Unconventional well shut-in and reopening: Multiphase gas-oil interactions and their consequences on well performance

Abstract

Recent field reports show the uplift in oil production rate (qo), after the shut-in period, referred to as “flush production”. The conventional hypotheses for explaining this phenomenon are based on water-oil-rock interactions such as counter-current oil production and water-blockage reduction due to imbibition of fracturing water. Here, we hypothesize other drive mechanisms responsible for the uplift in qo: 1) pressure buildup near matrix-fracture interface during the shut-in period, 2) increasing oil saturation (So) and compressibility (co) due to an increase in solution-gas content at higher pressures, and 3) gas expansion (solution-gas drive) during pressure drawdown after restarting the well.We analyzed the production data of two unconventional wells which were shut-in for 194 and 20 days after the primary-production period. Analysis of production data indicates that pressure buildup is the primary mechanism responsible for higher post-shut-in qo, followed by an increase in oil relative permeability (kro). The results of our compositional simulations show that by increasing the pressure near the fracture face during the shut-in period, a fraction of the free gas is dissolved in the oil phase, leading to an increase in So which is considered as the primary factor for kro enhancement. Increasing co because of increasing solution-gas content is the secondary factor that improves post-shut-in kro. However, gas relative permeability (krg) drops after the shut-in period while kroincreases. The reduction of gas saturation because of pressure buildup during the shut-in period and trapping of the gas phase due to hysteresis effect are the two reasons that explain krg reduction.