Multi-scale rheology and strain localization of the continental lithosphere
Final Report Abstract
This project examined the relationship of mechanical properties, micro-damage, and deformation structures in granitic rocks from the scale of grains to the scale of the continental crust. To this end, a new workflow was established combining laboratory deformation experiments observed with Synchrotron computed micro-tomography (SCMT) and novel numerical simulations of rock deformation on multiple time and length scales. SCMT permits fast three-dimensional in-situ observations of how individual minerals of a rock sample respond to deformation, with an image resolution of less than one micron. The resulting tomograms do not only provide unprecedented insight into the microphysics of deformation and the opportunity to derive material properties at the sample scale. They also yield excellent data for the calibration of numerical simulations. The calibrated numerical simulations can then be employed to investigate the rock’s response to deformation under geological conditions, most of which cannot be realised in the laboratory. Geological deformation of continental crust, for example, entails length scales of up to hundreds of kilometres and time scales of up to millions of years. To account for the time- and length-scale-ependent physics of geological deformation, an innovative geomechanical framework was used in the numerical computations. This framework is based on the concept of non-equilibrium thermodynamics and thus considers scale-dependent processes and energy dissipation explicitly. The new workflow was tested with granitic rocks, which are characteristic of the continental crust. The initial step entailed the first in-situ heating experiment on Westerly granite observed in a Synchrotron (Advanced Photon Source, Argonne National Laboratories, USA). Granites and most other rocks contain minerals with different and anisotropic thermal-expansion and elastic properties. Therefore, upon heating, space problems arise between grains. These space problems lead to large contact stresses that may fracture the rock. The Synchrotron heating experiment provided the first three-dimensional time-lapse observations of thermal cracks, improving our understanding of thermal micro-cracking, an important phenomenon in, e.g., nuclear waste storage. The tomograms were successfully used to calibrate numerical grain-scale simulations of heated granite. In the next step, the calibrated numerical models were employed to examine if thermal-elastic contact stresses matter during long-term geological deformation of continental crust or if they are relaxed too quickly by high-temperature flow. Surprisingly, the grain-scale computations showed that burial-induced heating and compression of rocks can induce very large differential stresses (>100 MPa) with lifetimes from hundred thousands to even millions of years, even in the middle crust. This implies that thermal elasticity may serve as a booster for criticality in the crust. In the third step, the numerical simulations were used to investigate large-scale faulting and mechanical properties of a crustal-scale, buried granite body (more than 50 km wide and more than 10 km thick) targeted for geothermal-energy exploitation. A geothermal-energy company provided access to its rich geophysical data set for model verification. The models successfully predicted the location and geometry of observed km-scale fractures sets. They also predicted that the fractures are connected to a fluid reservoir below the bottom of the brittle crust, which was confirmed just recently by isotope studies. Additional exciting implications of these simulations include a new mechanism for diffuse fluid transport through the lower crust, advances in our understanding of the interplay between seismicity of the crust and slow ductile creep, and new insights into the physical controls of crustal rheology and the mechanical nature and effects of the brittle-ductile transition. It is expected that the workflow established here will be of great interest not only to the academic community but also to reservoir engineers. It permits to characterise the mechanical properties of reservoir rocks on multiple scales and provides novel computational tools for thermodynamically consistent simulations of exploitation scenarios.
Publications
- 2010, Digital rocks: Upscaling from microstructure to hand-specimen scale. SGTSG 2010 Conference in Port Macquarie, Australia, February 1 – 5
Schrank, C., Regenauer‐Lieb, K., Poulet, T., Karrech, A., Liu, J., Fusseis, F., Gaede, O.
- 2010, Thermal cracking of Westerly granite: from physical to numerical experiment. EGU General Assembly 2010, Geophysical Research Abstracts, Vol. 12, EGU2010-8191‐1
Schrank, C., Fusseis, F., Karrech, A., Revets, S., Regenauer-Lieb, K., Liu, J.
- 2011, Predicting fracture zones in Cooper-basin granites: insights from continuum damage mechanics finite-element modelling. 33 p. Research project with Geodynamics Ltd.
Schrank, C. E., Karrech, A., Regenauer‐Lieb, K.
- 2011, Thermal elasticity promotes failure in the brittle-ductile crust. EGU General Assembly 2011, Geophysical Research Abstracts, Vol. 13, EGU2011-5478
Schrank, C., Fusseis, F., Karrech, A., Regenauer‐Lieb, K.
- 2012, Deep‐seated fractures in hot granites: a new target for enhanced geothermal systems? Proceedings, Thirty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30 - February 1, 2012 SGP-TR-194
Schrank, C., Karrech, A., Regenauer-Lieb, K., Wyborn, D.
- 2012, Pore formation during dehydration of polycrystalline gypsum observed and quantified in a time‐series synchrotron radiation based X-ray micro‐tomography experiment: Solid Earth, v. 3, no. 2, p. 857-900
Fusseis, F., Schrank, C., Liu, J., Karrech, A., Llana-Funez, S., Xiao, X., and Regenauer‐Lieb, K.
