Field and laboratory studies have shown that EOR Huff-n-Puff (HnP) in tight rocks can mobilize additional hydrocarbon. However, these previous studies have not investigated potential changes to the microstructure of shales due to the EOR process. The present study focuses on the study of microstructural changes during EOR HnP on shale samples from the Duvernay and Montney.
To evaluate how EOR operations could potentially alter the microstructure of unconventional formations, HnP experiments were conducted with a mixture of methane and ethane (C1:C2/72:28mol%) were performed on crushed samples (7-8 mm) from Duvernay and Montney (oil window). These experiments were carried on samples in their preserved state (without recombination with dead oil) at 1000 psi above minimum miscibility pressure (MMP) and 150 °F using 1-hour soak and 1-hour production time. Changes to the microstructure were evaluated by the integration of measurements from Nuclear Magnetic Resonance (NMR), Hydrocarbon Analyzer with Kinetics (HAWK) pyrolysis, Mercury Injection Capillary Pressure (MICP), isothermal adsorption based on Brunauer Emmett Teller theory (BET) and SEM (Scanning Electron Microscopy).
Significant microstructural alterations were observed after the HnP experiments. We observed an increase in the proportion of pore throat size between 3-10 nm radius and an increase in surface area by a factor of 2 in the Duvernay sample. SEM images also confirm MICP and BET observations where the organic matter pore size increased by a factor of 2 in the same sample after EOR. Oil composition analysis from HAWK shows that hydrocarbons up to C24 were efficiently mobilized during the process for both samples. However, in the Duvernay sample heavier hydrocarbons up to C27 were produced.
This experimental study shows that gas injection leads to measurable changes in the microstructure and organic component of the rock. It also gives an insight into reservoir selection for potential field HnP candidate.
The successes reported by EOG in the Eagle Ford have renewed interest in EOR HnP; additional 30-70% recovery could be unlocked after primary production (Hoffman, 2018). The EOR HnP process is designed to release hydrocarbons through miscible injection, injected gas moves into the fractures and diffuses through the rock at the matrix/fracture interface (Fragaso et al., 2015; Sheng 2017); driven by pressure the oil swells and its viscosity reduces, as the pressure gradient decreases the oil is moved from pores to the fractures (Hawthorne et al., 2014).