Production from unconventional reservoirs using enhanced oil recovery (EOR) has gained much attention in recent years due to its ability to dramatically increase oil recovery. This paper reviews all EOR methods which have been applied in unconventional reservoirs, with the aim of constructing a roadmap for the conditions at which different EOR methods are applicable. The most applicable unconventional techniques that have been reported to enhance oil recovery are Carbon Dioxide (CO2) gas injection, low salinity water flooding, and surfactant. Various tools are employed to study EOR methods in unconventional reservoirs including simulation, experimental, and field cases. Most of the reviewed studies utilize simulation and experimental tools. Notably, we discovered that gaseous EOR methods, such as huff-n-puff gas injection, were widely used to investigate the applicability of such methods. The most widely used gas that was reportedly used is CO2. Of the chemical methods, surfactant is the material with the most potential to improve oil recovery due to its ability to change the wettability of the reservoir rocks to water wet. We present an analysis of the most common fluid and rock properties of unconventional reservoirs.

1. INTRODUCTION

Unconventional reservoirs have significantly changed oil and gas production in recent years. More than 50% of U.S. oil production comes from shale and tight reservoirs (Energy Information Administration (EIA), 2019). Production from tight reservoirs is predicted to increase significantly due to current developments and advancements in the area. The EIA predicted that U.S. tight oil production will increase significantly in the coming decades, as shown in Figure 1. In terms of current production output, more than 4 million barrels of oil were produced daily between 2011 and 2014 in the U.S (Todd et al., 2016). Alvarez et al. (2016) reported that the Bakken formation only delivers about 10% of total U.S. production, with around 1.1 million barrels per day. Primary recovery methods of horizontal wells and hydraulic fracturing have the main impact on increasing oil and gas production from shale plays (Balasubramanian et al., 2018). Understanding the EOR methods in conventional plays is much easier than unconventional ones due to the abundance information and applications of conventional reservoirs. The applications are dissimilar in unconventional plays due to the ultra-small porosity and permeability of unconventional reservoirs. All unconventional reservoirs share these characteristics; ultra-small permeability and small porosity. The permeability of tight formation is less than 0.1 mD (Jia et al., 2012). Figure 2 shows the types of oil and gas reservoirs according to the permeability cut offs.

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