A Comprehensive Model for Simulation of Gas Transport in Shale Formation with Complex Hydraulic Fracture Geometry

Abstract

AbstractA 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 non-planar fractures with varying fracture width and fracture permeability along fracture length due to very complicated gridding issues, an expensive computational cost, and complexities in development of computational codes. Second, conventional gas mass balance 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 semi-analytical model by dividing fractures into several fracture segments to represent the complex non-planar fracture geometry and present a revised mass balance equation to consider all the important gas transport mechanisms for shale gas reservoirs. We verified this model against a numerical reservoir simulator for planar fracture by considering the effects of gas desorption, non-Darcy flow, and geomechanics separately. Transient flow behavior between planar fractures and non-planar fractures with and without considering the natural fractures was compared. Furthermore, a well with actual production data from Marcellus shale was analyzed. First, we use a three-dimensional fracture propagation model to generate more-realistic non-planar fracture geometry. Then, we apply the fracture geometry into a semi-analytical model to evaluate well performance. Contributions of each mechanism to gas recovery are then examined. Based on history matching and production forecasting, the difference of cumulative gas production between the realistic non-planar fractures and ideal planar fractures is about 20% at 30 years of production. In addition, the contribution of flow mechanisms such as gas slippage, gas diffusion, and gas desorption to gas recovery at 30 years of production compared to that without considering them is about 13%, 17%, and 22%, respectively. Totally, the contribution of all the important mechanisms is about 52%. Hence, modeling of gas production from the realistic non-planar fractures as well as modeling the important gas transport mechanisms in shale gas reservoirs is significant.This work, for the first time, combines a complex non-planar fracture geometry with varying fracture permeability and the important gas transport mechanisms to efficiently analyze field production data from Marcellus shale. The model can provide significant insights into understanding the impacts of gas slippage, gas diffusion, gas desorption, and non-planar fracture geometry on long-term gas recovery prediction.