Recent Advances in Enhanced Oil Recovery Technologies for Unconventional Oil Reservoirs

摘要

Unconventional resources have significantly transformed the landscape of the oil and gas industry. The primary recovery factor ranges anywhere from 2% to 8% for the various shale plays throughout the United States. Hence, it is imperative to exploit the vast potential of unconventional reservoirs and increase the recovery factors beyond primary depletion by implementing improved and enhanced oil recovery (IOR/ EOR) methods. This paper presents detailed review of the advances in IOR/EOR technologies applied to unconventional oil reservoirs. A thorough review of the pertinent published literature on IOR/EOR was performed. Results of EOR application to unconventionals shared by various operators in their investor presentations and press reports were also analyzed. The IOR/EOR studies were classified into laboratory experiments, numerical modeling and field laboratory trials (pilots). In addition, the field trials were also analyzed based on the representative shale plays. Most of the studies performed for the application of EOR technologies to unconventional oil reservoirs have been limited to experimental investigations and numerical simulation studies. The research revealed that miscible gas injection (produced gases, CO_2, etc.) is the most promising method among the EOR techniques (miscible gas, water flooding, surfactant, chemical and polymer). Experimental studies showed that CO_2 injection had the highest potential of improved recovery in unconventionals followed by produced gas injection and that diffusion was the most predominant mechanism. Surfactant injection showed the next best potential to increase oil recovery by altering the wettability of rock in laboratory experiments. The gas injection pilots showed that sufficient injectivity was achieved mainly due to the injection induced fractures and did not exhibit any significant effect of diffusion. Conformance control remains a big challenge especially due to the channeling of the gas through the fractures. Produced gas injection pilots in the Eagle Ford formation have demonstrated the greatest success in increasing oil recovery. However, there are many inconsistencies between the laboratory investigations and field trials that needs reconciliation. Further research is necessary to bridge the gap and improve the scaling from laboratory to field. This methodical study elicits the learnings and challenges from the application of different IOR/EOR technologies to unconventionals at various scales (micro to macro to field scale). In addition, ideas for future research are recommended to improve the understanding of the complex mechanisms of EOR in unconventional oil reservoirs. These include optimizing gas injection schemes (huff-n-puff, continuous injection) based on key parameters such as permeability and investigating fracture placement for improving the drainage area and inter-well communication.