Unstable seepage modeling and pressure propagation of shale gas reservoirs

Pressure disturbance propagation was investigated using the steady state replacement method, the relationship between moving boundary and time was obtained. An unstable seepage model in shale gas reservoirs was established considering the effects of desorption, diffusion, slip and moving boundary. Using Laplace transform, the pressure characteristics equation was solved for the condition of internal boundary being constant production and outer boundary being the moving boundary. Subsequently, combining the parameters of shale gas in southern China, unstable seepage pressure characteristics and its influence factors of shale gas reservoir were analyzed using MATLAB software. The results indicate that the pressure propagation is characterized by moving boundary effect during shale gas exploitation, which means that moving boundary is propagated outwards with the propagation velocity decreasing gradually. Under the effect of moving boundary or shale gas desorption, the pressure propagation velocity decreases and the reservoir pressure drop slows down. With the increasing of the diffusion coefficient, the reservoir pressure drop slows down and the effect of diffusion coefficient decreases gradually. In the process of gas reservoir exploitation, diffusion and slip contribute more and more to gas production, acting as the dominant factors, while the contribution of flow and desorption level off after decreasing.