Earlier studies show that huff-n-puff injection is preferred to continuous gas flooding to improve liquid oil production in shale gas condensate reservoirs. Compared to gas flooding, huff-n-puff has more operation parameters to optimize so that liquid oil production can be maximized. This paper focuses on the effects of cycle time and soaking time under practical operation conditions and reservoir conditions. The operation conditions include injection and production pressures, injection and production rates. The reservoir conditions include reservoir permeability and initial reservoir pressure. Different fluid PVT data are tested. Different injection gases like methane, CO2 and nitrogen are compared.
The simulation results and discussions further show that huff-n-puff injection of produced gases can produce more liquid oil in gas condensate reservoirs than gas flooding or primary depletion. The advantages of huff-n-puff over gas flooding are attributable to high drawdown pressure and oil saturation decrease near the wellbore by evaporation, hence overcoming the condensate blockage and pressure transport problem owing to ultra-low permeability. The simulation results show that an optimum soaking time is so short (3 to 5 days) that it is not necessary to design a soaking time in practice because such short soaking time can be met when changing huff mode to puff mode. Although longer soak time results in reduction of oil recovery, the gas utilization (oil recovered per unit volume of gas injected) increase as soak time gets longer. Similarly, the effect of cycle time is studied. It is found that optimum cycle times are in months in typical shale reservoir conditions. Analysis of simulation data about the changes in pressure, oil saturation and oil viscosity, empirical soak time and cycle time are proposed in this paper.
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