Comparison of the Effectiveness of CO2 and Hydrocarbon Gas for Shale Huff ’n’ Puff

Abstract

Summary The gas huff ’n’ puff (HnP) technique holds great potential for enhancing oil recovery from shale reservoirs following primary recovery. Both CO2 and hydrocarbon (HC) gases are commonly considered for HnP processes, each offering distinct advantages and disadvantages. In this study, we propose a novel experimental approach to systematically evaluate the enhanced oil recovery (EOR) potential of different injectants in shale cores and the potential carbon storage capacity for CO2 HnP. The HC gas mixture utilized consists of 72% methane (C1), 16% ethane (C2), and 12% propane (C3), closely mirroring the composition of available HC gas in oil fields. In each test, shale cores were first saturated with the dead oil, and then high-pressure methane diffusion was utilized to make in-situ live oil. Primary recovery was performed before HnP cycles. Next, six cycles of gas HnP were conducted with a huff pressure of 6,000 psi, a soaking time of 24 hours, and a puff pressure of 2,000 psi. The huff pressure was selected to be much higher than the calculated minimum miscibility pressure (MMP) to enable the development of miscibility of injected gases with the oil in the matrix. The EOR performance of each injectant was evaluated by analyzing oil recovery through nuclear magnetic resonance (NMR), core mass measurements, and effluent fluid collected at each step. The produced gas and liquid effluent were carefully collected and analyzed by gas chromatography (GC) analysis. The experimental results revealed that both CO2 and HC gas enhance oil recovery from the studied shale cores. After six cycles of HnP process, oil recoveries reached 84% for CO2 and 74% for HC gas. Oil recovery efficiency was about 58 STB/MMscf of gas injected. NMR results confirmed that most of the injected oil could be extracted after six cycles of gas injection. However, incremental oil recovery decreased significantly with each subsequent cycle for both gases. In addition, oil produced in later cycles generally had a higher fraction of heavy HC components, as evidenced by density measurements and GC analysis. CO2 can be stored in the pore space of the shale matrix, with up to 1.56 kg/ft³ of the shale bulk volume if the production is ceased in the final HnP cycle.

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