Abstract
In this work, a triple-porosity, two-phase flow model was established to fill the knowledge gap of previous models focusing on gas production characteristics while ignoring the impacts of water injection. The proposed model considers the water flow in the fracture systems and clay minerals and the gas flow in the organic matter, inorganic pore, and fracture systems. The proposed model is solved using a finite element approach with COMSOL Multiphysics (Version 5.6) and verified with field data. Then, the evolutions of the intrinsic and relative permeabilities during water injection and gas production are examined. Furthermore, the impacts of water injection time and pressure are investigated. Good verification results are obtained; the goodness-of-fit value is 0.92, indicating that the proposed model can replicate both the water stimulation and the gas production stages. The relative gas permeability declines during water injection but recovers in the gas depletion stage. Furthermore, the intrinsic permeability increases during the water injection stage but decreases during the gas production stage. A higher water injection pressure and longer injection time would enlarge the intrinsic permeability, thus improving flow capacity. However, it would reduce gas relative permeability, thereby hindering gas flow. The shale gas production characteristic is controlled by the two abovementioned competing mechanisms. There exists a perfect combination of water injection pressure and injection time for achieving the maximum profitability of a shale gas well. This work can give a better understanding of the two-phase flow process in shale reservoirs and shed light on the field application of hydraulic fracturing.
Highlights
Compared with other fossil fuels, shale gas plays an important role in the global energy mix with the exhaustion of conventional reservoirs [1, 2]
This study proposes a coupled two-phase flow model to comprehensively examine the effect of injected water on reservoir production
A triple-porosity, two-phase flow model was established and applied in this paper to examine the impact of hydraulic water on shale gas production characteristics
Summary
Compared with other fossil fuels, shale gas plays an important role in the global energy mix with the exhaustion of conventional reservoirs [1, 2]. The two-phase flow process during shale gas production has been widely investigated analytically and semianalytically with the influence of complexity and half-length of hydraulic fracture [26, 27], increased permeability, and fracture conductivity in stimulated reservoir volume (SRV) [28, 29] considered. Only the gas production stage is replicated in the simulation work, and the water injection process is seldom studied In such approaches, the impacts of water injection pressure and injection time are not fully understood. The impacts of water injection pressure and injection time are not fully understood To address these shortcomings, this study proposes a coupled two-phase flow model to comprehensively examine the effect of injected water on reservoir production.
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