The Utility of Multilevel Pressure Measurements in Monitoring Geologically Stored Carbon Dioxide

Advisor

Sally M. Benson

Abstract

Emissions of carbon dioxide (CO₂) due to the burning of fossil fuels are cited by the Intergovernmental Panel on Climate Change (IPCC) as virtually certain to have a dominant influence on atmospheric concentrations of CO₂ in the 21st century. A growing concern is that the presently rapid increase of greenhouse gases in the atmosphere contributes to climate change. CO₂ being the predominant greenhouse gas, much emphasis has been placed on how to reduce CO₂ emissions. One promising approach is carbon dioxide capture and storage (CCS). CCS can be summarized as the process through which CO₂ is captured from a stationary emission source (e.g. a power plant) and stored permanently in an underground geologic formation overlain by a "caprock‟ formation with sealing properties. CCS has received a great deal of attention because it has the potential to significantly reduce anthropogenic CO₂ emissions while at the same time utilizing technology used in other industries. Critical to large-scale implementation of CCS is the ability to monitor the injected CO₂ to ensure permanent sequestration. A monitoring method currently under investigation involves having multiple, vertically distributed pressure sensors in a monitoring well that extends down to the depth of injection. This study examines the extent to which multilevel pressure measurements in the storage reservoir, seal, and overlying aquifer can provide information on where the CO₂ as well as the displaced brine migrate in the reservoir. The study is conducted using the TOUGH2 multiphase flow simulator. Based on assumed geology at a CCS demonstration project site in Illinois, we investigate multilevel pressure measurements in a 30-layer system where supercritical CO₂ is injected in the bottom layers of a 23-layer storage reservoir. A six-layer shale formation comprises the seal and a one-layer sandstone aquifer overlies the seal. Porosity, permeability, and the capillary pressure curve are uniquely defined for each layer. A total of one million metric tons are injected over three years. Four basic scenarios for the constructed geologic system are studied: 1) homogeneous and isotropic, 2) homogeneous and anisotropic, 3) heterogeneous and vi isotropic, and 4) heterogeneous and anisotropic. For practical purposes, the fourth scenario is the most realistic scenario, as every real reservoir will have some degree of heterogeneity and anisotropy. Because it is critical to know whether the presence of CO₂ in the system gives rise to a distinct pressure response compared to when there is just water flowing, we also conduct a sensitivity study on the effects of CO₂ injection versus pure water injection, assuming equivalent volumetric injection flow rates. Examination of the pressure responses shows that large, detectable pressure changes can be observed for all of the scenarios. Distinct pressure transients for the different system scenarios suggest that heterogeneity greatly impacts the pressure response. Normalized vertical pressure gradients nevertheless appear to be more diagnostic of the nature of the system heterogeneity and CO₂ plume location rather than pressure transients from individual monitoring points alone. Normalized vertical pressure gradients provide 1) a clear representation of the system heterogeneity soon after start of injection, 2) distinct features depending on whether CO₂ is present in the system, and 3), a strong indication of where the CO₂ is in the reservoir, i.e. at what depth, prior to the CO₂ arriving at the monitoring well. Anomalous vertical pressure gradients can be attributed to anomalous vertical aqueous flow caused by water displacement due to the advancing CO₂ plume. Based on these results it seems beneficial to place as many pressure monitors as possible in the system; especially at the depth of injection and in low permeability layers above. The study presented here confirms the basis for an inverse method for reservoir characterization and CO₂ plume migration detection.