Enhanced Oil Recovery Techniques in Low Permeability Unconventional Shale Reservoirs

Oil and gas production from very low permeability shale reservoirs became a reality in the 1990s when Mitchel Energy developed novel fracking techniques, but the road ahead was very challenging until around 2008 when oil and gas production from low permeability shale reservoirs became a significant component of total oil and gas production in the US The US Energy Information Administration (EIA) estimates that in 2023, about 3.04 billion barrels (or about 8.32 million barrels per day) of crude oil were produced directly from tight-oil resources in the United States. This was equal to about 64% of total US crude oil production in 2023. Also, the current ultimate amount of oil production from such reservoirs is in the range of 3 to 8% of the in-place oil. Furthermore, the production rates from such reservoirs decline rapidly; thus, there is a compelling need for techniques to produce at least some of the enormous amounts of the remaining reservoir oil. The classical waterflooding or gas flooding used in conventional reservoirs is not plausible in unconventional shale reservoirs because the injection fluid displacement velocities are extremely small which prohibits oil displacement through any injection-production well pattern. Therefore, creative and imaginative approaches are needed to increase oil production without relying on injecting large quantities of water or gas to produce additional incremental oil. The current techniques include two vastly different cyclic injection-production methods—known as the huff-n-puff process. The first method involves injecting either CO2 or rich gas in a multi-stage, hydraulically fractured, production well and soaking for a few days, then producing the same well which generally would yield substantially increased oil flow rates. The second method involves injecting low concentrations of aqueous solutions containing surfactants or mutual solvents. This method is also applied as a huff-n-puff process similar to the gas injection huff-n-puff process. The gas injection huff-n-puff process requires expensive high-pressure injection compressors while solvent injection huff-n-puff requires simpler and less expensive water injection pumps. The mechanism of oil mobilisation by gas injection is generally due to mass transfer across gas-oil interface by molecular diffusion resulting in oil swelling, miscibility, viscosity reduction, and favourable shift in residual oil saturation in the oil relative permeability function. On the other hand oil mobilisation by surfactant solutions or mutual solvents includes reduction of the oil-water interfacial tension in the surfactant case, wettability alteration by surfactant and mutual solvents, and shift in residual oil saturation in the relative permeability function. Additionally, using a brine containing a mutual solvent or a surfactant results in substantial additional oil recovery first by cleaning the micro- and macro-fracture flow paths in the stimulated reservoir volume followed by changes in IFT, wettability, and irreducible oil solubilisation. In this paper we will present laboratory results, mathematical modelling concepts, and field results to illustrate each of the above processes.