THE EFFECT OF SALINITY OF INJECTION BRINE ON WATER ALTERNATING GAS PERFORMANCE IN TERTIARY MISCIBLE CARBON DIOXIDE FLOODING: EXPERIMENTAL STUDY

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 (oil recovery, tertiary recovery factor, and CO2 utilization factor) are determined. Coreflood experiments are conducted at 60oC and miscible condition. 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 the experiments with model oil, artificial injection brines are made by dissolving NaCl into distilled water with different salinities ranging from 1000 to 32000ppm (mg/L); artificial brines containing 4000ppm NaCl and 4000ppm CaCl2 are also used to investigate the effect of divalent salt in the injection brine. Artificial brines containing 20000ppm NaCl and 10000ppm CaCl2 are used to mimic connate brines, and artificial injection brines containing 66.67wt% of NaCl and 33.33wt% of CaCl2 with salinities ranging from 10000 to 32000ppm are used in experiments with Cottonwood Creek oil. 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. 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 the model oil and the crude oil 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. Comparisons of WAG and continuous gas injection are also made on both model oil and crude oil.