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.

Shale has ultra-low matrix permeability, and consequently requiring the creation of hydraulic fracturing to maximize the contact area with the reservoir. The key to successful fracturing treatment in shale formation is the identification of the sweet spots. Productive shale consists of quartz, feldspar or carbonate and clays, in addition to organic matter (Kerogen). Thus it is challenging process to map the best zones to fracture and locate horizontal wells. A new Fracturability Index based on mineralogy has been developed to locate the best areas along horizontal wells to fracture. A good Mineralogical Index would prolong production plateau for shale plays.Current technology follow two schools, first one through equally spaced fracturing, and the more fractures the better production, second; guide through brittleness index, which distinguish brittle versus ductile zones along the well path, supporting the second school, we have a better correlation of Fracturability index, the new correlation indicates the most brittle parts of the reservoir (MI>0.6) versus the most ductile parts (MI <0.6), it became easier to map the producing shale with sweet or unattractive spots leading to the effective fracture locations. It is a new sweet spot identifiers, which guide the fracture design and fracture allocation along horizontal wellbore path, it may optimize well placement and hydraulic fracturing positioning in unconventional resources.A new Minerological Index is developed ranging from 0 to 1 which helps optimize the fracturing and shale development, in addition to guaranteeing fracture treatments are in the right place. This will facilitate connectivity with natural fracture network, priority of guiding fracture design start with High FI which corresponds to higher mineralogy index. MI value of 0.6 is a good starting point to map sweet spots. This may enhance far field Fracturing Complexity and help getting a branched fracture. Based on a sub category of quartz, type of treatment may be recommended.

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The Wells Placement Problem (WPP) consists in choosing well locations within an oil reservoir grid to maximize the reservoir total oil production, subject to distance threshold between wells and number of wells cap constraints. A popular approach to WPP is Genetic Algorithms (GA). Alternatively, WPP has been approached in the literature through Mathematical Optimization. Here, we conduct a computational study of both methods and compare their solutions and performance. Our results indicate that, while GA can provide near-optimal solutions to instances of WPP, typically Mathematical Optimization provides better solutions within less computational time.

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