(296d) Optimization of Huff-n-Puff Gas Injection in Shale Condensate Reservoirs to Improve Liquid Oil Production

Abstract

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.