An image-based equation for estimating the prospective CO2 storage resource of organic-rich shale formations

Abstract

An image-based volumetric equation for estimating the prospective CO2 mass storage resource potential for organic-rich shale formations has been developed using data obtained from advanced image analysis of Bakken Formation shale samples. The equation is a modification of the U.S. Department of Energy (DOE) National Energy Technology Laboratory’s (NETL’s) volumetric equation methodology for estimating the prospective CO2 mass storage resource for shale formations. The current equation enhances DOE’s version by systematically deriving expressions for calculating efficiency factors based on analysis of high-resolution field-emission scanning electron microscopy (FESEM) shale images. FESEM images are used to obtain improved nanoscale porosity estimates for the shale matrix and OM. The calculation of the efficiency factors associated with free-phase storage and CO2 adsorption onto solid surfaces are based on a ratio of connected-to-nonconnected porosity and the newly developed shared border analysis (SBA) method, respectively. The image-based equation described herein is structured so that it can be adapted and applied to other formation types by considering the specific mineralogy or matrix characteristics. Preliminary data used to illustrate the newly developed equation show that the Upper Bakken Shale (UBS) may have a lower prospective CO2 storage resource potential than the Lower Bakken Shale (LBS) by about 65 %. Although these results are neither optimized for each lithofacies nor represent the Bakken formation regional scale, they are encouraging for unfractured shale samples and serve as a useful starting point for future evaluations at the regional scale.