Numerical simulation of water alternating gas flooding (WAG) using CO2 for high-salt argillaceous dolomite reservoir considering the impact of stress sensitivity and threshold pressure gradient

Recently, special attention has been drawn to the high-salt argillaceous dolomite reservoir in Jianghan basin, China due to its abundant oil resources with complex geological conditions. However, the current production methods used to develop this reservoir are not efficient with low throughput. Hence, the purpose of this paper is to explore the main possible reasons affecting the oil well productivity and determine suitable field development method for the target oilfield through experimental and numerical simulation approaches. The mercury injection capillary pressure (MICP) test results indicate that the average pore radius of samples is 0.0144 μm, and pore size distribution (PSD) results from MICP indicate that single peaks appeared in the PSD curves of the pore type samples. Permeability peaks of the samples range from 0.01 to 0.03 μm, and mesopores make most of the permeability contribution of the samples, which also reveals the low-permeability and porosity characteristics of the target oilfield. The results of core stress sensitivity tests show that the largest permeability damage rate is 98.97% with an average permeability damage rate of 98.37% while the maximum porosity damage rate is less than 10% with an average damage rate of 5.57% during the process of depressurization. The results of Non-Darcy percolation experiment indicate that threshold pressure gradient testing by water is about 0.03 MPa/m, which is close to the actual situation of the target oilfield. Numerical simulation method that considers the impact of threshold pressure gradient and stress sensitivity were established to study flooding patterns deployment for the research area. The simulation results show that performing water alternating gas flooding (WAG) by using CO2 is recommended for the development of the high-salt reservoir with inverse five-spot flooding pattern made up of 400 m well spacing in X-direction and 150 m row spacing.