The objective of LBNL’s work in Crystalline Disposal R&D focuses on advancing our understanding of long-term disposal of spent fuel in crystalline rocks and to develop necessary experimental and computational capabilities to evaluate various disposal concepts in such media. LBNL’s FY20 work specifically focuses on reducing the uncertainties of key flow parameters in the EDZ, which will support the GDSA and PA model directly or through better parameterization of a process model that feeds either GDSA/PA. Specifically, we have the following research activities:
Long-term laboratory EDZ characterization
Research highlight: Development of a customized high-pressure (up to 10,000 psi), high-temperature (up to 200˚C) triaxial loading system that enables (1) long-term (days to months) simultaneous lab experiments on multiple core-scale samples under temperature-controlled flow, mechanical, and chemical conditions, and (2) localized heating of rock samples in the triaxial vessel, while maintaining low-temperature exterior environment through a cooling and isolation system. This unique design allows for (1) shortened heating distance of injected fluid in the vessel before contacting with sample, and (2) a better control on the thermal gradient between rock and fluid. The system can be applied to characterize the EDZ rock/fracture deformation, permeability and fluid chemistry evolution subject to site-relevant THMC conditions.
Selected publication: Crystalline Disposal R&D at LBNL: FY20 Progress Report
Contact: Chun Chang
Characterization of Transmissive Fractures in Crystalline Rocks and Development of Flow Models
Research highlight: Low permeability crystalline rocks have long been considered as one of the important potential geologic environments for long term disposal of nuclear waste (e.g., Witherspoon et al., 1981; Bredehoeft and Maini, 1981). However, one key concern relating to this geologic disposal option is the presence of hydraulically conductive fractures that could result in transport of radionuclides (Cherry et al., 2014). Extensive field studies conducted at a number of underground research laboratories in crystalline rocks, such as the Stripa Mine in Sweden and the Grimsel Test Site in Switzerland, have revealed that often a small subset of measured fractures is responsible for the bulk of the observed fluid flow (e.g., Witherspoon and Gale, 1982; Olsson and Gale, 1995; Carbonell et al., 2010). Discontinuities such as faults, joints, and fractures serve as the primary controls for groundwater flow in low permeability crystalline rocks; thus the frequency, size, orientation, and transmissivity of fractures are key attributes that need to be determined during site characterization (Patera, 1986; Rechard et al., 2011). These fracture attributes then can be used to develop discrete fracture network and equivalent continuum models for simulating flow and transport (Hadgu et al., 2017). Fracture characterization remains a critical component for evaluating the safety case for potential crystalline repositories.
The COSC-1 borehole was drilled as part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific deep drilling project in central Sweden (Lorenz et al., 2015). The well was drilled to a depth of 2.5 km through the Seve Nappe, which contains high grade metamorphic rocks indicative of deep (100 km) crustal levels. The main lithologies encountered in the borehole consist of felsic gneisses, amphibolite gneisses, calc-silicate gneisses, amphibolite, migmatites, and garnet mica schists, with discrete zones of mylonite and microkarst. The primary objectives of the COSC project were to gain insights into the tectonic evolution of the area, calibrate high quality surface geophysics through deep drilling, characterize present and past deep fluid circulation patterns, determine current heat flow to constrain climate modeling, and characterize the deep biosphere (e.g., Lorenz et al., 2015; Hedin et al., 2016; Wenning et al., 2016, 2017).
Our research team at LBNL has been collaborating with the COSC project to use information from this deep borehole research study as an analog for the deep borehole environment and to develop insights into fluid flow in crystalline basement rocks. Our team has previously used Flowing Fluid Electrical Conductivity (FFEC) logging as a means of identifying hydrologically transmissive fractures in the deep borehole (e.g., Tsang et al., 2016; Doughty et al., 2017). Our team, in collaboration with colleagues at Uppsala University, used the SIMFIP tool to characterize three specific intervals within the COSC-1 borehole — a zone with a flowing fracture, a zone with a mineralized (sealed) fracture, and a zone of mainly intact rock. These three zones were located in the borehole at a depth of ~0.5 km. Modeling of the SIMFIP data was conducted to simulate stress data using two approaches – an inversion of the displacement data, using the approach of Kakurina et al. (2019 and submitted), and a fully coupled hydromechanical analysis of fracture movements using the numerical modeling code 3DEC, which employs the distinct element method. We have also conducted laboratory and modeling studies to characterize the hydraulic properties of crystalline rock cores from the COSC-1, which were determined using a unique laboratory-scale apparatus developed at LBNL. This apparatus was used to measure multi-directional transmissivity of core samples, collected at selected intervals in the COSC-1, and the results can be used to assess fracture anisotropy under confining stress conditions. To validate the results of laboratory-scale tests, we used synthetic core samples and a newly obtained core sample from the flowing fracture interval that was tested by the SIMFIP in the field.
Doughty, C., Tsang, C.F., Rosberg, J.E., Juhlin, C., Dobson, P.F., and Birkholzer, J.T. (2017) Flowing fluid electrical conductivity logging of a deep borehole during and following drilling: estimation of transmissivity, water salinity and hydraulic head of conductive zones. Hydrogeol. J. 25, 501–517.
Contact: Pat Dobson
DECOVALEX-2023 Task G
Research highlight: DECOVALEX-2023 Task G involves three steps:
Step 1: Mechanical behavior of rough fractures before and after mechanical shear
Step 2: HM flow through rough fractures
Step 3: Thermal-slip: Thermally induced rock fracture slip
LBNL participation in DECOVALEX-2023 Task G includes modeling for Step 1 and Step 3.
Geometric representation of fractures: continuum, interfaces for discrete fractures, and explicit microscale asperities for detailed fracture geometry.
Modeling tools: the numerical manifold method (NMM), TOUGH-FLAC
Hu, M., Rutqvist, J. Microscale mechanical modeling of deformable geomaterials with dynamic contacts based on the numerical manifold method. Computational Geosciences 2020a. https://doi.org/10.1007/s10596-020-09992-z
Hu, M., Rutqvist, J. Numerical manifold method modeling of coupled processes in fractured geological media at multiple scales. Journal of Rock Mechanics and Geotechnical Engineering 2020b. https://doi.org/10.1016/j.jrmge.2020.03.002
Rutqvist, J. An overview of TOUGH-based geomechanics models. Computers & Geosciences 2017, 108, 56–63.
Contact: Mengsu Hu