The Effect of Salinity of Injection Brine on Water Alternating Gas Performance in Tertiary Miscible Carbon Dioxide Flooding: Experimental Study

Abstract

Abstract The effect of salinity of the injection brine on Water Alternating Gas (WAG) performance in tertiary miscible carbon dioxide (CO2) flooding is investigated. Coreflood experiments are performed in Berea sandstone core, from which the WAG performance, such as percent oil recovery, tertiary recovery factor, and CO2/Gas utilization factor are determined. The core flooding experiments are conducted at 60 °C and at miscible condition, i.e., 20% above the minimum miscible pressure (MMP) of the oil sample. A model oil, which is a mixture of 50 wt% n-decane and 50 wt% n-hexadecane, and a crude oil from Cottonwood Creek are used. For experiments with model oil, artificial injection brines are made by dissolving NaCl into distilled water with different salinities ranging from 1000 to 32000 ppm (mg/L); artificial brines containing 4000 ppm NaCl and 4000 ppm CaCl2 are also used to investigate the effect of divalent salt in the injection brine on WAG performance. For experiments with Cottonwood Creek oil, artificial brines containing 20000 ppm NaCl and 10000 ppm CaCl2 are used as the connate brines and artificial brines containing 66.67 wt% of NaCl and 33.33 wt% of CaCl2 with salinities ranging from 10000 to 32000 ppm are used as the injection brines. Six alternate cycles of brine and CO2 with a half-cycle slug size of 0.25 pore volumes (PV) and a CO2/water ratio (volume ratio) of 1:1 are injected in every core flood test. Comparisons of WAG and continuous gas injection (CGI) are also made on both model and crude oils. At the same miscible condition, the tertiary recovery factor of WAG is demonstrated to be higher than that of continuous CO2 flooding. The tertiary oil recovery and recovery factor of both model and crude oils are found to increase slightly with the salinity of the injection brine due to the decrease in the CO2 solubility in brines. The CaCl2 in the injection brine is found to have similar effect as NaCl. The increased oil recovery of WAG over water flooding can be higher than 50% of Original Oil in Place (OOIP) for model oil and 35% of OOIP for crude oil.