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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, CO2, etc.) is the most promising method among the EOR techniques (miscible gas, water flooding, surfactant, chemical and polymer). Experimental studies showed that CO2 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.

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This paper presents a comprehensive workflow to screen, rank and evaluate improved and enhanced oil recovery (IOR/EOR) opportunities for given reservoirs. This multidisciplinary workflow was applied to multiple oil reservoirs, aiming to improve volumetric sweep and recover bypassed oil. This comprehensive screening workflow integrates the static and dynamic reservoir conditions, and fully couples with geological and geophysical characterization. Firstly, a parametric screening and ranking for various IOR/EOR techniques for the target reservoirs are performed. Secondly, an advanced EOR screening process incorporating the evaluation of geological framework, facies distribution, flow barrier, reservoir production performance and other key reservoir rock and fluid characteristics is applied. The impacts of critical parameters to production performance are rigorously investigated, including various diagnostic plots of waterflood. The oil reservoirs were pre-screened by oil initially in place (STOIIP) and current production rate to screen out the less attractive reservoirs for EOR. Five top-ranked reservoirs were evaluated by integrating geological characteristics (static) and dynamic reservoir indicators. Diagnostic plots such as water oil ratio (WOR), oil rate, number of active oil producers, GOR, reservoir pressure, water injection and production profiles were analyzed to investigate the reservoir performance. Through this workflow, additional opportunities were identified to increase production and/or extend the reservoir life. The reservoir screening for IOR/EOR assessment revealed a high side opportunity of adding over 5.2 MMBOE of recovery through effective reservoir management. A portfolio of most attractive EOR targets was generated for decision making by higher management. The integrated IOR/EOR screening workflow, evolved from practical field experience, is beneficial for revitalizing mature waterfloods, effective reservoir management, recovering bypassed oil, and more importantly the design and implementation of suitable EOR pilots.