This paper studies the technical and economic viability of this EOR technique in Eagle Ford shale reservoirs using natural gas injection, generally after some period of primary depletion, typically through long, hydraulically fractured horizontal-reach wells. The Eagle Ford formation has produced approximately 2 billion bbl of oil during the last 7 years, yet its potential may be even greater. Using improved oil-recovery (IOR) methods could result in billions of additional barrels of production.
Cyclic gas injection (CGI), also known as huff-n-puff process, has been demonstrated as the most effective and promising IOR solution in shale reservoirs. Such process involves many operating parameters that can affect the recovery performance in different degrees. As soaking time and pressure depletion rate (PDR) are the two crucial factors, this study aims to further investigate their roles in the oil recovery process of CGI experimentally and numerically.
A total of ten series of the N2 huff-n-puff tests were performed on oil-saturated Eagle Ford shale plugs in a matrix-fracture system. With the injection pressure (Pin) of 1,000 psi, two groups of tests were conducted: 1) under a constant PDR in blowout and five soaking times changed from 0.25-hr to 48-hr; 2) under a constant soaking time of 12-hr and four pressure depletion times changed from 0.05-hr to 48-hr. With the Pin of 5,000 psi, the CGI tests were performed under 12-hr soaking time and blowout pressure depletion to study the effect of Pin on recovery efficiency. Lab-scale simulation models were built and tuned to history match the experimental data.
The experimental results show that recovery factor (RF) from a single cycle increases with soaking time within a certain range, and a longer time has no effect on improving oil recovery. For 1,000 psi Pin, during the soaking phase, the system pressure declined rapidly at the first 3 hr and then became stabilized. The pressure drop was relatively low (10 psi) in the first cycle compared with the following cycles. It tended to increase with the number of cycles performed, which was caused by the decrease of oil saturation thus more gas can be injected into the core sample. It reveals that a soaking period is necessary to the oil recovery process. On the other hand, increasing the pressure depletion rate can raise the incremental RF from each cycle. The blowout condition showed the highest cumulative RF. The performance of N2 huff-n-puff with 5,000 psi Pin outperforms the case of 1,000 psi, which can increase the ultimate RF with less cycles. The well-tuned simulation models were used to analyze and optimize the CGI recovery process.
Gamadi, T.D. (Texas Tech University) | Sheng, J.J. (Texas Tech University) | Soliman, M.Y. (Texas Tech University) | Menouar, H. (Texas Tech University) | Watson, M.C. (Texas Tech University) | Emadibaladehi, H. (Texas Tech University)
An article by Hart (2011) quoted one of EOG resources recent reports that recovery factor for the Eagle Ford shale play during primary drive reservoir depletion will be roughly 5%. The vast oil remaining stimulates our efforts to investigate the application of enhanced oil recovery methods in shale oil reservoirs. Recent numerical studies have indicated that cyclic gas injection could be an effective method to increase the oil recovery of shale oil reservoirs.
Cyclic gas injection could be an effective technique because it is a single-well process; well-to-well connectivity is not required. The hydraulic fracturing provides a large contact area for the injected gas to penetrate and diffuse into the low-permeability matrix. The payback period of the cyclic gas injection process is rather short compared with that of the field- scale flooding process. This makes the single-well cyclic injection process a low-risk process.
This paper presents our experimental verification and quantification of the potential of using cyclic CO2 injection in shale oil reservoirs. This work is the first experimental work to investigate the performance of CO2 huff-n-puff on shale cores. Core plugs of Mancos and Eagle Ford shales were used. A special lab set up was built to conduct this study. The effects of cyclic time (Soaking Period) and injection pressure on oil recovery, among other parameters, were investigated. Our experimental results showed that cyclic CO2 injection could increase the recovery factor from 30% to 70% depending on the shale core type. It also shows the effect of the pores connectivity and heterogeneity on the performance of CO2 huff-n-puff. This study shows that one of the important mechanisms of cyclic gas injection is the pressure effect that causes a large pressure drawdown during the production phase.