Various 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 a horizontal well with multiple hydraulic fractures. Despite a few analytical models as well as a small number of published numerical studies there is currently little consensus regarding the large-scale flow behavior over time in such systems, particularly regarding the dominant flow regimes 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: ultra-tight matrix permeability, hydraulically fractured horizontal wells, multiple porosity and permeability fields, and desorption. These features cover the production 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 identify/illustrate the various flow regimes which progressively occur over time. We visualize the flow regimes using both specialized plots of rate and pressure functions, as well as maps of pressure distributions.
We use pressure maps to demonstrate the various flow regimes and their transitions in tight gas systems. In a typical tight gas system, we illustrate the initial linear flow into the hydraulic fractures (i.e., formation linear flow), transitioning to compound formation linear flow, and eventually transforming into elliptical flow.
We explore variations of possible shale gas system models. Based on diffusive flow (with/without desorption), we show that due to the extremely low permeability of shale, the flow behavior is dominated by the extent of and configuration of the fractures.
This work expands our understanding of flow behavior in tight gas and shale gas systems, where such an understanding may ultimately be used to estimate reservoir properties and reserves in these types of reservoirs.