Shale Gas Seepage And Numerical Simulation

The researches of shale gas molecular adsorption-desorption phenomenon, diffusion mechanism, nonlinear seepage theory and numerical simulation technology are the basis and precondition for effective developing shale gas reservoir. At present, the traditional seepage theory ignores the shale gas adsorption phenomenon, slippage and diffusion mechanism, which causes the numerical simulation technology based on the traditional seepage theory-difficult to predict the development dynamic. The researches of shale gas accumulation and migration mechanism and numerical simulation technology, can explore the shale gas inherent seepage law in different scale pore and micro cracks, create conditions and provide guidance for improving the gas rate and ultimate recovery, which will be of great significance for the effective and reasonable development of shale gas reservoir.In this paper, according to the characteristics of shale gas in adsorption-desorption, diffusion, slippage and seepage during accumulation, migration and production, a mathematical model of unstable seepage in dual-porosity sealed shale gas reservoir is developed while considering Knudsen diffusion, slip flow effect and Langmuir desorption effect. The effects of several parameters on the pressure dynamics are discussed. Eclipse software for numerical simulation research of shale gas reservoir is used for sensitivity analysis of the impact of gas production respectively on the reservoir parameters and the horizontal well fracturing parameter. Through the orthogonal experimental range method, sensitivity tests are performed to identify the influential parameters on the gas production of primary and secondary order effects.Main research results obtained in this paper are as follows:1. According to the characteristics of shale gas in adsorption-desorption. diffusion, slippage and seepage during accumulation, migration and production, a mathematical model of unstable seepage in dual-porosity sealed shale gas reservoir is developed while considering Knudsen diffusion, slip flow effect and Langmuir desorption effect. By solving the model utilizing the Stehfest numerical inversion and computer programming in Laplace space, several typical curves of bottom-hole pressure are obtained. The effects of several parameters on the pressure dynamics are discussed.2. From the influence of shale gas reservoir sensitivity parameters on the pressure dynamics, we can conclude that the pressure change of unsteady seepage in vertical wells in sealed shale gas reservoir can be divided into three stages:①Early wellbore storage;①Transition stage:matrix-fracture coupling factor appears. Concavity of the curve appears in this stage;③The radial flow of dual porosity system:this stage presents the homogeneous characteristics of formation. Pressure derivative curve shows a horizontal line, whose value is0.5.3. The changes of Langmuir volume and Langmuir pressure associated with desorption and adsorption effect are the internal causes of the storativity ratio change; the desorbed gas extends the time for fluid to flow from matrix system to fracture system; when the tangential momentum accommodation coefficient decreases, the time for pressure wave to spread to the border reduces; matrix-fracture coupling factor determines the occurrence time of the transition stage; boundary range restricts the time for pressure wave to spread to the border.4. The numerical simulation researches of shale gas reservoir production performance show that the influence of the fracture permeability to the gas production is strong, by increasing permeability of fracture system, the gas production of horizontal well shows a sharp rise; matrix-fracture coupling factor, gas diffusion coefficient and matrix sub-grids numbers on the sensitivity of the gas production appears only within a specific interval, beyond the particular interval, the three on the sensitivity of gas production will become very small; the influence of the adsorption gas content, Langmuir pressure and rock compaction to the gas production is weak.5. Through the orthogonal experimental range method, sensitivity tests are performed to identify the influential parameters on the gas production of primary and secondary order effects:primary hydraulic fracture spacing, matrix sub-grids numbers, primary hydraulic fracture conductivity, reservoir fracture permeability, primary hydraulic fracture half-length, matrix-fracture coupling factor, gas diffusion coefficient, adsorption gas content, Langmuir pressure, rock compaction.