This study examines the impact of methane molecular diffusion for different geometries in shale gas reservoirs. In shale gas formations matrix is composed of inorganic and kerogenic parts. Kerogenic material in shales is the source of methane molecules. Gas is stored as free and desorbed gas, and as dissolved gas in the body of the kerogen. The rate of gas mass flux is different in these processes. Although methane-molecular diffusion is low in kerogen, mass flux is considerable, owing to the small distances that the dissolved molecules need to travel to reach the pore spaces. After having inspected published SEM images of the kerogenic material of different shale strata, we used two models (linear and radial) to calculate mass flux from the body of the kerogenic material into pore spaces. We then calculated mass flux using the numerical solution to the second Fick’s law. Inward diffusion from the kerogen body and outward diffusion from the kerogen external boundary were evaluated separately. In addition, we studied the dual pore model and effect of natural fractures for different geometries. Results show that it is necessary to know the geometry and direction of flow, which strongly affect diffusion flux. We concluded that the geometric type, alignment of flow and fractures have considerable effect on shale gas recovery.
July 3, 2014
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