The dynamic convection-diffusion effect of CO2 Huff-n-Puff on Pre-Darcy flow behavior

Abstract

Many reports have presented that in tight matrix, oil phase obeys Pre-Darcy flow. However, for the CO2 Huff-n-Puff development, it is an outstanding challenge to understand the changing oil properties could influence the nonlinear degree in the Pre-Darcy percolation process. As the dynamic Pre-Darcy flow model has been presented for describing the nonlinear degree variation, the dynamic convection-diffusion effect of the injected gas on the percolation behavior has become a key issue. In this paper, based on the fluid-rock experiments of the studied area, the near miscible system of CO2-injected mixtures in nanopores is firstly investigated. Then, the dynamic Pre-Darcy flow numerical simulation is launched for gas diffusion coefficient evaluation, convection-diffusion effect on Pre-Darcy flow behavior and field development program optimization. The results indicate that considering the dynamic change of the nonlinear degree in CO2 injection process, the diffusion coefficient for the studied area is set as 0.00093 cm2/s by fitting the experiment and field data. Additionally, based on the detailed discussion on the DDPC, pore pressure, and oil saturation, it could be seen the simulation prediction accuracy will reduce without linking this gas convection-diffusion effect. Finally, for maximizing the field oil recovery in two Huff-Puff cycles, the optimal re-opened time with this effect is when the near-well pressure drops to 14.7 MPa. This investigation could provide further insight into the gas enhanced oil recovery of the Pre-Darcy flow.