Multi-scale Petrography and Fluid Dynamics of Caprocks Associated with Geologic Co2 Storage

Implementation of the underground storage of anthropogenic CO2 in geologic media requires containment of the CO2. The general concept for onshore, deep (> 800 m) geologic environments includes both a permeable reservoir and an overlying low permeability, high capillary-breakthrough-pressure “caprock”. Significant migration of the buoyant CO2 from the reservoir may lead to inadequate performance in terms of storage goals and possibly impact underground sources of drinking water. Consequently, the sealing efficiency of a caprock is critical. Carbon dioxide can migrate as a separate phase (e.g., supercritical or gaseous depending on pressure and temperature conditions) and as a dissolved aqueous species through caprock features that range in size. These features vary from the microscopic (e.g., pore networks with sub-μm pore sizes) to some appreciable fraction of the caprock thickness (e.g., mesoscopic fractures) and up to the full thickness of the caprock (e.g., faults, connected fracture networks, and large-scale sedimentary discontinuities). Caprocks must impede CO2 migration at all these scales for effective storage. The largerscale, high permeability “seal bypass systems” are a major concern due to the potential of relatively rapid, significant loss of CO2. Thus, the evaluation of caprock for CO2 storage is a multi-scale challenge. It requires determination of the spatial scale of the feature that dominates the overall response (i.e., both temporal and spatial) of the system. The research objective of this study is to assess performance of caprocks from the pore-scale up to that of the entire thickness and the field-scale (i.e., projected areal extent of storage site). Methods of assessment combine novel, high resolution three-dimensional imaging techniques of pore networks of caprocks, fracture characterization, and collection of noble gases from the top and bottom of a caprock. Mudstones are the focus as they constitute a primary caprock type in sedimentary basins. Noble gases are especially key for elucidating dominant transport processes of a caprock, which indicate the impact of seal bypass features on fluid fluxes. Sub-μm reconstructions of pore networks from continental to shallow to deeper marine mudstones indicate that primary depositional environmental and burial history strongly control sealing efficiency (i.e., capillary breakthrough pressures). The sealing efficiency generally increases from proximal to distal mudstone facies, which is indicated by differences in geometric pore structure, pore types, connectivity of pores, and mercury intrusion pressures. Burial history can influence organic materials through mobilization and filling of pores, resulting in higher breakthrough pressures. Therefore, in general, choice of a high quality caprock seal involves identification of caprocks with primary distal environments and that have undergone deep burial. Examination of pore-lining solid phases indicates that capillary sealing is governed by pore shapes and phases that are not identifiable except through high resolution direct characterization of the pores. Pore-lining phases are not directly indicated by bulk X-ray diffraction data. Organics can line pores and such linings may be the remains of once-mobile organics that modified the wettability of an originally claylined pore system. For shallow formations (i.e., ~800 m depth or shallower) interfacial tension and contact angles indicate that breakthrough pressures would be sufficient to fracture the rock—thus, sealing by capillarity is indicated. Deeper seals have poorer capillary sealing if mica-like wetting dominates the wettability. However, little information on wetting properties of the pore-lining phases observed in this study is available in the literature. The results of this study thus show that there is a high degree of uncertainty in prediction of capillary sealing behavior. The wettability properties of CO2 and brine in contact with common caprock minerals (i.e., clays) and organics should be a major focus of future research. Study of a proposed regional, overpressured caprock—the Kirtland Formation, San Juan Basin, USA—via multi-scale methods suggests that although pore network properties can contribute to very high capillary sealing capacity and low permeabilities, larger-scale high-permeability features may exist. Fracture mineralization and gas saturation within and directly above the caprock evidence multiple episodes of fluid flow. Interpretation of He concentrations, measured at the top and bottom of the caprock, suggests low fluid fluxes through the caprock: 1) Of the total He produced in situ (i.e., at the location of sampling) by U and Th decay since deposition of the Kirtland Formation, a large portion still resides in the pore fluids. 2) Simple advection-only and advectiondiffusion models, using the measured He concentrations, estimate low permeability (~10 m or lower) for the thickness of the Kirtland Formation. These findings, however, do not guarantee the lack of a large-scale bypass system. The measured data, located near the boundary conditions of the models (i.e., the overlying and underlying aquifers), limit our testing of conceptual models and the sensitivity of model parameterization. Thus, we suggest approaches for future studies to better assess the presence or lack of a seal bypass system at this particular site and for other sites in