Publications

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Subsurface containment of CO2 is predicated on effective caprock sealing. Many previous studies have relied on macroscopic measurements of capillary breakthrough pressure and other petrophysical properties without direct examination of solid phases that line pore networks and directly contact fluids. However, pore-lining phases strongly contribute to sealing behavior through interfacial interactions among CO2, brine, and the mineral or non-mineral phases. Our high resolution (i.e., sub-micron) examination of the composition of pore-lining phases of several continental and marine mudstones indicates that sealing efficiency (i.e., breakthrough pressure) is governed by pore shapes and pore-lining phases that are not identifiable except through direct characterization of pores. Bulk X-ray diffraction data does not indicate which phases line the pores and may be especially lacking for mudstones with organic material. Organics can line pores and may represent once-mobile phases that modify the wettability of an originally clay-lined pore network. For shallow formations (i.e., < {approx}800 m depth), interfacial tension and contact angles result in breakthrough pressures that may be as high as those needed to fracture the rock - thus, in the absence of fractures, capillary sealing efficiency is indicated. Deeper seals have poorer capillary sealing if mica-like wetting dominates the wettability.

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This report evaluates the feasibility of high-level radioactive waste disposal in shale within the United States. The U.S. has many possible clay/shale/argillite basins with positive attributes for permanent disposal. Similar geologic formations have been extensively studied by international programs with largely positive results, over significant ranges of the most important material characteristics including permeability, rheology, and sorptive potential. This report is enabled by the advanced work of the international community to establish functional and operational requirements for disposal of a range of waste forms in shale media. We develop scoping performance analyses, based on the applicable features, events, and processes identified by international investigators, to support a generic conclusion regarding post-closure safety. Requisite assumptions for these analyses include waste characteristics, disposal concepts, and important properties of the geologic formation. We then apply lessons learned from Sandia experience on the Waste Isolation Pilot Project and the Yucca Mountain Project to develop a disposal strategy should a shale repository be considered as an alternative disposal pathway in the U.S. Disposal of high-level radioactive waste in suitable shale formations is attractive because the material is essentially impermeable and self-sealing, conditions are chemically reducing, and sorption tends to prevent radionuclide transport. Vertically and laterally extensive shale and clay formations exist in multiple locations in the contiguous 48 states. Thermal-hydrologic-mechanical calculations indicate that temperatures near emplaced waste packages can be maintained below boiling and will decay to within a few degrees of the ambient temperature within a few decades (or longer depending on the waste form). Construction effects, ventilation, and the thermal pulse will lead to clay dehydration and deformation, confined to an excavation disturbed zone within a few meters of the repository, that can be reasonably characterized. Within a few centuries after waste emplacement, overburden pressures will seal fractures, resaturate the dehydrated zones, and provide a repository setting that strongly limits radionuclide movement to diffusive transport. Coupled hydrogeochemical transport calculations indicate maximum extents of radionuclide transport on the order of tens to hundreds of meters, or less, in a million years. Under the conditions modeled, a shale repository could achieve total containment, with no releases to the environment in undisturbed scenarios. The performance analyses described here are based on the assumption that long-term standards for disposal in clay/shale would be identical in the key aspects, to those prescribed for existing repository programs such as Yucca Mountain. This generic repository evaluation for shale is the first developed in the United States. Previous repository considerations have emphasized salt formations and volcanic rock formations. Much of the experience gained from U.S. repository development, such as seal system design, coupled process simulation, and application of performance assessment methodology, is applied here to scoping analyses for a shale repository. A contemporary understanding of clay mineralogy and attendant chemical environments has allowed identification of the appropriate features, events, and processes to be incorporated into the analysis. Advanced multi-physics modeling provides key support for understanding the effects from coupled processes. The results of the assessment show that shale formations provide a technically advanced, scientifically sound disposal option for the U.S.

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A combined experimental and constitutive modeling program for weak porous sandstone deformation is described. A series of axisymmetric compression tests were performed over a range of mean stresses to study dilatational, compactional and transitional regimes. Experimental results were used both to derive constitutive parameters for testing localization theory and to parameterize a poroelastic-plastic model. Observed strain localization, imaged syn-deformationally using acoustic emissions, includes high- and low-angle shear and low angle compactional features or 'bands'. Isotropic elastic moduli measured via unloading loops show a progressive degradation pre-failure as decreasing functions of work-conjugate plastic strains and increasing functions of stress magnitude. The degradation pathway is unique for samples which underwent localization versus those that underwent spatially pervasive pore collapse. Total shear and volume strains are partitioned into elastic and plastic portions including the ''coupling'' strain associated with modulus degradation. Plastic strain calculated with and without the coupling term is compared with regard to localization predictions. Both coupled and uncoupled cases predict high angle shear bands for uniaxial and low mean stress conditions on the dilatational side of the yield surface. Uncoupled predictions show progressively lower angle shear bands approaching the transitional regime (stress conditions approaching the 'cap' surface). When elastic-plastic coupling is accounted for, compaction bands are predicted for the transitional regime, as are observed in the experiments. Finite element modeling efforts are described using a 3-invariant, mixed-hardening, continuous yield surface, elasto-plasticity model that includes several features important for porous sandstone constitutive behavior and observed experimentally, including non-associativity, nonlinear elasticity, elastic-plastic coupling, and kinematic hardening. Modeled deformational behavior attending stress paths relevant for several reservoir production scenarios are described.

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Natural helium is a screening tool for identifying the presence or absence of caprock imperfections. Imperfections can be manifested as a variety of features or processes, including insufficiently low permeability, preferential flowpaths such as fractures and faults, and the propensity for capillary breakthrough. Theory and simulations detail how various types of imperfections affect the spatial distribution of natural helium above, within, and below caprock in a single-phase, brine-saturated system. Specifically, the distribution of natural helium can reveal the presence of preferential flowpaths through formations with low matrix permeability. The distribution patterns of helium shed insight on the size, shape, location, and connectedness of imperfections in caprock. We show how imperfections associated with characteristic distributions of natural helium will affect the retention of CO2. We discuss the advantages of natural helium, together with temperature distributions, for revealing imperfections and the optimum locations for sampling the natural tracers. This research is being carried out to support design and interpretation of ongoing field-testing by the Southwest Regional Partnership on Carbon Sequestration. Specifically, we are evaluating seal integrity of the Partnership's Pump Canyon Enhanced Coalbed Methane- CO2 Storage Demonstration, located in the San Juan Basin, New Mexico. The caprock at this site is the Kirtland Formation. This formation is composed of a variety of continental deposits (sandstones, siltstones, mudrocks, and shales) and is ideal for investigating the capability of helium to characterize sealing integrity of a very heterogeneous caprock. We present results of analyses of noble gases and a variety of petrological and petrophysical analyses on core through this caprock. These results are used to investigate the presence of imperfections and their potential impact on CO2 migration and the overall viability of utilizing natural helium as a screening tool. The authors gratefully acknowledge the U.S. Department of Energy and the National Energy Technology Laboratory for sponsoring this project. © 2009 Elsevier Ltd. All rights reserved.

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