Comprehensive Experimental Study of Huff-n-Puff Enhanced Oil Recovey in Eagle Ford: Key Parameters and Recovery Mechanism
Byeungju Min, Sidi Mamoudou, Son Dang, Ali Tinni, Carl Sondergeld, Chandra Rai
Abstract
Abstract Huff-n-puff gas injection enhanced oil recovery has received increased attention especially in the unconventional plays like the Eagle Ford, where oil recovery is as low as 5 - 10%. An increase in 1% of recovery could realize a potential of 2.3 billion barrels of oil, which has an enormous economic value. Through a laboratory investigation of huff-n-puff conducted on preserved Eagle Ford samples; we evaluate different factors that can affect the recovery performance such as minimum miscibility pressure (MMP), surface area, soaking time, injection pressure, composition of injection gas and injection gas rate. In addition, different recovery mechanisms such as a vaporization (concentration gradient) and a miscible flowback (pressure gradient) were also investigated. Two sets of experiments were conducted utilizing a high-pressure chamber: one with Eagle Ford oil, providing MMP values using a VIT technique and vaporization test with different soaking times (2 days, 4 days, and 6 days). Another set of experiment were performed with preserved Eagle Ford samples. Different types of gas: carbon dioxide (CO2,) methane (C1), ethane (C2), C1:C2 (72:28) mixture, C1:C2 (95:5) mixture, and field gas were injected at various pressures from 1000 psi below MMP, MMP to 1000 psi above MMP with various soaking time of (1 hr, 3 hr and 6 hr). Nuclear Magnetic Resonance (NMR), HAWK pyrolysis, isothermal nitrogen adsorption tests (BET), Mercury Injection Capillary Pressure (MICP) and Gas Chromatography (GC), were performed to qualitatively and quantitively monitor the changes in Eagle Ford hydrocarbons recovered from shale samples. The experimental results demonstrated that: 1) as methane concentration in gas is increased, MMP also increased, 2) residence time (soaking time + production time) controls the recovery, 3) injection pressure determines the fraction of hydrocarbons mobilized, 4) surface area variation studies showed that the samples with higher surface area have greater recoveries, 5) ethane showed the best performance of all the gases tested (40% recovery). CO2 performed the second best (32%). C1:C2(72:28) mixture and field gas exhibit the similar efficacy in recovery (24% and 21%). C1:C2(95:5) mixture showed the worst recovery (11%). 6) high injection rate yielded better recovery (37%) than low injection rate (24%), 6) Increase in pore surface area by factor of 2.5 was observed from the opening of small pores and pore- throat on post huff-n-puff sample. In addition, recovery mechanism study shows that miscible flow back mobilized hydrocarbon up to C30, vaporization at 1000psi above MMP mobilized hydrocarbon up to C23 and vaporization 1000psi below MMP mobilized hydrocarbon up to C15. The results also indicated that longer soaking times increased diffused oil concentration in vapor phase.