Effect of light hydrocarbon solvents on the performance of CO2-based cyclic solvent injection (CSI) in heavy oil systems

Abstract

Cyclic Solvent Injection (CSI) has gained significant interest as a viable method among solvent injection processes to increase recovery factor of heavy oil reservoirs. The type of solvent, operating pressure, and oil viscosity markedly affect the performance of CSI.In this study, series of cyclic injection tests of pure CO2, CH4-CO2, and C3H8-CO2 were conducted at various operating pressures and mixture compositions to different (in term of viscosity) heavy oil samples. Sand pack models with absolute permeability of kabs = 6–10 D and porosity of ø = 27–32% were used as representative of a typical heavy oil reservoir. In each cycle, the produced oil and pressured drop during soaking time were measured and CO2/solvent apparent solubility was calculated using the pressure decay method.Results showed that there is an optimum pressure in cyclic CO2 injection process in heavy oil systems; the pressure beyond which the ultimate oil recovery factor (RF) did not notably improve (near 4.82 MPa in this study). Higher concentration of C3H8 in the CO2 stream improved the oil recovery during cyclic injection. However, ultimate RF was not noticeably increased when C3H8 concentration in the mixture exceeded a certain value (near 50% in this study). Although C3H8 showed effective role on the performance of CSI, it was found that the recovery factor reduces with increased CH4 concentration in the CO2 stream. The highest recovery factor of RF = 73.8% was obtained by injecting mixture of 50% C3H8 - 50% CO2 under the operating pressure of Pinj = 1.72 MPa. The results of CSI tests on the heavy oil with viscosity of 6 430 mPa s were lower by almost 20% since the solvent solubility was noticeably lower. No oil production was observed after conducting the first cycle injection of optimum quantified solvents on oil with the viscosity of 22 000 mPa s. Small values of SUF, relatively high values of SOR, and low quality of the produced oil suggested that higher cycle numbers of CSI in heavy oil reservoirs is depending on the economic limits and might be conducted cautiously.