In recent years, gas huff-n-puff has proven to be a potential EOR method in unconventional reservoirs. The main mechanism is that the injected gas will penetrate the reservoir matrix and diffuse into the oil phase to extract oil. Gas penetration into the reservoir and oil phase is fundamentally important to enhance oil recovery. However, the investigation of gas penetration depth during the huff-n-puff process has been overlooked. The purpose of this study is to improve understanding of the mechanism during gas huff-n-puff process and to investigate the gas penetration depth during the huff period.
In this paper, methane and Wolfcamp cores saturated with Wolfcamp oil were used to conduct huff-n-puff experiments. Then lab scale numerical simulation model was built and validated with the experimental data. The model was employed to quantitatively describe the gas penetration process and measure the penetration depth in the core during the huff period. Diffusion effect was also investigated. As our previous study states that the core size will affect the oil recovery, a history matched field scale model was applied to evaluate the field production performance. The effects of different parameters on gas penetration depth were investigated including reservoir properties such as permeability, natural fracture spacing, and operation properties such as huff-n-puff start time, number of huff-n-puff cycles, injection pressure, and huff and puff time.
The experimental results show that after eight huff-n-puff cycles, the oil recovery of the core is around 48.57%. During one huff-n-puff cycle, gas penetrated fast during the injection time and slowly during the soaking time, which illustrates that longer soaking time is not necessary. With diffusion, injected gas will penetrate deeper area during huff period. For the field scale study, most gas is penetrated to the stimulated reservoir volume (SRV). For the reservoir with the hydraulic fracture spacing of 600ft, the CO2 penetration depth is about 105.6 ft at 100 days’ huff time in the first huff-n-puff cycle, covering about 36% of the SRV region. In the gas penetrated region, the oil viscosity decreases by 30% to 70%, and the average injected CO2 mole fraction in oil phase reaches about 40%. A sensitivity study result shows that the most important parameter that can affect penetration depth is natural fracture spacing, followed by injection pressure, gas diffusion rate in oil phase, huff-n-puff time, huff-n-puff start time, and reservoir permeability. Gas diffusion rate in gas phase has little effect on penetration depth.