Mechanism and Main Control Factors of CO2 Huff and Puff to Enhance Oil Recovery in Chang 7 Shale Oil Reservoir of Ordos Basin

Abstract

The Chang 7 shale oil reservoir has low natural energy and is both tight and highly heterogeneous, resulting in significant remaining oil after depletion development. CO2 huff and puff (huff-n-puff) is an effective way to take over from depletion development. Numerous scholars have studied and analyzed the CO2 huff-n-puff mechanism and parameters based on laboratory core sample huff-n-puff experiments. However, experimental procedures are not comprehensive, leading to more general studies of some mechanisms, and existing CO2 huff-n-puff experiments struggle to reflect the effect of actual reservoir heterogeneity due to the limited length of the experimental core samples. In this paper, CO2 huff-n-puff laboratory experiments were performed on short (about 5 cm) and long (about 100 cm) core samples from the Chang 7 shale oil reservoir, and the microscopic pore fluid utilization in the short samples was investigated using a nuclear magnetic resonance (NMR) technique. We then analyzed and discussed the seven controlling factors of CO2 huff-n-puff and their recovery-enhancing mechanisms. The experimental results show that the cumulative recovery increased with the number of huff-n-puff cycles, but the degree of cycle recovery decreased due to the limitation of the differential pressure of the production. The significant increase in recovery after the CO2 mixed-phase drive was achieved by increasing the minimum depletion pressure as well as the gas injection amount. The soaking time was adjusted appropriately to ensure that the injected energy was thoroughly utilized; too short or too long a soaking time was detrimental. The pressure depletion rate was the main factor in the CO2 huff-n-puff effect in shale. If the pressure depletion rate was very high, the effective permeability loss was larger. In the CO2 huff-n-puff process of the Chang 7 shale oil reservoir, the improvement in oil recovery was mainly contributed to by mesopores and small pores. The huff-n-puff experiments using long cores could better characterize the effect of heterogeneity on the huff-n-puff effect than short cores.

Insights into Enhancing Shale Oil Recovery Postfracturing with CO2 Huff and Puff and Elastic Depletion Development Strategies

“…Increasing the soaking time was found to be beneficial for enhancing the oil recovery rate. Coupled with nuclear magnetic resonance testing, it was analyzed that mesopores and micropores played a significant role in improving the oil recovery rate . Li et al analyzed the feasibility and potential of CO 2 huff and puff-enhanced shale oil recovery by investigating the interaction between CO 2 and formation of oil and the effects of injection pressure, soaking time, matrix permeability, fracture, and diffusion coefficient.…”

Preparation and Application of CO2-Resistant Latex in Shale Reservoir Cementing

“…The current research gradually focuses on the development of unconventional oil and gas resources, particularly the development of shale oil resources [1,2], as a result of the growing demand for oil and gas resources in social development and the significantly declining amount of exploitable conventional reservoir resources. At present, shale reservoirs frequently used formation fracturing techniques to boost output.…”

Compositional Simulation of CO2 Huff-n-Puff Processes in Tight Oil Reservoirs with Complex Fractures Based on EDFM Technology Considering the Threshold Pressure Gradient

“…Zhao et al studied the impacts of different operating parameters on EOR during CO 2 -HnP using NMR technology [54]. Sun et al investigated the effects of injection parameters such as CO 2 injection amount, injection rate, and time on EOR through experimental methods [55,56]. Simultaneously, experiments can effectively reflect the process and mechanism of CO 2 injection, providing a foundation for numerical simulation [54,57,58].…”