Optimizing Cyclic CO2 Injection for Low- permeability Oil Reservoirs through Experimental Study

Abstract

Abstract Cyclic CO2 huff ’n’ puff process in diverse reservoir conditions, such as immiscible, near-miscible, and miscible, has been tested in the laboratory and applied in the fields as a viable secondary or tertiary means to recover the residual oil in water flooded reservoirs or pressure depleted reservoirs. Recently, petroleum operators have shown increasingly interest in taking CO2 huff ’n’ puff process as a preferred option to extract light oil in low-pressure and low-permeability reservoirs. In this paper, experimental results from a series of coreflood tests are presented and operation strategies for cyclic CO2 injection are optimized to maximize the light oil recovery factor in a low- permeability reservoir with low original reservoir pressure and while, minimize CO2 utilization. 6 cyclic coreflood tests, 20 cycles in total, are conducted in a 973 mm-long composite core with an average porosity of 9.6% and an average permeability of 2.3mD. The reservoir pressure is 12.9MPa, far below the measured MMP value of 23MPa, which indicates that the reservoir has no sufficient energy support to do primary production. The impacts of primary operational parameters, such as slug size, injection pressure, chasing gas (N2) and CO2 injection rate on the performance have been investigated. It is concluded that, on the basis of experimental data, 0.1PV seems to be an optimal slug size for the first cycle, with a cycle recovery factor as high as 14.52% when reservoir pressure depleted to 8MPa. The recovery factor is suggested to be sensitive to the maximum pressure and the maximum pressure should be built up to as high as formation permits. In the following cycles, injecting N2 as chasing gas after CO2 injection seems to improve the cycle performance significantly and concurrently reduce the CO2 utilization. The optimal operation should have three cycles and the ultimate recovery factor for these three cycles could reach above 30%. The findings of this paper extend the understanding of cyclic CO2 operation and may be employed as technique reference for cyclic CO2 process operations in low-pressure and low-permeability closed boundary reservoirs.