Physical characteristics of caprock formations used for geological storage of CO2 and the impact of uncertainty in fracture properties on CO2 transport through fractured caprocks

Capture and geological sequestration of CO2 from energy production is proposed to help mitigate climate change caused by anthropogenic emissions of CO2 and other greenhouse gases. Performance goals set by the US Department of Energy for CO2 storage permanence include retention of at least 99% of injected CO2 mass. Part of meeting these goals will be detailed assessments of each potential storage site’s geologic environment, especially properties of the storage reservoir(s) and caprock(s) that may affect permanence of CO2 storage.
The overall goals of this research were to examine the physical and lithologic characteristics of caprock formations considered for saline CO2 sequestration, and to investigate the impact of uncertainty in hydraulic properties of a fractured caprock on the ability to meet long-term CO2 storage goals. To accomplish these goals three specific objectives were pursued: 1) Review the current state of knowledge on the physical and lithologic characteristics of caprocks in areas considered for CO2 sequestration, and identify common features that may impact long-term CO2 storage. 2) Develop an integrated analytical model to investigate the influence of fracture hydraulic properties on the transport of CO2 through caprocks. 3) Investigate the impact of uncertainty in fracture aperture and density on predicting CO2 loss and caprock hydraulic fracture properties associated with meeting long-term storage goals.
Review of the caprock properties revealed that they were generally thick and exhibited low permeability. However, they were not continuous or uniform in lithology throughout the regions examined. Caprocks exhibited lateral facies changes, fractures, and spatial variability in thickness, permeability, porosity, and other physical properties that could affect CO2 storage. Fractures reported in caprock formations were not fully characterized and had unknown regional extent and interconnectivity.
An integrated analytical model was developed to estimate the limits of hydraulic fracture properties within a caprock that are consistent with storage performance criteria, and with observed ranges for aperture size and density within field studies on fracture networks. Results showed hydraulic fracture properties, consistent with performance objectives, to be low in comparison to reported measurements. In particular, 1) microfractures (e.g. 10-7 to 10-6 m range) yielded CO2 loss rates of concern given certain conditions. (2) Fracture permeability was in the nano- to micro-Darcy (μD) range (i.e. 10-21 – 10-18 m2), and 3) Fracture porosities were below 0.02 %.
For the third objective, a stochastic framework was applied to the integrated analytical model to examine the impact of uncertainty in caprock fracture aperture and density on predicting CO2 loss and hydraulic fracture properties meeting CO2 storage criteria. Major findings include: 1) combinations of parameters meeting the CO2 loss criteria were rare events and more data would be needed to characterize caprock fractures. 2) Fracture porosity was identified as a good diagnostic parameter for caprock screening. (3) Fracture permeability had the strongest association with CO2 loss, with a high probability (>90%) that caprocks which met performance goals had values < 10-17 m2. (4) Correlations between reservoir parameters and caprock fracture properties became stronger as the CO2 loss from the system became more constrained.
Overall, the results of this study showed that selected caprocks in the U.S, currently investigated for CO2 storage, exhibit significant variability in their structural, lithologic, and fluid transport characteristics. Pre-existing fractures can occur in caprocks, which is of interest for impact on long-term CO2 storage. Modeling results suggest a low tolerance for microfractures in overlying caprocks, where acceptable hydraulic fracture properties were low in comparison to reported measurements. In addition, the interdependence of the transport parameters showed that the storage reservoir and caprock fracture properties needed to be modeled together in order to assess the potential to meet CO2 storage criteria.