Publications

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Wellbore integrity of abandoned wells is of high priority for ensuring the containment of sequestered CO₂. Carbonic acid formed when injected CO₂ mixes with subsurface brines has the potential to damage well cement so as to compromise the seal integrity of the wellbore. Bench-scale experiments reported in the literature indicate that the well cement reaction rates are initially fast enough to constitute a potential threat to wellbore integrity. However, it has also been suggested that the formation of calcium carbonate within the cement effectively arrests the acid attack by forming a passivation layer (so called “self -sealing”) that prevents further leaching of cement minerals. As a result, a broader theoretical context is presented here that delineates brine composition regimes that will instigate self-sealing in cement during carbonic acid attack.

Subsurface geologic formations used for extracting resources such as oil and gas can subsequently be used as a storage reservoir for the common greenhouse gas CO₂, a concept known as Carbon Capture and Storage (CCS). Pre-existing wellbores penetrate the reservoirs where supercritical CO₂ is to be injected. These wellbores can potentially be a pathway for contamination if CO₂ leaks through wellbore flaws to an overlying aquifer or the atmosphere. Characterizing wellbore integrity and providing zonal isolation by repairing these wellbore flaws is of critical importance to the long-term isolation of CO₂ and success of CCS. This research aims to characterize the microannulus region of the cement sheath-steel casing interface in terms of its compressibility and permeability. A mock-up of a wellbore system was used for lab-scale testing. Specimens, consisting of a cement sheath cast on a steel casing with microannuli, were subjected to confining pressures and casing pressures in a pressure vessel that allows simultaneous measurement of gas flow along the axis of the specimen. The flow was interpreted as the hydraulic aperture of the microannuli. Numerical models are used to analyze stress and displacement conditions along the casing-cement interface. These numerical results provide good agreement with closed-form elastic solutions. Numerical models incorporating flaws of varying dimensions along the casing-cement interface were then developed to describe the microannulus region. A joint model is used to describe the hydraulic aperture of the microannulus region, whose mechanical stiffness is altered in response to the imposed stress state across the joint interface. The aperture-stress behavior is based upon laboratory measurements of hydraulic aperture as a function of imposed stress conditions. This investigation found that microannulus permeability can satisfactorily be described by a joint model and that the constitutive model imposed in a numerical simulation can play a significant role in the solution behavior and agreement to experimental data. Recommendations for future work include an application of the joint model with a thermally active large-scale reservoir coupled with pore pressure caused by dynamic CO₂ injection and subsequent microannulus region affects.

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At the initiation of the Used Fuel Disposition (UFD) R&D campaign, international geologic disposal programs and past work in the U.S. were surveyed to identify viable disposal concepts for crystalline, clay/shale, and salt host media. Concepts for disposal of commercial spent nuclear fuel (SNF) and high-level waste (HLW) from reprocessing are relatively advanced in countries such as Finland, France, and Sweden. The UFD work quickly showed that these international concepts are all “enclosed,” whereby waste packages are emplaced in direct or close contact with natural or engineered materials . Alternative “open” modes (emplacement tunnels are kept open after emplacement for extended ventilation) have been limited to the Yucca Mountain License Application Design. Thermal analysis showed that if “enclosed” concepts are constrained by peak package/buffer temperature, that waste package capacity is limited to 4 PWR assemblies (or 9 BWR) in all media except salt. This information motivated separate studies: 1) extend the peak temperature tolerance of backfill materials, which is ongoing; and 2) develop small canisters (up to 4-PWR size) that can be grouped in larger multi-pack units for convenience of storage, transportation, and possibly disposal (should the disposal concept permit larger packages). A recent result from the second line of investigation is the Task Order 18 report: Generic Design for Small Standardized Transportation, Aging and Disposal Canister Systems. This report identifies disposal concepts for the small canisters (4-PWR size) drawing heavily on previous work, and for the multi-pack (16-PWR or 36-BWR).

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At the initiation of the Used Fuel Disposition (UFD) R&D campaign, international geologic disposal programs and past work in the U.S. were surveyed to identify viable disposal concepts for crystalline, clay/shale, and salt host media (Hardin et al., 2012). Concepts for disposal of commercial spent nuclear fuel (SNF) and high-level waste (HLW) from reprocessing are relatively advanced in countries such as Finland, France, and Sweden. The UFD work quickly showed that these international concepts are all “enclosed,” whereby waste packages are emplaced in direct or close contact with natural or engineered materials . Alternative “open” modes (emplacement tunnels are kept open after emplacement for extended ventilation) have been limited to the Yucca Mountain License Application Design (CRWMS M&O, 1999). Thermal analysis showed that, if “enclosed” concepts are constrained by peak package/buffer temperature, waste package capacity is limited to 4 PWR assemblies (or 9-BWR) in all media except salt. This information motivated separate studies: 1) extend the peak temperature tolerance of backfill materials, which is ongoing; and 2) develop small canisters (up to 4-PWR size) that can be grouped in larger multi-pack units for convenience of storage, transportation, and possibly disposal (should the disposal concept permit larger packages). A recent result from the second line of investigation is the Task Order 18 report: Generic Design for Small Standardized Transportation, Aging and Disposal Canister Systems (EnergySolution, 2015). This report identifies disposal concepts for the small canisters (4-PWR size) drawing heavily on previous work, and for the multi-pack (16-PWR or 36-BWR).

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This report examines the technical elements necessary to evaluate EBS concepts and perform thermal analysis of DOE-Managed SNF and HLW in the disposal settings of primary interest – argillite, crystalline, salt, and deep borehole. As the disposal design concept is composed of waste inventory, geologic setting, and engineered concept of operation, the engineered barrier system (EBS) falls into the last component of engineered concept of operation. The waste inventory for DOE-Managed HLW and SNF is closely examined, with specific attention to the number of waste packages, the size of waste packages, and the thermal output per package. As expected, the DOE-Managed HLW and SNF inventory has a much smaller volume, and hence smaller number of canisters, as well a lower thermal output, relative to a waste inventory that would include commercial spent nuclear fuel (CSNF). A survey of available data and methods from previous studies of thermal analysis indicates that, in some cases, thermo-hydrologic modeling will be necessary to appropriately address the problem. This report also outlines scope for FY16 work -- a key challenge identified is developing a methodology to effectively and efficiently evaluate EBS performance in each disposal setting on the basis of thermal analyses results.

The purpose of the two projects discussed in this report is to use the cohesive zone method to evaluate fracture properties of geomaterials. Two experimental tests, the push-out test and the notched three-point bend test, were modeled computationally using finite element analysis and cohesive zone modeling to extract load and displacement information and ultimately determine failure behavior. These results are to be compared with experimental tests when they are available. The first project used the push-out test to investigate the shear bond strength at the cement- shale interface. The second project explored the effects of scaling a notched three-point bending specimen to study fracture toughness characteristics. The bond strength and fracture toughness of a material and its interfaces are important parameters to consider in subsurface applications so that zonal isolation can be achieved.

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