Geomechanical Characterization of the Mount Simon Sandstone and Eau Claire Formation of Northern Illinois Basin

ABSTRACT The purpose of this paper is to analyze publicly available geomechanical data from two carbon storage sites in the Illinois Basin: the Illinois Basin-Decatur Project (IBDP) in Macon County, IL and the FutureGen2.0 project in Morgan County, IL, which was cancelled before injection began. This study estimates the magnitudes and directions of the least compressive principal stress gradient using hydraulic fracture-based stress measurement tests, borehole breakouts, and sonic logs in the Eau Claire Formation (primary confining zone) and the Mount Simon Sandstone (targeted reservoir). The range of stresses expected in the underlying Precambrian basement is also investigated. The evaluation of the state of stress is carried out in a probabilistic manner using the State of Stress Analysis Tool (SOSAT) developed under the National Risk Assessment Partnership (NRAP) program. Finally, the paper also provides a discussion on distinctions between fracture initiation pressure, propagation pressure, and fracture closure pressure, given that hydraulic fracture testing results show a significant difference between these three values. The discussion also addresses why the fracture closure pressure should be preferred in injection pressure determinations, a critical point in geological carbon storage projects. INTRODUCTION In this paper, geomechanical data from two geological carbon storage (GCS) sites in the northern Illinois basin are analyzed and compared to each other. The GCS projects are the Illinois Basin-Decatur Project (IBDP) in Macon County, IL, and the FutureGen2.0 project in Morgan County, IL. While operations in IBDP were fully completed, the FutureGen2.0 project was interrupted before injection began. Both sites underwent extensive subsurface characterization and made their geomechanical data publicly available. The geomechanical datasets have been analyzed separately. For example, Appriou (2019) assesses the geomechanical risks associated with CO2 injection at the FutureGen2.0 site and Gilmore et al. (2016) describe and analyze the comprehensive characterization program at the FutureGen 2.0 site and refer the readers to essential publications regarding further characterization efforts, modeling of carbon dioxide (CO2) injection and transport, monitoring, verification, and accounting plan. For IBDP, Bauer et al. (2016) and references therein provide an overview of the geologic characterization of the site and analyze the microseismic response to CO2 injection at the IBDP site.