Impact of Water Dynamics in Fractures on the Performance of Hydraulically Fractured Wells in Gas-Shale Reservoirs

Slickwater fracturing has been increasingly applied to stimulate unconventional shale-gas reservoirs. Comparing with crosslinked fluids, slickwater used as a fracturing fluid has several advantages, including low cost, a higher possibility of creating complex fracture networks, less formation damage, and ease of cleanup. An enormous amount of water is injected into the formation during the treatment. Even with a good recovery of injected water from flow-back, large quantities of water are still left within the reservoir. The dynamics of the water phase within the created hydraulic fractures and reactivated natural fractures (induced fractures) has significant impact on both short- and long-term performance of a hydraulically fractured well. The dynamics of the water phase within fractures is controlled by many mechanisms, such as relative permeability, capillary pressure, gravity segregation, and stress-sensitive fracture conductivities. In this paper, reservoir-simulation models for a generic gas-shale reservoir are constructed to investigate the changes of water-saturation distribution in fractures over time during production and their impact on gas-production performance. It is demonstrated that water imbibitions caused by capillary pressure and gravity segregation can play important roles in water-saturation distribution and redistribution, particularly during extended shut-in, which in turn affects gas flow significantly. Moreover, an unfavorable combination of relative permeability, capillary pressure, stress-sensitive fracture conductivities, and invasion-zone permeability damage can lead to water-blockage problems.