Pressure transient behavior of horizontal gas injection well in low permeable reservoirs

This work addresses the pressure transient behavior of horizontal gas injection well in low permeable reservoirs. Low permeable reservoirs such as shale oil reservoirs have been receiving great attentions lately which normally require hydraulic fracturing and horizontal well development to maximize the oil production. However, the primary recovery factor of shale oil reservoirs is still low and has been estimated to be below 10-15 due to tight nature of the shale formations. Enhanced oil recovery method such as miscible carbon dioxide (CO2) injection is said to be one of the most efficient and effective methods used to increase the oil recovery factor of a low permeable shale oil reservoir. The objective of this paper is to study the pressure transient behavior of the horizontal gas injection well in low permeable shale oil reservoirs using numerical simulator, CMG-GEM. Flow regimes and its significant reservoir parameters are investigated from the log-log plot of pressure-derivatives. It is found that a unit-slope line is developed on pressure-derivative log-log plot at early time due to the gas compressibility effect, followed by early radial flow and early linear flow regimes. The effect of various parameters such as gas injection rate, duration of gas injection, well location and well perforation length are studied and analyzed on the changes of pressure transient characteristics. It is identified that gas injection rate affects the pressure-derivative response significantly at middle time due to gas mobility and viscosity; whereas well location and well perforation length affect the late time pressure-derivative response which relate to dominant boundary effect; however, duration of gas injection is not able to show or prove any impacts on the pressure-derivative behavior due to numerical instability issue. Reservoir characteristics such as average permeability and skin can be identified from the flow regimes equations similar to the horizontal production well. © 2023 The Authors.