Laboratory study on oil recovery characteristics of carbonated water huff-n-puff process in tight cores under reservoir condition

Although CO2 huff-n-puff process has been widely employed as an efficient oil recovery and carbon storage technique in unconventional reservoirs, it suffers from the disadvantages of lack of availability near a large field and formation leakage due to the high CO2 mobility. Carbonated water injection (CWI) has been extensively studied as the alternative of CO2 injection process, but the laboratory studies concerning on CW huff-n-puff process is scarcely reported. In this paper, the oil recovery characteristics of CW huff-n-puff process has been scrutinized under reservoir conditions of 28 MPa and 65 °C together with NMR tests to reveal the oil mobilization rules in the tight cores. Comparative studies on CO2 huff-n-puff and water huff-n-puff process were also performed. It is found 4 rounds of CW huff-n-puff processes could recover considerable amount of 36.2% of the oil from the tight core. Although the recovery rate is lower than 55.4% for CO2 process but it is much higher than 14.4% of the water process. NMR T2 results reveal the oil in the large size pores of the tight cores are mobilized and recovered in early huff-n-puff cycles, whereas the oil from small size pores are collected in the later stage of the CW and CO2 huff-n-puff processes. The research results indicate the CW huff-n-puff process is promising for tight reservoir development.