Field Gas Huff-n-Puff for Improving Oil Recovery from Eagle Ford Shale Reservoirs
Authors: Sie Chao-yu , Nguyen Quoc P. 
Published: August 2020

Gas huff-n-puff in unconventional reservoirs has been studied extensively in the past five years due to a large number of shale resources available and their dramatic production decline. With the increasing natural gas availability and its low price at the wellhead, there exists a unique opportunity for produced gas huff-n-puff in shale oil reservoirs. However, the injection gas compositions found in literature are oversimplified (e.g., methane, ethane, or their mixtures), and the dominated production mechanisms, with or without full miscibility between the oil and the injection gas established, remain unclear. This work provides a pioneer laboratory study with unique experimental procedures tailored to investigate the production mechanisms during the shut-in stage and depressurization stage. The composition of the injection gas applied in this study is the same as an associated gas produced in the field.

Capillary rise experiments were conducted to determine the minimum miscibility pressure (MMP) for which the injection gas can develop miscibility with the crude oil at reservoir temperature. An equation-of-state (EOS) model developed based on the constant composition expansion and swelling tests were used to confirm the MMP obtained from the capillary rise experiments. Huff-n-puff experiments were conducted on different reservoir shale plugs retrieved from Eagle Ford. The oil produced during the soaking stage and depressurization stages were collected separately and analyzed by gas chromatography- mass spectrometry. The recovery efficiency and production mechanisms were evaluated by the ultimate recovery factor as well as the mass and the composition of the effluents produced in each fluid stage.

The MMP obtained from the capillary rise experiments matched the prediction by the EOS model. The results of the huff-n-puff experiments showed that more than 80% of oil could be recovered in just one huff-n-puff cycle when the huff pressure was at MMP. However, the ultimate recovery factor decreased non-linearly with the huff pressure for the same puff pressure depletion rate of 1000 psi/day. For the experiments operated above MMP, a large fraction of produced oil was observed from the huff process, while very little oil was produced during pressure depletion. In contrast, more oil was produced during the depressurization for huff pressures below MMP, suggesting the strong variation of recovery mechanisms with huff-n-puff pressure.

This work investigated the efficiency of shale oil extraction with the produced gas through the huff- n-puff process in preserved shale plugs, which is rarely discussed in literature. It identified critical experimental design problems in the quantitative evaluations of process performance and provided techniques to solve these problems. The results from this study have advanced our understanding of key production mechanisms involved in field gas huff-n-puff and can be used to determine important transport properties such as hydrocarbon component diffusivities required for process modeling.