Laboratory Investigations on Field Gas Huff-n-Puff for Improving Oil Recovery in Eagle Ford Shale-Effect of Operating Conditions

This work aims to advance the understanding of the natural gas huff-n-puff process for enhanced oil recovery in shale formations with a focus on the impact of operating conditions (i.e., huff pressure, soaking time, and pressure depletion rate) on oil production. Eight huff-n-puff experiments were conducted with a unique single-cycle process designed to identify the production mechanisms during soaking and puffing. The crude oil and core plugs used in the experiments were retrieved from an Eagle Ford shale reservoir, and the injection gas was an associated gas obtained from the same reservoir. Oil extraction efficiency was evaluated by the ultimate recovery factor (URF) and produced oil properties (i.e., composition, mass, and density). A marginal increase in URF (5.4-6.2%) was observed when the huff pressure was further increased above the minimum capillary pressure (MCP), at which the interfacial tension between the oil and injection gas diminished. This pressure dependence of URF is much weaker than that observed at pressures below MCP as a result of the less-selective enhanced phase partitioning of crude oil components at elevated pressures. It was also found that a lower pressure depletion rate during the puff led to a modest increase in URF. Such incremental oil productions were obtained only when the puff pressure fell below 2500 psia. The latter observation could be attributed to reduced fluid compressibility at higher pressures (i.e., above 2500 psia) and advancement of the high oil content region of the mixing zone toward the fracture. The effect of soaking time on the huff-n-puff performance was also evaluated by comparing four experiments with different soaking times. The average URF increased by 9.5% with two additional soaking days. Also, the produced oil composition became heavier over time as a result of preferential partitioning of light oil components into the injection gas and molecular-weight dependency of component diffusivity.

Automated summary

This study focuses on the natural gas huff-n-puff process for enhanced oil recovery in shale formations, examining the impact of operating conditions on oil production. Results show a marginal increase in oil recovery factor with higher huff pressure and a modest increase with lower pressure depletion rate during the puff. The study also highlights the impact of soaking time on incremental oil production and changes in produced oil composition over time.