Experimental and Numerical Study on CO₂ Sweep Volume during CO₂ Huff-n-Puff Enhanced Oil Recovery Process in Shale Oil Reservoirs

Abstract

CO2 huff-n-puff has been proven to be the most effective enhanced oil recovery (EOR) method in shale oil reservoirs. The injected CO2 will replenish reservoir energy and penetrate the reservoir matrix to extract oil. However, the CO2 sweep volume during the huff-n-puff process has not been accurately evaluated by existing studies. In this paper, the CO2 sweep volume was investigated through experimental and numerical simulation methods. In the experimental study, the CO2 sweep areas were depicted by X-ray computed tomography scan technology. The results indicated that the ratio of the CO2 sweep area was 78.63% in the seventh huff-n-puff cycle, leading to a total oil recovery of 56.80%. The numerical simulation considered the mechanisms of molecular diffusion and nanopore confinement. The results showed that in the first huff-n-puff cycle, the gas sweep volume percentage was 9.47% after 100 days of huff period. In the gas swept volume, oil viscosity was reduced by 25.9% to 68.2%. After three cycles of CO2 injection, the oil recovery manifested a 1.5% increase compared to the case without huff-n-puff. The contributions of different parameters on gas sweep volume and cumulative oil recovery were investigated. The results illustrated that the nanopore confinement effect and molecular diffusion had significant impacts on the gas sweep volume and cumulative oil recovery. Higher injection pressure, longer huff time, and more huff-n-puff cycles lead to larger gas sweep volume, as well as cumulative oil recovery. A suitable primary depletion period and a huff-n-puff schedule should be determined based on the requirements of field production. The investigations in this study provide insights into better understanding of the EOR mechanisms and optimizing the design of CO2 huff-n-puff operations in shale oil reservoirs.

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

“…In an effort to replenish reservoir pressure and improve oil recovery, authors such as Sheng and Chen investigated confirmed conventional field injection processes, including gas flooding schemes (e.g., continuous CO 2 injection and water-alternating-CO 2 (CO 2 –WAG) flooding) in shale oil reservoirs. Unfortunately, those approved methods were found to consume a considerable amount of gas to recover the desired oil volume . Another persistent challenge to researchers is that the gas applied into a tight formation is subjected to an early breakthrough, especially in densely fractured reservoirs, thus causing poor oil displacement .…”

“…The cost of pilot studies is another existing challenge which is in excess of several million U.S. dollars, compared with laboratory research that can be performed at significantly lower costs. Researchers initiating valuable attempts to increase oil productivity of shale/tight oil reservoirs include the reports given in refs , , , , , , , , − . Most of the authors confirmed the success of the H-n-P application; however, they disagreed over the influence of some crucial mechanisms and key parameters, because of the lack of actual field data.…”

A Review of Gas Injection in Shale Reservoirs: Enhanced Oil/Gas Recovery Approaches and Greenhouse Gas Control

“…The above mentioned mechanisms may play important roles in tight (e.g., Middle Bakken formation) or conventional reservoirs, where gas is relatively easier to diffuse into the matrix and to make contact with oil. Recent studies visualized the gas sweep volume in ultra-tight shale plugs by using CT images [33,58], indicating that gas could make contact with the oil that is trapped in nanosized pores. Furthermore, the nanoconfinement effect may influence the estimations of MMP and alter the fluid properties, so the inclusion of capillary pressure effect and the shift in critical properties results in more accurate recovery prediction [52,54].…”

Thermodynamic effects of cycling carbon dioxide injectivity in shale reservoirs

“…The interaction of carbon dioxide with formation fluid has led to reduce oil viscosity and enhance the mobility ratio. The profound impact of crucial parameters such as temperature, pressure, carbon dioxide soaking time, and core stimulation on the oil recovery enhancement should be taken into consideration before performing recovery processes (Zhang et al, 2019;Li et al, 2019;Alfarge et al, 2018;Pranesh, 2018).…”