Assessing the Effective CO2 Storage Capacity of a Reservoir Using a Geomechanical Framework: A Case Study of a Site in the Arches Province of the Midwest U.S

Geological storage in deep saline formations has gathered much attention over last two decades as an effective strategy to combat rising anthropogenic emissions of CO₂. With Carbon Capture and Storage (CCS) initiatives gathering momentum, focus has recently shifted into examining the geomechanical risks associated with injecting CO₂ into formations on the scale of megatons. For many of the deeper formations in the Midwest, there are only limited studies analyzing the effects of CO₂ injection using an approach of multiphase fluid flow simulations coupled with geomechanics. This paper addresses this gap by describing the results of a reservoir simulation study performed on the Mount Simon and the Eau Claire formations. First, the framework for conducting a study that assesses the twin risks of surface deformation and the possibility of induced fractures, is described. Subsequently, we demonstrate the utility of this framework by quantifying the effective CO₂ storage capacity of a formation, in a case-study of a CO₂injection test well drilled into the Mount Simon formation in Boone County, Kentucky, U.S.A.

We begin with an outline of the methodology for obtaining the relevant parameters, emphasizing the uncertainty inherent in quantifying the geomechanical rock properties in particular (e.g. Young’s Modulus, Poisson’s Ratio). We then describe the details of the coupled reservoir-geomechanics simulation study required to comprehensively estimate the effective storage capacity of the formation, within allowable thresholds. This section will include a discussion of coupled reservoir-geomechanics model construction, choice of scenarios to adequately model the uncertainty in the geomechanical properties, as well as an interpretation of the results for surface deformation and the effective stress changes that can lead to fracturing in the relevant formations.

This framework will be illustrated via the study performed for a CO₂ injection test well drilled through the Cambrian Eau Claire Formation (caprock) into the Mount Simon Sandstone (storage formation). Given that only a limited number of geomechanical rock tests were performed, this data served as valuable calibration points for geophysical log analysis. Well-logs were studied to understand the distribution of geomechanical properties as well as to determine the stress magnitude for each formation. These considerations were important in the design (e.g. lateral extent, gridding size, layering) of the coupled reservoir-geomechanics model. The reservoir model was comprised of the caprock overlying the aquifer and the geomechanical model consisted of layers from the basement up to the surface in the simulation. Since most of the parameters describing the formations were derived from a predominantly log-based analysis, significant uncertainty was deemed to be inherent in the model – and as a consequence, in the estimates for the effective storage capacity of the formation. The sensitivity study designed to address this uncertainty, encompassed a variety of scenarios with the best-case and worst-case combination of property values, and by considering different bottom-hole pressure constraints. In addition to the standard examination of formation pressures and CO₂plume migration, simulation results were also interpreted to quantify the range of injected volumes such that there was minimal risk of fault activation, hydraulic fracturing, and that the surface deformation was less than 30mm.

This research was supported by the U.S. DOE / National Energy Technology Laboratory (Contract DE-FE0023330) and the Ohio Development Services Agency Ohio Coal Dev. Office (Grant CDO-D-14-17).