A dynamic model of CO2 diffusion coefficient in shale based on the whole process fitting

Abstract

CO2 diffusion is of great importance for enhanced oil recovery (EOR), production prediction, and performance forecast of carbon capture and storage (CCS) in shale reservoirs. This work aims to determine the diffusion coefficient of the shale reservoir during CO2 huff and puff development. Firstly, combining pressure decay experiments and mathematical modeling studies, this paper developed a dynamic method to determine the diffusion coefficients of CO2 in cores under shale reservoir conditions. Based on Fick's law, MATLAB software was used to build the fitting chart under different diffusion coefficients. The diffusion coefficient was determined by matching experimental data. During the experiment, the reservoir's pore pressure and confining pressure were simulated, and a saturation-depletion-diffusion experiment method was established to simulate the natural reservoir conditions. The effects of different initial pressures on diffusion were compared in pressure decay experiments. The mathematical model of the diffusion coefficient of CO2 was established, and the fitting chart of diffusion coefficient considering time step was derived. The diffusion distance of different diffusion coefficients and different times was compared. The results show that the diffusion coefficient of CO2 in shale is between 5*10-8 ~ 5*10-7 cm2/s, and the diffusion rate is 0.28 cm/d under the condition of 20 MPa and 319.55 K. In addition, when the diffusion coefficient increases by one order of magnitude, the diffusion distance increases by 200%.