Nuclear-Magnetic-Resonance Study on Mechanisms of Oil Mobilization in Tight Sandstone Reservoir Exposed to Carbon Dioxide

Nuclear magnetic resonance (NMR) was used to investigate the exposure between carbon dioxide (CO2) and the sandstone matrix with a permeability of 0.218 md and a porosity of 9.5% at 40 degrees C and 12 MPa (immiscible condition). Minimum miscibility pressure (MMP) between oil and CO2 was 17.8 MPa, determined by slimtube test at 40 degrees C. The exposure process between CO2 and the sandstone matrix included first, second, third, and fourth exposure experiments. Before each exposure experiment started, there was a CO2-injection stage with a CO2 injection under a constant pressure of 12 MPa and at a constant rate to keep fresh CO2 (concentration of CO2 is 100% in gas phase) in the system. Each exposure experiment ended when the obtained T-2 spectrum was unchanged (total amount of oil in tight matrix remains constant). These processes were similar to CO2 huff 'n' puff. The results showed that (1) oil in all pores could mobilize as exposure time increases in the first exposure experiment. (2) The total original-oil-in-place (OOIP) recovery is 46.6% for oil in big pores (29 ms < T-2 <= 645 ms)-this result is higher than the recovery (12.8%) for oil in small pores (T-2 <= 29 ms). (3) Oil is mobilized fast in the initial exposure hours, and then the rate drops gradually until no more oil is produced. (4) Initially, the oil exists in pores with maximum relaxation times of 645 ms in the originally saturated core. After the CO2 injection, oil flows to pores with relaxation times slower than 645 ms, suggesting that oil in tight matrix is mobilized to the surface of core by swelling caused by CO 2 diffusion. (5) The final OOLP recoveries of first, second, third, and fourth exposure experiments are 23.7, 7.2, 2.6, and 1.5%, respectively, and they decline exponentially. Oil mobilization in a tight-sandstone reservoir exposed to CO2 was observed by NMR T-2 spectra under multiple exposure experiments. Mechanisms of oil mobilization were investigated (i.e., oil swelling, concentration-driven diffusion of hydrocarbons, and extraction of light components). The CO2 enhanced oil recovery (EOR) with multiple injections under immiscible conditions is acceptable and satisfactory in a tight-sandstone reservoir. CO2 huff 'n' puff with optimized injection, soaking, and production process is an economic development method in a tight sandstone reservoir.