud point pressures were measured to ensure that the PFA did not come out of the solution due to a phase change during IFT, gas diffusion coefficient, and gravity drainage experiments. Results showed that the CO2-PFA thickener (20000 ppm) could decrease the IFT from 56 to 24 dyne/cm compared to the pure CO2 scenario, improving the gravity drainage mechanism in the gas invaded zone. In addition, the CO2 diffusion coefficients were increased approximately more than two times during CO2-PFA injection in comparison with pure CO2 injection in both porous media and bulk oil phase scenarios at reservoir conditions. Also, an incremental oil recovery of 16 percent was achieved during PFA/CO2 compared to pure CO2 injection in the gas invaded zone. Therefore, gas gravity drainage is the most important mechanism once gas thickener or CO2 enters the fractures in the gas invaded zone

NOVEL CO2-THICKENERS FOR IMPROVED MOBILITY CONTROL

The first carbon dioxide thickeners have been successfully designed. Each thickener is characterized by a highly carbon dioxide-phobic functionality that imparts CO{sub 2}-solubility and a carbon dioxide-phobic group that facilitates viscosity-enhancing intermolecular associations. The design of each thickener required that appropriate balance of these groups to yield a compound that was at least several weight percent soluble in CO{sub 2} and capable of thickening the carbon dioxide by a factor of 2-20. Four types of thickeners were identified, fluoroacrylate-styrene copolymers (polyFAST), fluorinated telechelic ionomers, semi-fluorinated trialkyltin fluorides and small, fluorinated hydrogen-bonding compounds. Although significant viscosity increases (e.g. doubling the viscosity) were evidenced for each thickener during falling cylinder viscometry analysis, the polyFAST thickener provided the most dramatic increases at dilute concentration. PolyFAST is a bulk-polymerized, random copolymer of fluoroacrylate and styrene with a number-average molecular weight of about 500,000. It appears as a white, slightly waxy solid at ambient conditions. The fluoroacrylate enhances the CO{sub 2} solubility, while the styrene promotes intermolecular stacking of the aromatic groups. Although concentrations between 20-29 mol% styrene yield a thickener, the optimal composition of polyFAST for thickening was 29mol% styrene and 71mol% fluoroacrylate. Mobility measurements with a Berea sandstone core indicated that at a superficial velocity of one foot per day, a 0.5wt% concentration of 29%styrene--71%fluoroacrylate polyFAST tripled the viscosity. At concentrations of 1% and 1.5wt%, the CO{sub 2} viscosity increased by a factor of 8 and 19, respectively. If lower proportions of styrene are used, the compound will dissolve more readily in carbon dioxide but the viscosity enhancement will diminish. At higher proportions of styrene, the CO{sub 2} solubility decreases and the thickening capability also decreased, apparently due to the increased number of non-viscosity enhancing intramolecular interactions between the aromatic groups. The high price, environmental persistence, and lack of availability of bulk amounts of the fluoroacrylate monomer guided our final efforts of this work (and all of our efforts in its continuation) toward the development of inexpensive non-fluorous compounds. We have therefore initiated the design highly CO{sub 2} soluble polymers that can replace the fluoroacrylate. These hydrocarbon-based CO{sub 2}-philic compounds will then be incorporated into the structure of a compound that contains CO{sub 2}-phobic associating groups, yielding a commercial thickener

INEXPENSIVE CO{sub 2} THICKENING AGENTS FOR IMPROVED MOBILITY CONTROL OF CO{sub 2} FLOODS

The objective of this research was the design, synthesis and evaluation of inexpensive, nonfluorous carbon dioxide thickening agents. We followed the same strategy employed in the design of fluorinated CO{sub 2} polymeric thickeners. First, a highly CO{sub 2}-philic, hydrocarbon-based monomer was to be identified. Polymers or oligomers of this monomer were then synthesized. The second step was to be completed only when a CO{sub 2}-soluble polymer that was soluble in CO{sub 2} at pressures comparable to the MMP was identified. In the second step, viscosity-enhancing associating groups were to be incorporated into the polymer to make it a viable thickener that exhibited high CO{sub 2} solubility at EOR MMP conditions. This final report documents the CO{sub 2} solubility of a series of commercial and novel polymers composed of carbon, hydrogen, oxygen and, in some cases, nitrogen

Decreasing Asphaltene Precipitation and Deposition during Immiscible Gas Injection Via the Introduction of a CO2-Soluble Asphaltene Inhibitor

In this study, the ability of dilute concentrations of toluene to act as a CO2-soluble asphaltene stabilization agent capable of inhibiting asphaltene precipitation during immiscible CO2 injection was assessed. Phase behavior results indicated that 1,000 to 20,000 ppm toluene could readily dissolve in CO2 at cloudpoint pressures that are well below the formation pressure and typical CO2 minimum miscibility pressure (MMP) values during gas-based enhanced oil recovery (EOR). Single-phase solutions of the modified gas (CO2/toluene) were then combined with asphaltenic oils in oil swelling phase behavior tests to demonstrate that the presence of toluene increased the amount of CO2 that dissolved into reservoir crude oil at a specified temperature and pressure. However, asphaltene precipitation diminished, apparently because the effect of the increased asphaltene solvent strength of toluene was more significant than the increased amount of CO2 (an asphaltene antisolvent) that entered the oil-rich phase. During the injection of CO2/toluene solution into cores initially saturated with crude oil and brine, compared to the injection of pure CO2, asphaltene deposition declined during the injection of CO2/toluene mixtures for asphaltenic volatile and intermediate oils from 3.7 wt% to 0.7 wt% and 5.9 wt% to 1.7 wt%, respectively. Based on the asphaltene particle-size analysis, the CO2/toluene mixtures can stabilize oil particles and simultaneously reduce asphaltene aggregation more effectively than pure CO2

Novel CO2-Thickeners for Improved Mobility Control

The objective of this contract was to design, synthesize, and characterize thickening agents for dense carbon dioxide and to evaluate their solubility and viscosity-enhancing potential in CO2