Abstract
Recent field reports show the uplift in oil production rate (q_o), 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 q_o: 1) pressure buildup near matrix-fracture interface during the shut-in period, 2) increasing oil saturation (S_o) and compressibility (c_o) 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 q_o, followed by an increase in oil relative permeability (k_(ro)). 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 S_o which is considered as the primary factor for k_(ro) enhancement. Increasing cG because of increasing solution-gas content is the secondary factor that improves post-shut-in k_(ro). However, gas relative permeability (k_(rg)) drops after the shut-in period while k_(ro)increases. 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 k_(rg) reduction.
NSTL