Gas huff-n-puff is an attractive enhanced oil recovery process for shale oil and is being field tested by several companies. In this work, laboratory core-scale huff-n-puff studies were conducted using CO2 and a hydrocarbon gas containing 60% methane, 35% ethane and 5% propane. The liquid saturation and pore occupancy was studied by nuclear magnetic resonance spectrometry. The study showed that the oil recovery is very similar for these two gases, even though the calculated minimum miscible pressure differed by 800 psi. The oil recovery was independent of pressure above 2800 psi. The oil recovery decreased sharply with subsequent cycles, the first cycle recovering 75% of the recovery in 3 cycles.
Rapid increase of energy demand has brought the focus of petroleum industry toward vast shale resources worldwide. As self-sourced formations, shales have large amounts of hydrocarbon-in-place. The advances in drilling and fracturing techniques have unlocked shale formations and enabled shales to become a reliable hydrocarbon source in US. In the last decade, oil production from shale resources went from 12% of the total US oil production to more than 62%, and it is expected to increase in the decades to come (US EIA 2019). Shale reservoirs are known by their extremely low permeability (in the order of nd to μd).
Horizontal drilling and multi-stage hydraulic fracturing techniques have made oil production possible in shale resources. During a typical hydraulic fracturing operation, the slickwater or viscous polymeric solution is injected into the well to create fractures. The hydraulic fractures and connected natural fracture networks can significantly improve the hydrocarbon productivity by enhancing the effective permeability, the matrix-wellbore connectivity, and the stimulated volume. The reservoirs are depressurized through these fractured horizontal wells. However, the primary production recovers only 2-10% of initial oil in place (Clark 2009, Hoffman 2012). In addition, the production declines rapidly which requires the infill drilling or subsequent fracturing to maintain profitable production rates (Lee et al. 2011, Alfarge et al. 2017). Therefore, the implementation of enhance oil recovery (EOR) techniques in shale resources is necessary for sustainable development. So far, several EOR techniques have been investigated to improve oil recovery from shale resources including gas injection (Gamadi et al. 2013, Chen et al. 2014, Sanchez-Rivera et al. 2014, Habibi et al. 2017, Zeng et al. 2020, Tovar et al. 2021), chemical injection (Nguyen et al. 2014, Wang et al. 2012, Alvarez and Schechter 2016, Mohanty et al. 2019), and thermal treatment (Egboga et al. 2017).
