Nuclear magnetic resonance study of CO2 flooding in tight oil reservoirs: Effects of matrix permeability and fracture

The implementation of CO2 enhanced oil recovery (EOR) projects for tight reservoir development has the dual effects of economic benefits and carbon storage. Matrix permeability and fractures in tight reservoirs have a significant impact on the performance of CO2 flooding. In this study, CO2 flooding experiments were carried out using natural tight cores with different matrix permeabilities and artificial fracture conditions. The pore size range variation of oil-producing pores and the distribution of residual oil were explored by using nuclear magnetic resonance (NMR) technique. The results indicated that the increase of matrix permeability was beneficial to increase oil recovery, but it also reduced the displacement pressure and thus weaken the ability of CO2 diffusion into small pores. Specifically, with the matrix permeability increased from 0.1 to 0.2 mD to 0.4–0.5 mD and 0.8–0.9 mD, the oil recovery of fracture-free tight cores increased from 15.59% to 19.78% and 24.73%, and the pore size range of oil-producing pores decreased obviously. Conversely, the fractures aggravate the channeling flow of CO2 and reduce the swept area but facilitates the recovery of crude oil from smaller pores. In tight cores of 0.8–0.9 mD with single fracture and multiple fractures, the oil recovery decreased to 21.3% and 20.12%, but the oil-producing pore size range expands from 0.2-6 μm to 0.05–6 μm and 0.04–6 μm. The NMR imaging results showed that the residual oil content gradually increased from the injection end to the production end, and the distribution of the residual oil in the core with high permeability and non-fracture was more even.