Abstract
Gas flow mechanisms in shale reservoirs with multiscale pores and fractures are extremely complex. In this study, a dual fracture framework model was adopted to describe gas flow in multiscale shale reservoirs. Gas flow through a shale reservoir occurs through both the shale matrix and hydraulic fractures. This study considered bulk phase and adsorbed gas flow in the shale matrix. Next, a series of partial theories were combined to derive a fully coupled model simulating gas flow in multiscale shale reservoirs: (1) fractal theory was adopted to obtain the pore distribution within shale reservoirs; (2) mechanical equilibrium equations were used to investigate the stress-sensitivity of permeability and porosity; and (3) a Langmuir adsorption model was applied to describe the effects of gas adsorption/desorption. The proposed model was validated using traditional models as well as field data on gas production from Marcellus Shale, and was subsequently applied to study variations of mass flux in various flow regimes with respect to reservoir pressure. We found that mass flux in the slip flow regime decreased at first and then increased with decreasing reservoir pressure, while in the continuum regime, Knudsen diffusion and surface diffusion the mass flux decreased with decreasing reservoir pressure. Stress-sensitivity has a significant impact on bulk phase gas flow, while adsorption/desorption influence both the bulk phase gas flow and adsorbed gas flow. At high pressures, the impact of stress-sensitivity on total gas mass flux is greater than that of adsorption/desorption, while the reverse was true for low pressures. The proposed model shows promising applications for analyzing various gas flow regimes in multiscale pores/fractures, and accurately evaluating in situ apparent permeability.
Highlights
With its wide distribution and high resource potential, shale gas has received increasing attention and plays a more important role around the world [1,2,3,4]
The effects of stress-sensitivity on gas flow within shale reservoirs incorporate two aspects: (1) matrix and fracture permeability, as well as porosity, which decreases with decreasing reservoir pressure; and (2) matrix shrinkage with increasing effective stress decreases the effective pore diameter
∆p, ZRT μ where Mc is the mass flux in the continuum regime, M is the molar mass of the gas, Z is the compressibility factor, k is the matrix permeability, μ is dynamic viscosity of the fluid, and ∆p is the gradient in reservoir pressure
Summary
With its wide distribution and high resource potential, shale gas has received increasing attention and plays a more important role around the world [1,2,3,4]. Gas flow through a shale reservoir is controlled by the coupled effects of multiscale pores/fractures, dynamic changes in reservoir pressure, adsorption/desorption, and stress-sensitivity. Studies of gas flow within shale reservoirs have mainly focused on gas flow in nano-scale pores by experimental analysis and numerical simulation [7,8,9,10,11,12] Most of those models ignored the coupled effects of dynamic behaviors and shale reservoir properties, such as multiscale pores/fractures, adsorption/desorption, and stress-sensitivity on both the bulk phase gas flow and adsorbed gas flow. A comprehensive gas flow model for describing gas flow in shale reservoirs with the coupled effects of multiscale pores, adsorption/desorption, and stress-sensitivity under a dynamic change in reservoir condition will be obtained by combining with those individual gas flows models
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