An Experimental Study of CO2-low Salinity Water Alternating Gas Injection in Sandstone Heavy Oil Reservoirs

Several studies have shown that oil recovery significantly increases by low salinity water flooding (LSWF) in sandstones. However, the mechanism of oil recovery improvement is still controversial. CO2 that develops buffer in the presence of water is expected as a deterrent factor in LSWF efficiency based on the mechanism of interfacial tension reduction due to pH uprising. No bright evidence in literature supports this idea. Herein, a set of core floods including a pair of CO2 water alternating gas (WAG) and a pair of water injection tests were conducted and the efficiency of LSWF and high salinity water flooding (HSWF) was compared for each pair. HSWF was followed by LSWF in tertiary mode. The results showed that not only did not CO2 deteriorate LSWF recovery efficiency, it improved recovery, because CO2-low salinity WAG showed the best performance among the other types at a constant pore volume injected. The positive results in both secondary and tertiary modes with Kaolinite free samples used herein showed that Kaolinite release was not the critical phenomenon in LSWF brisk performance. In addition, different pressure behaviors of CO2 WAG processes in comparison with the reported behavior of LSWF proves that LSWF performance may not depend on how pressure changes through flooding.