Gas Flow Tightly Coupled to Elastoplastic Geomechanics for Tight- and Shale-Gas Reservoirs: Material Failure and Enhanced Permeability

Abstract

We investigate coupled flow and geomechanics in gas production from extremely low permeability reservoirs such as tight and shale gas reservoirs, using dynamic porosity and permeability during numerical simulation. In particular, we take the intrinsic permeability as a step function of the status of material failure, and the permeability is updated every time step. We consider gas reservoirs with the vertical and horizontal primary fractures, employing the single and dynamic double porosity (dual continuum) models. We modify the multiple porosity constitutive relations for modeling the double porous continua for flow and geomechanics. The numerical results indicate that production of gas causes redistribution of the effective stress fields, increasing the effective shear stress and resulting in plasticity. Shear failure occurs not only near the fracture tips but also away from the primary fractures, which indicates generation of secondary fractures. These secondary fractures increase the permeability significantly, and change the flow pattern, which in turn causes a change in distribution of geomechanical variables. From various numerical tests, we find that shear failure is enhanced by a large pressure drop at the production well, high Biot's coefficient, low frictional and dilation angles. Smaller spacing between the horizontal wells also contributes to faster secondary fracturing. When the dynamic double porosity model is used, we observe a faster evolution of the enhanced permeability areas than that obtained from the single porosity model, mainly due to a higher permeability of the fractures in the double porosity model. These complicated physics for stress sensitive reservoirs cannot properly be captured by the uncoupled or flow-only simulation, and thus tightly coupled flow and geomechanical models are highly recommended to accurately describe the reservoir behavior during gas production in tight and shale gas reservoirs and to smartly design production scenarios. IntroductionUnconventional natural gas such as tight and shale gas has become an increasingly important source of natural gas in the United States over the past decade and the estimates of the natural gas resource potential for shale gas range from 14.16×10

An Independent Scientific Assessment of Well Stimulation in California Volume II: Potential Environmental Impacts of Hydraulic Fracturing and Acid Stimulations

“…This paper, however, also stated that earlier work found orientations dependent on the unique stress profiles and rock fabric of a given location (Walker et al, 2002). Coupled flow-geomechanical modeling (Kim and Moridis, 2012) found inherent physical limitations to the extent of fracture propagation-for example, the presence of overlying confining formations may slow or stop fracture growth in the vertical direction, thus containing fractures within the reservoir (Kim et al, 2014). Likewise, find that the majority of induced fractures (with data focused on high-volume fracturing operations in the Barnett Shale in Texas) range from less than 100 m (330 ft) to about 600 m (2,000 ft) in vertical extent, with approximately a 1% probability of a fracture extending 350 m (1,100 ft) vertically.…”

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“…We employ the multiple porosity (continuum) model in thermoporo-mechanics, which can allow more realistic flow simulation for the fracture and rock matrix systems [19,29]. Since fracturing induces a fracture-rock matrix system from the rock matrix only, we employ the dynamic dual continuum approach, shown in Fig.…”

“…In this study, we focus on physical responses related to hydraulic fracturing, while those during production are analyzed elsewhere [19]. For hydraulic fracturing, creation, propagation, and the aperture of the fractures depend on several factors such as initial reservoir condition of saturation, injected fluid pressure or injection rate, geomechanical moduli, heterogeneity, criteria of tensile failure, a type of fluid within the fractures, and permeability models.…”

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