A Comprehensive Model for Simulation of Gas Transport in Shale Formation With Complex Hydraulic-Fracture Geometry
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SummaryA comprehensive model for simulating gas transport in shale formation with complex fracture geometry is still lacking in the petroleum industry. First, the current models are challenging to efficiently handle the complex nonplanar fractures with varying fracture width and fracture permeability along fracture length caused by very-complicated gridding issues, a high computational cost, and complexities in development of computational codes. Second, the conventional gas-diffusivity equation needs to be revised to include all important gas-transport mechanisms such as gas slippage, gas diffusion, and gas desorption. Hence, the goal of this study was to develop a new model to fill this gap.We present an efficient semianalytical model by dividing fractures into several fracture segments to represent the complex nonplanar-fracture geometry and present a revised diffusivity equation to consider all the important gas-transport mechanisms for shale-gas reservoirs. We verified this model against a numerical reservoir simulator and an analytical solution for a single planar fracture. Transient-flow behavior between planar fractures and nonplanar fractures with and without considering the natural fractures was compared. Furthermore, a well with actual production data from the Marcellus Shale was analyzed. First, we use a complex hydraulic-fracture-propagation model to generate nonplanar-fracture geometry. Then, we apply the fracture geometry in the semianalytical model to evaluate well performance. Contributions of each mechanism to gas recovery are examined. By use of history matching and production forecasting, the difference of cumulative gas production between the nonplanar fractures and the converted planar fractures with equal fracture length is approximately 12% at 30 years of production. In addition, the contribution of gas diffusion, gas slippage, and gas desorption to gas recovery at 30 years of production compared with that without considering them is approximately 11, 16, and 25%, respectively. In total, the contribution of these three important mechanisms is approximately 56%. Hence, modeling of gas production from the nonplanar fractures and modeling the important gas-transport mechanisms in shale-gas reservoirs are significant.
