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

Numerical simulation of the environmental impact of hydraulic fracturing of tight/shale gas reservoirs on near‐surface groundwater: Background, base cases, shallow reservoirs, short‐term gas, and water transport

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Some key technical issues in modelling of gas transport process in shales: a review

“…The most commonly used modeling approach is based on the available reservoir simulators that include multiphase flow, sorption, and gas slip flow (e.g., Darishchev et al 2013). Recently, a number of researchers have also made an effort to develop models that couple reservoir fluid flow with rock deformation for both hydraulic fracturing and gas recovery processes (e.g., Gan and Elsworth 2016;Kim and Morridis 2014;Perera et al 2011;Wang et al 2014). Despite the variations of these modeling approaches, their successful application to a giving problem is eventually determined by how well the key physical mechanisms are captured.…”

A New Modeling Framework for Multi-Scale Simulation of Hydraulic Fracturing and Production from Unconventional Reservoirs

“…An example of the TOUGH+ mesh before and after coupling the GEOS aperture is shown in Figure 2. (6) As the pressures and stresses change in the course of production, the macro-scale properties of the matrix (porosity and permeability) are continuously adjusted using either the simplified of the full geomechanical capabilities built into TOUGH+ [5,35,36]. (7) Property changes due to micro-scale processes with fractures and at the fracturematrix interfaces are incorporated based on the imaging, testing, and modeling work described in Section 5.…”