A Review of Huff-and-Puff Enhanced Oil Recovery Technology in Unconventional Oil Reservoirs in China: Current Status and Future Perspectives

Abstract

Huff-and-puff enhanced oil recovery (HnP-EOR) has become an important means of developing unconventional oil reservoirs (UORs) in China. However, oil recovery performance varies across different injectants and production blocks, posing challenges for the large-scale promotion and process optimization of this technology. To further advance the application of HnP-EOR technology in UORs, this paper systematically reviews the major injectants currently in use along with their research status in oil-displacement mechanisms, influencing factors, and engineering applications. The results show that the technically recoverable UOR resources in China are on the order of several billion tons, yet their matrix pore sizes are at the micro- to nanoscale. The main HnP-EOR injectants include single-phase liquids and gases, and multiphase fluid synergy, with water and CO2 the most prevalent. A unified understanding has been achieved regarding the microscopic interaction mechanism of each single-phase injectant; however, in terms of pore-scale utilization, the utilization laws of gas have been basically clarified while those of water still need to be explored. The influence laws of some parameters on HnP-EOR have been basically clarified, such as fracture density, injection pressure, well shut-in time, and cycle number. However, the dominant microscopic interaction mechanism and the weight ranking of influencing factors under different injectants, reservoir properties, and operational conditions remain unclear. Multiphase fluid synergy and pressure-driven injection are novel HnP-EOR technologies that have been developed rapidly in recent years. Their excellent performance has been preliminarily verified in laboratory experiments, and further in-depth research is urgently needed. The incomplete evaluation system for HnP-EOR has become an important obstacle for its engineering applications; therefore, a ″five-element method″ evaluation system is proposed in this work. Lastly, the challenges and future development directions of the HnP-EOR technology in UORs in China are comprehensively summarized from seven perspectives.