Reservoir Modeling in Shale-Gas Reservoirs

Abstract

Summary The exploitation of unconventional gas reservoirs has become an ever increasing component of the North American gas supply. The economic viability of many unconventional gas developments hinges on effective stimulation of extremely low-permeability rock by creating very complex fracture networks that connect huge reservoir surface area to the wellbore. In addition, gas desorption may be a significant component of overall gas recovery in many shale-gas reservoirs. The widespread application of microseismic (MS) mapping has significantly improved our understanding of hydraulic fracture growth in unconventional gas reservoirs (primarily shale) and has led to better stimulation designs. However, the overall effectiveness of stimulation treatments is difficult to determine from MS mapping because the location of proppant and the distribution of conductivity in the fracture network cannot be measured (and are critical parameters that control well performance). Therefore, it is important to develop reservoir-modeling approaches that properly characterize fluid flow in and the properties of a complex fracture network, tight matrix, and primary hydraulic fracture (if present) to evaluate well performance and understand critical parameters that affect gas recovery. This paper illustrates the impact of gas desorption on production profile and ultimate gas recovery in shale reservoirs, showing that in some shale-gas reservoirs desorption may be a minor component of gas recovery. In addition, the paper details the impact of changing closure stress distribution in the fracture network on well productivity and gas recovery. In shale-gas reservoirs with lower Young's modulus rock, stress-dependent network-fracture conductivity may reduce ultimate gas recovery significantly. The paper includes an example that contrasts the application of numerical reservoir simulation and advanced decline-curve analyses to illustrate issues associated with conventional production-data-analysis techniques when applied to unconventional reservoirs. Selected examples from the Barnett shale are included that incorporate MS fracture mapping and production data to illustrate the application of production modeling to evaluate well performance in unconventional gas reservoirs. This paper highlights production modeling and analysis techniques that aid in evaluating stimulation and completion strategies in unconventional gas reservoirs.