A Numerical Study of Transport and Storage Effects for Tight Gas and Shale Gas Reservoir Systems
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AbstractVarious analytical, semi-analytical, and empirical models have been proposed to characterize rate/pressure behavior as a function of time in tight/shale gas systems featuring wells of various completion designs. Despite a few analytical models as well as a small number of published numerical studies there is currently little consensus regarding the importance of the various transport and storage mechanisms on performance over time and whether or not reservoir properties/volume can be estimated from well performance data.We constructed a fit-for-purpose numerical simulator which accounts for a variety of production features pertinent to these systems, specifically: hydraulically fractured horizontal wells possessing induced fractures with Forchheimer flow, stress dependent properties, multicomponent diffusive and Knudsen flow and desorption, and the presence of water, which introduces issues of relative permeability and wettability, as well as gas solution and phase change. These features cover several of the production and storage mechanisms which are currently believed to be relevant in tight/shale gas systems.We employ the numerical simulator to examine various tight/shale gas systems and to study the effects of reservoir and completion parameters on performance.We employ rate and auxiliary functions to illustrate the impact of water, desorption, thermal, and non-Darcy flow effects on performance. The relationship between these factors and the change in produced gas composition with time is explored.We explore variations of possible shale gas system models. Based on non-Darcy flow in the microscale and inertial regimes (with/without desorption), we show that due to the extremely low permeability of shale and various intrinsic permeability reducing effects, the flow behavior is dominated by the fracture system configuration and extent.This work expands our understanding of transport and storage mechanisms in tight gas and shale gas systems and their impact on performance, where such an understanding may ultimately be used to estimate reservoir properties and reserves in these types of reservoirs.
