Publications

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Polymer concrete (PC) is a commonly used material in construction due to its improved durability and good bond strength to steel substrate. PC has been suggested as a repair and seal material to restore the bond between the cement annulus and the steel casing in wells that penetrate formations under consideration for CO2 sequestration. Nanoparticles including Multi-Walled Carbon Nano Tubes (MWCNTs), Aluminum Nanoparticles (ANPs) and Silica Nano particles (SNPs) were added to an epoxy-based PC to examine how the nanoparticles affect the bond strength of PC to a steel substrate. Slant shear tests were used to determine the bond strength of PC incorporating nanomaterials to steel; results reveal that PC incorporating nanomaterials has an improved bond strength to steel substrate compared with neat PC. In particular, ANPs improve the bond strength by 51% over neat PC. Local shear stresses, extracted from Finite Element (FE) analysis of the slant shear test, were found to be as much as twice the apparent/average shear/bond strength. These results suggest that the impact of nanomaterials is higher than that shown by the apparent strength. Fourier Transform Infrared (FTIR) measurements of epoxy with and without nanomaterials showed ANPs to influence curing of epoxy, which might explain the improved bond strength of PC incorporating ANPs.

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Seal integrity of functional oil wells and abandoned wellbores used for CO2 subsequent storage has become of significant interest with the oil and gas leaks worldwide. This is attributed to the fact that wellbores intersecting geographical formations contain potential leakage pathways. One of the critical leakage pathways is the cement-shale interface. In this paper, we examine the efficiency of a new polymer nanocomposite repair material that can be injected for sealing micro annulus in wellbores. The bond strength and microstructure of the interface of Type G oil well cement (reference), microfine cement, Novolac epoxy incorporating Neat, 0.25%, 0.5%, and 1.0% Aluminum Nanoparticles (ANPs) with shale is investigated. Interfacial bond strength testing shows that injected microfine cement repair has considerably low bond strength, while ANPs-epoxy nanocomposites have a bond strength that is an order of magnitude higher than cement. Microscopic investigations of the interface show that micro annulus interfacial cracks with widths up to 40 μm were observed at the cement-shale interface while these cracks were absent at the cement-epoxy-shale interface. Fourier Transform Infrared and Dynamic mechanical analysis measurements showed that ANPs improve interfacial bond by limiting epoxy crosslinking, and therefore allowing epoxy to form robust bonds with cement and shale.

This research aims to describe the microannulus region of the cement sheath-steel casing interface in terms of its compressibility and permeability. A wellbore system mock-up was used for lab-scale testing, and was subjected to confining and casing pressures in a pressure vessel while measuring gas flow along the specimen's axis. The flow was interpreted as the hydraulic aperture of the microannuli. Numerical joint models were used to calculate stress and displacement conditions of the microannulus region, where the mechanical stiffness and hydraulic aperture were altered in response to the imposed stress state and displacement across the joint interface.

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This report summarizes the first stage in a collaborative effort by Sandia, Los Alamos, and Lawrence Berkeley National Laboratories to design a small-diameter borehole heater test in salt at the Waste Isolation Pilot Plant (WIPP) for the US Department of Energy Office of Nuclear Energy (DOE-NE). The intention is to complete test design during the remainder of fiscal year 2017 (FY17), and the implementation of the test will begin in FY18. This document is the result of regular meetings between the three national labs and the DOE-NE, and is intended to represent a consensus of these meetings and discussions.

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As the title suggests, this report provides a summary of the status and progress for the Preliminary Design Concepts Work Package. Described herein are design concepts and thermal analysis for crystalline and salt host media. The report concludes that thermal management of defense waste, including the relatively small subset of high thermal output waste packages, is readily achievable. Another important conclusion pertains to engineering feasibility, and design concepts presented herein are based upon established and existing elements and/or designs. The multipack configuration options for the crystalline host media pose the greatest engineering challenges, as these designs involve large, heavy waste packages that pose specific challenges with respect to handling and emplacement. Defense-related Spent Nuclear Fuel (DSNF) presents issues for post-closure criticality control, and a key recommendation made herein relates to the need for special packaging design that includes neutron-absorbing material for the DSNF. Lastly, this report finds that the preliminary design options discussed are tenable for operational and post-closure safety, owing to the fact that these concepts have been derived from other published and well-studied repository designs.

Salt formations represent a promising host for disposal of nuclear waste in the United States and Germany. Together, these countries provided fully developed safety cases for bedded salt and domal salt, respectively. Today, Germany and the United States find themselves in similar positions with respect to salt formations serving as repositories for heat-generating nuclear waste. German research centers are evaluating bedded and pillow salt formations to contrast with their previous safety case made for the Gorleben dome. Sandia National Laboratories is collaborating on this effort as an Associate Partner, and this report summarizes that teamwork. Sandia and German research groups have a long-standing cooperative approach to repository science, engineering, operations, safety assessment, testing, modeling and other elements comprising the basis for salt disposal. Germany and the United States hold annual bilateral workshops, which cover a spectrum of issues surrounding the viability of salt formations. Notably, recent efforts include development of a database for features, events, and processes applying broadly and generically to bedded and domal salt. Another international teaming activity evaluates salt constitutive models, including hundreds of new experiments conducted on bedded salt from the Waste Isolation Pilot Plant. These extensive collaborations continue to build the scientific basis for salt disposal. Repository deliberations in the United States are revisiting bedded and domal salt for housing a nuclear waste repository. By agreeing to collaborate with German peers, our nation stands to benefit by assurance of scientific position, exchange of operational concepts, and approach to elements of the safety case, all reflecting cost and time efficiency.

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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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