Investigation of the pore size and structure in organic-rich shales

Abstract

Permeability in source rocks allows the flow of reservoir fluids during production and is dependent on the pore size distribution. In organic shales, the level of porosity of organic material (OM) is based on its range of pore sizes. Scanning electron microscope (SEM) images are commonly used to examine OM-hosted pores, but this technique is limited by resolution, which is in the order of ~5 nm. This study seeks to increase this range of pore size distribution (PSD) to ~ 0.38 nm, in organic-rich shales by using low-pressure carbon dioxide (CO₂) adsorption coupled with density functional theory (DFT). In addition, we coupled low-pressure nitrogen (N₂) adsorption with the Barrett-Joyner-Halenda (BJH) and DFT models to quantify pore sizes between ~2 to 170 nm. To characterize the entire range of pore sizes, we used high-pressure mercury intrusion because it is commonly used to quantify larger pores. The samples used in this study include a bulk sample and isolated kerogen of Green River shale (Eocene, Utah), Woodford shale (Upper Devonian, Oklahoma), and Cameo Coal (Cretaceous, Colorado). These samples represent type I, II and III, kerogen, respectively, at similar maturity levels and thus provide a good experimental basis for evaluating the PSD. The methodology consisted of four steps: i) Kerogens were isolated from the bulk samples by demineralization, ii) Samples were divided into sizes of ~ 0.5 grams into test tubes and degassed, iii) Samples were analyzed in the Porosimeter using low-pressure N₂ and CO₂ adsorption techniques, iv) Isotherm data from the adsorption measurement were extracted to create the PSD. Our results showed the presence of pore sizes as small as ~ 0.38 nm, based on combining techniques of N₂ adsorption at 77 K and CO₂ adsorption at 273 K in all three samples. Hence, we have expanded our understanding of the range of pore sizes contained in organic-rich material. In addition, the majority of pores in Green River shale and cameo coal fell below the SEM resolution limit of ~5 nm. Lastly, the kerogen and bulk samples of the Green River and Woodford shales showed a variation in the PSD, with the larger pores in the kerogen, which indicates that kerogen constitutes the majority of the pores in the samples. In conclusion, we developed a novel approach to investigate OM-hosted pore sizes. This approach increased the range of pore sizes from ~ 5 nm to ~ 0.38 nm, thus improving the estimation of flow rates during production in shale and in applicable reservoirs.