Field Gas Huff-n-Puff for Improving Oil Recovery in the Eagle Ford Shale─The Effect of Injection Gas Composition and Cycle

Abstract

This work is a continuation of our previous works aiming to advance the understanding of field gas huff-n-puff for enhanced oil recovery in shale reservoirs, which focuses on the effect of injection gas composition and the huff-n-puff cycle (i.e., huff time and puff time). Three injection gases (methane and two synthetic field gases from two different shale formations) were used in the single-cycle huff-n-puff experiments. In addition, a series of multicycle huff-n-puff experiments were conducted with varying huff and puff times to investigate the effect of cycle duration on production. The core plugs and crude oil used in this work were retrieved from an Eagle Ford shale formation. Oil production was evaluated based on the crude oil properties as well as the ultimate recovery factor (URF). The average URF for the (34.0 mol % of C2 +) enriched gas huff-n-puff is 41.4% higher than that of methane huff-n-puff. Additionally, more than 90% of crude could be recovered by a very rich field gas with a composition similar to natural gas liquid. Such an increase in URF with the gas composition enrichment could be attributed to the enhanced phase partitioning as well as the increased molecular diffusion. The results of multicycle huff-n-puff show that the cycle duration has a minor impact on URF. However, an increased puff time could be beneficial to the oil production rate due to the redistribution of the saturation and composition profile in the matrix during the low-pressure shut-in. Additionally, a more frequent puff operation has a positive impact on the oil recovery rate, which could be attributed to injection gas refreshing and crude component removal in the matrix. Moreover, a comparison between single- and multicycle huff-n-puff further confirms that a long soaking without puff may not be favorable for the oil recovery rate.