Abstract
There is a number of shale gases exsiting in the form of absorbed gas in shale gas reservoir. In the process of the development of gas reservoir, desorption and diffusion of absorbed gas will occur with the dropping of gas reservoir pressure. This will impact the pressure change process of the reservoir. Applied Langmuir isothermal adsorption curves and Fick diffusion law of quasi-steady state, the features of desorption and diffusion of adsorbed gas are charicterized mathematically in this paper. Combined with the principle of gas-water two-phase flow, a mathematical model of gas-water two-phase flow was built up for the shale gas reservoir. The finite difference method was used to get the numerical model and the computer programming was used to work out the analytic solutions of bottomhole pressure. At last, the influences of desorption and diffusion characteristics of adsorbed gas on the variation of gas well bottomhole pressure was analyzed. The results show that the fluid is supplied with adsorption gas, which increases the formation energy in turn. Thus, the decreasing of the bottomhole pressure becomes slower and pressure difference curve moves down. Due to matrix shrinkage and porosity increasing, physical property and formation flow property are improved, which leads to more obvious prophase depression of the pressure difference derivative curve.
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