NUCLEAR MAGNETIC RESONANCE EXPERIMENTS ON ENHANCED OIL RECOVERY BY CO ₂ FLOODING IN TIGHT RESERVOIRS

CO 2 displacement and storage technology has been widely used in the exploration and development of tight oil reservoirs, and has a very impressive effect on oil displacement and CO 2 emission reduction. However, CO 2 injection conditions will inevitably affect the physical parameters and seepage laws of formation fluids, thus affecting the oil displacement effect. Therefore, in this paper, the heterogeneity and complexity of the pore system are characterized by the fractal theory. The effects of different injection conditions on displacement results were analyzed by NMR [Formula: see text] spectrum and 2D NMR images using a high temperature and high pressure NMR on-line monitoring system. The results show that the fractal dimension shows that the pore distribution related to porous pores in tight cores is more complex than that related to adsorption pores. Under the initial CO 2 displacement pressure, there is a starting pressure gradient, and the oil in the pores and micro-pores of the core is produced to different degrees, mainly using the residual oil in the medium and large pores. With the increase of CO 2 displacement pressure, the degree of oil recovery in the pore area of the core increases continuously and the cumulative degree of oil recovery is different. With the increase of displacement speed, the capillary number will increase, and the displacement efficiency will increase rapidly at first, and then increase slowly after reaching a certain value. For the low permeability reservoir, there is an optimal displacement velocity interval. If the displacement velocity is too low, the oil in the low permeability reservoir is not conducive to displacement; if the displacement velocity is too high, the ineffective cycle is formed, and the gas drive development effect becomes worse. Nuclear magnetic resonance technology can deeply study the extent of oil recovery and the distribution of remaining oil in the pores and micro-pores of the core at different CO 2 displacement pressure stages in tight reservoirs, which is of great value for the study of microscopic oil displacement mechanism in tight reservoirs.