Experimental and Numerical Validation of Cooling-Induced Stress Change Within a Caprock Shale – Seal Integrity Insights from the Northern Lights CCS Project

Abstract

ABSTRACT: As Carbon Capture and Storage (CCS) is experiencing rapid growth, there is increased focus on the geomechanical integrity of the storage system(s). In addition to pressure limitations, thermal influences (due to injection of cold CO2 into a warmer subsurface) can play an important role in defining the geomechanical integrity of the caprock seal. Through analytical investigation, Thompson et al. (2021) recognized that undrained pore pressure effects in low permeability caprocks alter the effective stress picture of the caprock, as the caprock may be cooled significantly due to conduction processes. This paper builds on that work and presents laboratory data from specially designed thermal uniaxial strain testing in caprock shale material, as a validation exercise for the analytical investigation. The results of these tests are directly compared to confirm the analytical approach and to determine lab-calibrated parameters that can be used for further simulation and assessment of caprock integrity. Where the experimental and analytical approaches serve as end member descriptions of the effective stress evolutions (drained versus undrained) from cooling, numerical simulations are also performed to quantify additional factors that may affect the effective stress response. All three approaches are found to be consistent, qualifying the analytical approach as a robust means of thermal influence investigation for caprock integrity evaluation. 1. INTRODUCTION In the challenge to meet net zero ambitions, the Carbon Capture and Storage (CCS) industry is developing rapidly, with a high number of large-scale projects planned and/or coming on stream in the coming years (Global CCS Institute [2023]). Integrity assurance of the subsurface storage systems is paramount and, subsequently, geomechanical characterization and risk assessment is recognized as first-order contribution to storage site feasibility and storage volume potential (Rutqvist [2012]). Inherent to CO2 injection and storage are changes in pore pressure that need be assessed in terms of geomechanical integrity. Thermal changes can also be equally as important, as the injected CO2 is often significantly colder than the temperature of the storage formations. Such thermal influences may result in significant changes in effective stress both in the reservoir/storage and caprock/seal formations and therefore must be fully assessed in order to fully quantify the geomechanical risk for a given storage project.