Characteristics of the pressure and pressure derivative response of the CBM well based on the outer boundary condition with parallel faults

Abstract

In general conditions, there are faults developing in CBM (coalbed methane) reservoirs. Because the faults affect the dynamic response of the pressure and pressure derivative of gas-oil wells, research examining the pressure and pressure derivative response of CBM wells has become a difficult problem. The methods to solve seepage models of CBM reservoirs are also numerous and complicated. In this paper, a seepage model of CBM reservoir with an outer boundary condition of parallel faults is studied using both the CBM seepage model with infinite outer boundary and the mirror solution (the solution of the image well) of the CBM seepage model on parallel faults via the superposition principle. First, a seepage model of CBM with infinite outer boundary is built and solved by the Similar Constructive Method. Next, using the mirror reflection principle, the mirror solution of the CBM model on the parallel fault boundary is obtained. Finally, using the superposition principle of the mirror solution and similar structure solution of the CBM seepage model with infinite outer boundary, the solution of the CBM seepage model with the parallel fault boundary condition is obtained. After Laplace inversion, the diagnostic curves of log-log curve, semilog curve, Primary Pressure Derivative (PPD) curve and Second Logarithmic Pressure Derivative (SLPD) curve of the model are drawn. Next, the characteristics of the pressure and pressure derivative response of the CBM well are obtained based on the boundary conditions with three types of parallel faults. The solution method to solve the CBM seepage model can reduce the threshold and difficulty of the model solution. Four types of model diagnostic curves can replenish the diagnostic tools of the effect of the parallel fault boundary on the CBM well to analyze the effect of the parameters of the parallel fault boundary on the wellbore pressure and pressure derivative of CBM wells.