Primary
oil recovery from fractured unconventional formations,
such as shale or tight sands, is typically less than 10%. The development
of an economically viable enhanced oil recovery (EOR) technique applicable
to unconventional liquid reservoirs (ULRs) would lead to tremendous
increases in domestic oil production. Although injection techniques
such as waterflooding and CO2 EOR have proven profitable
in conventional formations for decades, EOR in ULRs presents a far
more difficult challenge. The extremely low permeability and mixed
wettability of unconventional formations are the foremost obstacles
to success. Because of the challenges associated with water-based
EOR techniques (a.k.a., chemical EOR) in shale, several nonaqueous
injection fluids have been considered, including CO2, natural
gas, and (to a lesser degree) nitrogen. All these fluids have significantly
lower viscosities than water, allowing them to more easily penetrate
shale nanopores. Unlike water, they also each possess some degree
of miscibility with oil, which enables the gas to extract oil through
a combination of mechanisms. Based on laboratory-scale experimentation,
CO2 and rich natural gas (methane-rich natural gas containing
high concentrations of ethane, propane, and butane) are the most promising
EOR fluids. The interpretation of results from field tests in the
Bakken and Eagle Ford formations have been complicated by interference
of frac-hits or well-bashing caused by hydraulic fracturing at nearby
wells. In this review we cover mechanisms, laboratory experiments,
numerical simulations, and field tests involving high-pressure CO2, natural gas, ethane, nitrogen, and water.
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