Gas Injection for EOR in Organic Rich Shale. Part I: Operational Philosophy

Abstract

We present the first comprehensive experimental evaluation of gas injection for EOR in organic rich shale. Experiments in preserved core demonstrated the potential of CO2 to extract the naturally occurring oil in organic rich shale reservoirs, whereas tests in re-saturated core plugs were used to compute accurate recovery factors, and evaluate the effect of soak time, operating pressure, and the relevance of slim-tube MMP on recovery. 18 core-flooding experiments were conducted in sidewall cores from different shale plays. The cores re-saturated with crude oil, were first cleaned by Dean-Stark extraction, and submitted to porosity and compressibility determination. The re-saturation, confirmed by CT-scanning, was attained by aging the core plugs at high pressure for two to four months. In all experiments, glass beads surrounding core samples were used to simulate the proppant and physically recreate in the laboratory a hydraulic fracture connected to the shale matrix. The slim-tube MMP was measured with CO2, and core-flooding experiments were performed below, close to, and above the MMP. The displacement equipment was coupled to a medical CT-scanner that enabled us to track the changes in composition and saturation taking place within the shale cores during the experiments. Continuous CO2 injection and huff-and-puff were evaluated using soak time from zero to 22 hours. Fixed reservoir temperature was used in all the experiments. Recovery factors ranged from 1.7 to 40%. The wide variation was the result of different experimental conditions for pressure and soak time. Both operational parameters were found to significantly affect the recovery. Increasing soak time at constant pressure consistently resulted in significant increase in recovery. The increase varied from 78 to 464% for different pressures and oil composition. Similarly, increasing operating pressure at constant soak time resulted in significant increase in recovery factor from 44 to 338% depending on soak time and oil composition. Unlike the typical response during CO2 EOR in conventional rocks, in organic rich shale, further pressure increases beyond the slim-tube MMP continued to increase the recovery factor significantly. In all runs, almost all oil recovery occurred within three days from the start of the experiment, and in all huff-and-puff tests the highest rate of recovery was observed in the first cycle, implying oil recovery with CO2 is a fast process, in comparison to oil re-saturation of the samples which occurs at a significantly slower rate. This investigation demonstrates CO2 EOR is a technically feasible method to extract significant amounts of crude oil from organic rich shale reservoirs and it provides operational understanding of how to manage pressure and soak time to maximize recovery. The recovery factors obtained in this investigation, in the context of the vast reserves of crude oil contained in organic rich shale, can sustain a second shale revolution and further capitalize oilfield infrastructure.

Huff-n-Puff Technology for Enhanced Oil Recovery in Shale/Tight Oil Reservoirs: Progress, Gaps, and Perspectives

“…Researchers such as the authors of refs and − attempted to inject CO 2 into a sample of shale core. They noticed that exceptionally high pressure decreases during the simulation, indicating that a stable state could not be achieved.…”

“…Few, if any, successful attempts at such practices from confined shale samples have been recorded because of the incredibly poor permeability of formations, such as shale. Limited trials of continuous methods using higher permeability “tight” cores have been executed. , Recently, some researchers, such as Tovar et al, attempted to insert CO 2 into a shale sample. They noticed incredibly high pressure drops and never entered a stable state.…”

Experimental study of enhanced oil recovery by CO2 huff-n-puff in shales and tight sandstones with fractures

“…The experiment and simulation results showed that the recovery factor of continuous gas injection is larger than huff-n-puff in the homogeneous formation (Wan et al 2013;Sheng 2015;Yu and Sheng 2015). However, Tovar et al (2018) demonstrated that the direct gas injection through the shale matrix is not possible in a reasonable time frame. Many technologies, including horizontal wells and multiple hydraulic fracturing, have been used to economically produce shale oil (Daneshy 2009;Gaurav et al 2012).…”

“…Third, a physical model, which is composed of an annular fracture and a cylindrical matrix, was used to study the CO 2 huffn-puff process in conventional fractured reservoirs (Torabi and Asghari 2010;Sang et al 2016). In some experiments, the fracture was filled with glass beads to model the fracture with proppant (Tovar et al 2018). However, these three types of assessment methods have their flaws if employed independently.…”

Recovery Mechanisms for Cyclic (Huff-n-Puff) Gas Injection in Unconventional Reservoirs: A Quantitative Evaluation Using Numerical Simulation

“…The value of enhanced oil recovery (EOR) for unconventional reservoirs like the Bakken and Eagle Ford was noted early in the production of these reservoirs [7,8], and there has been significant research over the last decade on this topic (summaries in Alfarge et al [9] and Kanfar et al [10]). Much of this early work was done in the laboratory (e.g., Jin et al [11] and Tovar et al [12]) or with numerical models (e.g., Li et al [13] and Gala and Sharma [14]), where they looked at questions of overall recovery, the type of gas, soak time, vaporization, and utilization.…”