document preview

Abstract
Various models featuring horizontal wells with multiple fractures have been proposed to characterize flow behavior over time in tight and shale gas systems. Currently, only very little is known about the effects of nonideal fracture patterns and coupled primary-secondary fracture interactions on reservoir performance in unconventional gas reservoirs.

We developed a 3D Voronoi mesh-maker that provides the flexibility to accurately represent various complex and irregular fracture patterns. A numerical model was developed based on such fracture concepts to assess the potential performance of unconventional gas reservoirs. We conducted simulations using up to a half-million cells and considered production periods that are orders of magnitude longer than the expected life of wells and reservoirs. Our aim is to account for a wide range of flow regimes that can be observed in irregular fracture patterns, and to fully assess even slight nuances in flow behavior.

We investigated coupled primary-secondary fractures, with multiple vertical hydraulic fractures intersecting horizontal secondary "stress-release" fractures. We studied irregular fracture patterns to show the effect of fracture angularity and nonplanar fracture configurations on production. The results indicate that the presence of high-conductivity secondary fractures results in the highest increase in production, while, contrary to expectations, strictly planar and orthogonal fractures yield better production performance than nonplanar and nonorthogonal fractures with equivalent propped fracture lengths.

Introduction
Various analytical and semi-analytical solutions have been proposed to model flow in shale-gas and tight-gas reservoirs. Gringarten (1971) and Gringarten et al., (1974) developed some of the early analytical models for flow through domains involving a single vertical fracture and a single horizontal fracture, while more accurate semi-analytical models for single vertical fractures were developed much later (Blasingame and Poe Jr., 1993). Prior to the development of models for multiply-fractured horizontal wells (Medeiros et al., 2006), it was common practice to represent these multiple fractures with an equivalent single fracture.

Several other analytical and semi-analytical models have been developed since Bello and Wattenbarger (2008); Mattar (2008); Anderson et al., (2010). Although these models are much faster than numerical simulators, they generally cannot accurately handle the very highly nonlinear aspects of shale-gas and tight-gas reservoirs because these analytical solutions address the nonlinearity in gas viscosity, compressibility and compressibility factor with the use of pseudo-pressures (an integral function of pressure, viscosity and compressibility factor) rather than solving the real-gas flow equation. Other limitations include the difficulty in accurately capturing gas desorption from the matrix, multiphase flow, multidimensional heterogeneities, unconsolidation, and several non-ideal and complex fracture networks (Houze et al., 2010).