This study investigates at high resolutions the system behavior caused by water invasion/imbibition during the short period of hydraulic fracturing in ultra-low permeability gas reservoirs, and during the subsequent long-term production period. The main objective is to assess the impact of this early water invasion/imbibition on the flow, fate and recovery of fluids during short- and long-term production from such reservoirs. The study accounts for all known flow and thermophysical processes and uses very fine discretization in order to capture the non-isothermal multi-scale system behavior in sufficient resolution. It focuses on two of the four general types of fractured systems (Types I and II), and describes complex processes and phenomena on length scales that cover the mm- to 15 m range. The high pressure during hydraulic fracturing, coupled with the generally low initial water saturation of such media, cause water imbibition into the matrix, with significant effects on the fluid flow behavior in this region. All key variables (pressure, temperature, phase saturations and relative permeabilities, capillary pressure, etc.) are monitored over time, and the effect on the fluid production rates is analyzed.
The results of the study indicate that penetration of water into the matrix during hydraulic fracturing depends on the type of the fractured system. Flow in the fractures of Type II systems is fast, but water flow and redistribution into the matrix is slow and continues for a long time after the end of hydraulic fracturing, and water evaporation during production may have a significant impact. The phase saturations and relative permeabilities exhibit a complex behavior and vary both spatially and temporally in the matrix (Types I and II) and in the fractures (Type II). Gas production exhibits a complex behavior that is affected by both the evolving relative permeability to gas and the changing pressure and temperature regime. The magnitude of water injection/recovery volumes and the recovery pattern appear to be indicator of the type of reservoir fractured system.