QUANTIFYING PORE SCALE GEOCHEMICAL INTERACTIONS OF SCCO2 WITH SHALE: SEALS OR STORAGE

As demand increases for an affordable energy source coupled with a reduction in greenhouse gas emissions, there is a growing consideration in shale production utilizing processes such as 1) enhancing hydrocarbon recovery (EOR) via carbon dioxide (CO₂) flooding, 2) using CO₂ as a fracturing agent, and 3) storing CO₂ in depleted shale formations to mitigate emissions to the atmosphere. Understanding the geochemical reactions and subsequent petrophysical alterations that occur as shale is exposed to fluids and CO₂ is necessary to develop and optimize each of these processes for field applications.

When CO₂ is introduced into shale reservoirs, reactivity with formation fluids, injected fracturing fluids, and the shale matrix may generate mechanical or chemical alterations that alter flow paths and properties of the formation. Shales tend to have characterization challenges due to their high heterogeneity and nano-sized pore networks. CO₂ storage, flooding, or EOR in shale will be impacted by mineralogy (including clay types), organic content, and pore scale variability. Pore scale variability is directly influenced by the amount and types of minerals and organics comprising the shale matrix. Therefore, to understand CO₂ storage potential in any given shale play, characterizing this complex nature is necessary.

Twelve shale samples from eight formations have been characterized using in situ Fourier Transform infrared spectroscopy (FTIR), feature relocation scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), and surface area and pore size analysis using volumetric gas sorption. Our studies show that CO₂ will physically interact with the organic matter and clay components in the shale. When water is available (either in the clay interlayers, inherent water found in the shale itself, or injected as hydraulic fracturing fluid) CO₂ dissolves in the water to form carbonic acid. This carbonic acid then dissolves carbonate causing the shale to be etched and pitted while simultaneously reprecipitating carbonate matter in other regions in the matrix. As a result, micrometer sized pores begin to open (freshly etched carbonate) while nanosized pores begin to close (clogged with precipitated carbonate). In addition, micro fracture apertures begin to increase during carbonic acid reactions.