Natural Tracers and Multi-Scale Assessment of Caprock Sealing Behavior: A Case Study of the Kirtland Formation, San Juan Basin

Abstract

The assessment of caprocks for geologic CO 2 storage is a multi-scale endeavor. Investigation of a regional caprock-the Kirtland Formation, San Juan Basin, USA-at the porenetwork scale indicates high capillary sealing capacity and low permeabilities. Core and wellscale data, however, indicate a potential seal bypass system as evidenced by multiple mineralized fractures and methane gas saturations within the caprock. Our interpretation of 4 He concentrations, measured at the top and bottom of the caprock, suggests low fluid fluxes through the caprock: 1) Of the total 4 He produced in situ (i.e., at the locations of sampling) by uranium and thorium decay since deposition of the Kirtland Formation, a large portion still resides in the pore fluids. 2) Simple advection-only and advection-diffusion models, using the measured 4 He concentrations, indicate low permeability (~10 -20 m 2 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 general. EXECUTIVE SUMMARYThe assessment of caprock for underground CO 2 storage is challenging due to geologic heterogeneity at multiple scales. Caprocks impede movement of fluids by means of viscous and capillary forces, due to their small pores. However, larger-scale fractures, faults, or other "seal bypass systems" can circumvent the pore networks and potentially lead to significant fluid migration through a caprock. This topical report presents a site-specific study by the Southwest Regional Carbon Sequestration Partnership (SWP) to develop multi-scale assessment methods that incorporate natural tracers. Natural tracers are already present at all sequestration sites and represent a resource for investigating the long-term and spatially-distributed transport properties of a caprock without the need for injecting applied or "exotic" tracers into the system. The multiscale assessment is not based on upscaling a variety of data sets, but simply comparing different types of data collected at different scales to determine if the data are coherent. As shown in this study, some small-scale data may indicate a different type of caprock sealing quality than data that apply at a larger scale.Study of a regional caprock-the Kirtland Formation, San Juan Basin, USA-at the porenetwork scale indicates high capillary sealing capacity and low permeabilities. Core and wellscale data, however, indicate a potential seal bypass system as evidenced by multiple mineralized fractures and methane gas saturations within the caprock. Our interpretation of 4 He concentrations, measured at the top and bottom of the caprock, suggests low fluid fluxes through the capr...