Background
The Department of Energy Office (DOE) of Fossil Energy and Carbon Management is working to refine priority research areas aimed at ensuring that CO2 sequestration (permanent storage) resources in the U.S continue to be utilized and managed efficiently and safely. Several research areas are being considered, including: (i) studies aimed at enhancing the current understanding of basin-scale geomechanical influences and impacts; and (ii) models and monitoring systems designed for assessing/forecasting/predicting basin-scale storage resource performance and impacts. In response to DOE prioritization of research into CO2 sequestration, SwRI Earth Science staff have been researching those processes and effects that need to be better characterized with respect to basin-scale geologic storage of CO2. We addressed the importance of understanding the evolution of subsurface pore pressure and stress in response to long-term CO2 injection. Long-term fluid injection may substantially alter subsurface pore pressures and stresses, leading to a range of potential risks and negative outcomes including (i) reactivation of pre-existing faults and the associated potential for earthquake triggering, (ii) slip or dilation (opening) of pre-existing faults and fractures leading to leakage of CO2, and (iii) compromised integrity of caprock sealing intervals due to large injection volumes. Because of these potential risks, numerical simulations are critical for understanding subsurface geomechanical responses to CO2 injection.
Approach
The research objective of this project was to evaluate the relative importance of selected geologic factors and their uncertainty on stress and pore pressure response to CO2 injection via finite-element-based geomechanical simulations. Finite-element-based geomechanical simulations were performed to evaluate the relative importance of selected geologic factors and their uncertainty on stress and pore pressure response to CO2 injection. The Illinois Basin Decatur project, which injected a total of ~1 million metric tons of CO2 into a saline reservoir over a three year period, served as the test case for the project. Our approach included a base case geomechanical simulation of the Decatur project, followed by parametric examination of key geologic factors in particular, mechanical rock properties on system geomechanical performance. Simulated pore pressure and stress responses to fluid injection are highly sensitive to geologic input parameters. These factors include the initial stress, pore pressure, and temperature distributions, the mechanical stratigraphic framework (e.g., layer thicknesses, rock properties) of the reservoir and sealing intervals (e.g., caprock), and the distribution, orientation, and intensity of pre-existing geologic structures (e.g., faults, fractures). Unlike operational factors, geologic factors are not controllable and are often poorly constrained due to the difficulty and high cost of collecting subsurface data. As such, the uncertainty in initial parameter values can negatively influence our ability to develop a robust geomechanical understanding of the impacts of CO2 injection.
Accomplishments
Our base case simulation was successful in capturing the spatial and temporal pore pressure changes induced by the 3-year injection load. After the base case analysis was developed and calibrated, we performed parametric investigation of the influence of alternative mechanical properties on system behavior under the CO2 injection loading conditions. Of the mechanical properties examined, Young’s modulus and void ratio (porosity) showed the largest influence on the stress state evolution due to injection, and therefore the most potential to negatively impact analyses of CO2 storage functionality (e.g., failure of the caprock). For the tested scenarios, the stress state evolution was less sensitive to friction angle and cohesion, likely due to injection-induced stress changes not reaching elastic limits. The stress state evolution was also found to be insensitive to the initial stress state, which likely reflects the limited initial stress state variability and low injection rate at the Decatur site.