Shale Gas Well Total Fracture Surface Area Calculation Re-Visited for a Dynamic Formation Permeability
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AbstractShale gas wells exhibit formation linear flow, which is in most cases the only transient flow regime available for engineering analysis. Production Rate Transient Analysis (RTA) methods have been developed to analyze the transient flow into shale gas well. These methods often assume homogeneous reservoir properties and neglect the dynamic nature of the formation. In shale gas reservoirs, however, the properties could change; in particular, the permeability due to stress-dependence of the formation and non-Darcian effects. In this work, a theoretical study is conducted to show the range of errors on the predicted total fracture surface area caused by the constant permeability assumption. The study proposes a modified RTA model, which accounts for dynamic permeability to eliminate the error in calculated area.Results from the sensitivity analysis show that the error on surface area calculation ranges from 1% 323% due to the constant permeability assumption. The total fracture surface area calculation is most sensitive to the geomechanical (stress-sensitive) parameters, which affect the permeability in the region away from fractures. Results show that the error in area is reduced when the initial value of the dynamic permeability is large. Sorption parameters are the second most influential category affecting the surface area calculation. Molecular diffusion of the adsorbed and free gas molecules enhance permeability near the fractures, wherein the slightest variations in dynamic permeability can cause the error in area to change significantly. For these parameters, higher permeability near the fractures translates to higher error in area.
