Fractures always play a significant role in tight oil reservoir development. Due to the high conductivity, the efficiency of mass transfer between fracture and matrix determines the ultimate development effectiveness especially during gas injection process. Convection and diffusion in fracture have been recognized as two dominant mechanisms affecting the mass transfer. In current studies, many authors illustrate this process by reducing the fracture to a lower-dimensional structure and imposing inconsistent boundary conditions on fracture surface. However, it hardly reveals the continuity of mass transfer between fracture and matrix in this kind of simplification.
In this paper, we studied the effect of convection and diffusion in fractures on the fracture-matrix mass transfer as well as the final recovery by developing a coupled model considering mass transfer continuity on the fracture wall. The model accounts for convection and two dimensional diffusions in the fracture, vertical convection and diffusion in the matrix. The interaction between matrix and fracture is expressed by exposing a consistent boundary condition. Subsequently, a numerical method was applied to solve the model. The model was validated by available experimental data. And then we compared the results generated by model under inconsistent boundary condition. Finally, we discussed the effect of injection rate and matrix permeability on convection-diffusion mass transfer efficiency.
The results show that convection and two dimensional diffusions in fracture promoted material transport between fracture and matrix during CO2 injection process. And convection in fracture favors transversal vertical diffusion both in fracture and matrix, which is the main mechanism for mass transfer in fracture-matrix system. In addition, with decreasing matrix permeability, transverse vertical diffusion in matrix dominated in mass transfer gradually.
The proposed model sheds some light on mechanism of mass transfer during gas injection in tight oil reservoirs. The method provides EOR practice for reference.