Investigation of Cyclic CO_2 Injection Process with Nanopore Confinement and Complex Fracturing Geometry in Tight Oil Reservoirs

摘要

Carbon dioxide is one of the capable processes in improving oil recovery from low-permeable oil reservoirs. However, the evaluation of cyclic $$text {CO}_{2}$$ CO 2 injection process with nanopore confinement effect in tight reservoirs is deficient in the oil industry. The nanopores confinement plays a significant role in low-permeable formations, creating a high capillary pressure which substantially influences fluid phase behavior and fluid flow in tight reservoirs. In order to consider the effect of nanopore confinement in unconventional reservoirs, the conventional methods need to be modified. Thus, we evolve an effective model for cyclic $$text {CO}_{2}$$ CO 2 injection process considering the effect of complex fracture geometry with nanopore confinement on the production from tight formations. Firstly, the reservoir fluid properties with nanopore confinement are estimated. Secondly, the minimum miscibility pressure is measured and verified with experimental data. Lastly, the well performance of cyclic $$text {CO}_{2}$$ CO 2 injection process in unconventional tight reservoirs is evaluated based on few parameters such as $$text {CO}_{2}$$ CO 2 diffusivity and matrix permeability. The results show that the combined effect of capillary pressure and $$text {CO}_{2}$$ CO 2 dispersion has significantly influenced the oil production efficiency from tight oil reservoirs. The oil recovery factor of cyclic $$text {CO}_{2}$$ CO 2 injection process at five years boosts from 14.67% as primary recovery to 22.54% due to $$text {CO}_{2}$$ CO 2 molecular diffusion effect with overall 7.87% incremental recovery. Moreover, the incremental oil recovery of cyclic $$text {CO}_{2}$$ CO 2 injection process is increased further by 1.7% while taking into consideration the combine effect of capillary pressure and $$text {CO}_{2}$$ CO 2 diffusion on the oil recovery. This study provides an appropriate method for predicting the production of cyclic $$text {CO}_{2}$$ CO 2 injection process with a complex fracture geometry, considering the capillary pressure and $$text {CO}_{2}$$ CO 2 diffusion on well performance of tight oil reservoirs.