Quantifying Oil-Recovery Mechanisms during Natural-Gas Huff ‘n’ Puff Experiments on Ultratight Core Plugs

Abstract

Summary Despite promising natural gas huff ‘n’ puff (HnP) field-pilot results, the dominant oil-recovery mechanisms during this process are poorly understood. We conduct systematic natural-gas (C1 and a mixture of C1/C2 with the molar ratio of 70:30) HnP experiments on an ultratight core plug collected from the Montney tight-oil formation, under reservoir conditions (P = 137.9 bar and T = 50°C). We used a custom-designed visualization cell to experimentally evaluate mechanisms controlling gas transport into the plug during injection and soaking phases and oil recovery during the whole process. The tests also allow us to investigate effects of gas composition and initial differential pressure between injected gas and the plug (ΔPi = Pg−Po) on the gas-transport and oil-recovery mechanisms. Moreover, we performed a Péclet number, NPe, analysis to quantify the contribution of each transport mechanism during the soaking period. We found that advective-dominated transport is the mechanism responsible for the transport of gas into the plug at early times of the soaking period (NPe = 1.58 to 3.03). When the soaking progresses, NPe ranges from 0.26 to 0.62, indicating the dominance of molecular diffusion. The advective flow caused by ΔPi during gas injection and soaking leads to improved gas transport into the plug. Total system compressibility, oil swelling, and vaporization of oil components into the gas phase are the recovery mechanisms observed during gas injection and soaking, while gas expansion is the main mechanism during depressurization phase. Overall, gas expansion is the dominant mechanism, followed by total system compressibility, oil swelling, and vaporization. During the “puff” period, the expansion and flow of diffused gas drag the oil along its flowpaths, resulting in a significant flow of oil and gas observed on the surface of the plug. The enrichment of injected gas by 30-mol% C2 enhances the transport of gas into the plug and increases oil recovery compared to pure C1 cases.

Propane–Bitumen Interactions during Cyclic Solvent Injection─A Cold Lake Case Study

“…However, the overall trend of the pressure profile is still relatively similar to Test 1, since there is a distinguishable pressure drop in the early soaking, followed by a stable pressure profile in the late soaking. Tran et al stated that similar pressure profiles during the soaking period suggest similar solvent transport mechanisms . Therefore, the same conclusion can be made for Test 2 and Test 3 such that advective transport by the pressure gradient dominates the early soaking, whereas diffusive transport by the concentration gradient is responsible for the late soaking.…”

Effects of Electro-Oxidation Process on Tight-Rock Wettability and Imbibition Oil Recovery

“…Our previous studies present more detailed properties of these core plugs. 39,40 All core samples and end pieces were aged before conducting the experiments. First, we dried the rock samples in an oven at 90 °C to evaporate the initial oil and water in the pores.…”

“…Table lists the core plugs’ mineralogy that is determined by X-ray diffraction (XRD) analysis. Our previous studies present more detailed properties of these core plugs. , …”

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“…The authors of that study also concluded that the use of methane above its minimum miscibility pressure with the crude oil reduced oil recovery by only 10%. It has also been well established that enriching methane with natural gas liquids in huff 'n' puff could significantly reduce the minimum miscibility pressure and enhance oil extraction (Tran et al 2021). Song et al (2021) conducted huff 'n' puff experiments on Upper Devonian Duvernay core samples and observed that adding propane to the injected mixture of methane and ethane resulted in an increase in oil recovery.…”