Simulation Study on the Displacement Characteristics of an Ultra-Low Permeability Reservoir by CO2-WAG Flooding

The oil production rate of the ultra-low permeability reservoir decreases rapidly after volume fracturing, and reservoir energy replenishment is in urgent need to enhance the production performance. However, due to the poor reservoir physical property, water injection into the ultra-low permeability reservoir is quite difficult, resulting in unsatisfactory development effect. To evaluate the effectiveness of water alternating CO2 (CO2-WAG) flooding in ultra-low permeability reservoir block Z, oil samples from a typical well were used to carry out laboratory experiments, including oil composition analysis, constant composition expansion and CO2 swelling test. After fitting the experimental results, a numerical simulation model containing four five-point patterns developed by CO2-WAG flooding was established. The production and seepage characteristics of CO2-WAG flooding were studied, and the parameter sensitivity analysis was conducted. The results showed that: (1) CO2-WAG flooding could maintain a longtime stable production with high oil rate, and significantly improve the production effect of the ultra-low permeability reservoir; besides, dynamic CO2 storage could be achieved with the storage percent of 70% during the flooding process. (2) CO2-WAG flooding has the following effects, including replenishing reservoir energy, expanding sweeping volume and improving oil displacement efficiency, and the development effect is obviously better than that of water flooding. (3) When the CO2-WAG flooding was implemented immediately at the reservoir starting production with 6-month alternating cycle and 1:1 CO2-Water ratio, good production performance could be achieved. The results obtained from this research can provide guidance in effectively developing block Z.