A Comparison Study on Huff-n-Puff Gas Injection and Chemical Relative Permeability Modification to Mitigate Condensate Blocking in Fractured Shale Gas Condensate Reservoirs

Abstract

In the literature, both huff-n-puff gas injection and chemical injection to modify relative permeabilities have been proposed to mitigate condensate blocking near the wellbore; and huff-n-puff gas injection has been proposed to improve liquid oil production in shale gas condensate reservoirs. This paper focuses on the potential of chemical relative permeability modification (RPM) to enhance hydrocarbon recovery in fractured shale gas condensate reservoirs using the simulation approach; and the potentials to improve hydrocarbon recovery by these two approaches are compared. The relative permeability model is studied and the dependence of gas/oil relative permeability curves on capillary number and interfacial tension is discussed. Chemical treatments to mitigate condensate and water blocking have been successfully implemented in the laboratory with sandstone and carbonate. Accordingly, a brief screening of RPM chemicals is conducted and the mechanism of potential surfactant molecules that may work in shale gas condensate reservoirs are investigated. In this study, a shale gas condensate reservoir model is set up and the reservoir is assumed to be treated with a hypothetic surfactant that improves both the relative oil and gas permeability within the near-wellbore area. The simulation results are juxtaposed with those of huff-n-puff gas injection production.It is assumed that the surfactant alters the end-point gas/oil relative permeability, residual oil saturation, gas connate saturation, etc. and is durable during the production years. In the surfactant treatment cases, an increase of oil recovery by 1%-3% is observed compared with the primary depletion. However, the optimized huff-n-puff process can achieve up to 8% more oil recovery than primary depletion. In terms of gas recovery, the surfactant treatment process increases gas recovery by 1-2%, while the optimized huff-puff gas injection leads to over 20% gas recovery reduction because of less total production time. To sum up, the huff-n-puff gas injection process achieve more liquid recovery but less gas recovery than the surfactant treatment process. Analysis of simulation data about the changes in drawdown pressure and liquid saturation are proposed. A simple economic analysis is conducted as well. It is concluded that the huff-n-puff process is preferred to the RPM process when the oil price is relatively high due to higher drawdown pressure and the oil saturation reduction near the wellbore by evaporation.The potential of RPM application in unconventional reservoirs has not been well investigated in the previous literature. This paper is to discuss this potential and to compare the advantages and disadvantages of the surfactant RPM process with those of huff-n-puff gas injection in shale gas condensate.