Nuclear magnetic resonance (NMR) was used to investigate the exposure between CO 2 and matrix with permeability of 0.218 mD at 40°C and 12 MPa. Before NMR experiment, the core was saturated with oil. To investigate the effects of exposure time on EOR, the saturated core was exposed to CO 2 and T2 test was continuously performed with NMR system until the obtained T2 spectrum was unchanged. After the first exposure, CO 2 and matrix reached equilibrium state. The second exposure started when CO 2 injection was under a constant pressure of 12 MPa and at a constant rate to keep fresh CO 2 in system. The procedure of T2 test was unchanged. The third and fourth exposures were conducted in sequence. The results showed that (1) Oil in all pores can mobilize as exposure time increases. (2) The recovery is 46.6% for oil in pores with the diameter of pore larger than 1 m, this result is higher than the recovery (12.8%) for oil in pores with the diameter of pore smaller than 1 m. (3) Recovery can be divided into two stages according to the exposure time: a fast-growing stage and a slow-growing stage. (4) Initially, the oil exists in pores with maximum radius of 21 m in the originally saturated core. After CO 2 injection, oil flows to pores with radius greater than 21 m, suggesting that oil in tight matrix ЉdiffusesЉ to the surface of core with exposure between CO 2 and matrix. (5) The final recoveries of 1 st , 2 nd , 3 rd , 4 th exposure experiments are 23.7%, 7.2%, 2.6% and 1.5%, respectively.