Manolis Veveakis

Modeling episodic fluid-release events in the ductile carbonates of the Glarus thrust

The exposed Glarus thrust displays midcrustal deformation with tens of kilometers of displacement on an ultrathin layer, the principal slip zone (PSZ). Geological observations indicate that this structure resulted from repeated stick-slip events in the presence of highly overpressured fluids. Here we show that the major characteristics of the Glarus thrust movement (localization, periodicity, and evidence of pressurized fluids) can be reconciled by the coupling of two processes, namely, shear heating and fluid release by carbonate decomposition. During this coupling, slow ductile creep deformation raises the temperature through shear heating and ultimately activates the chemical decomposition of carbonates. The subsequent release of highly overpressurized fluids forms and lubricates the PSZ, allowing a ductile fault to move tens of kilometers on millimeter-thick bands in episodic stick-slip events. This model identifies carbonate decomposition as a key process for motion on the Glarus thrust and explains the source of overpressured fluids accessing the PSZ.

Multi-Physics Modelling of Fault Mechanics Using REDBACK: A Parallel Open-Source Simulator for Tightly Coupled Problems

Faults play a major role in many economically and environmentally important geological systems, ranging from impermeable seals in petroleum reservoirs to fluid pathways in ore-forming hydrothermal systems. Their behavior is therefore widely studied and fault mechanics is particularly focused on the mechanisms explaining their transient evolution. Single faults can change in time from seals to open channels as they become seismically active and various models have recently been presented to explain the driving forces responsible for such transitions. A model of particular interest is the multi-physics oscillator of Alevizos et al. (J Geophys Res Solid Earth 119(6), 4558–4582, 2014) which extends the traditional rate and state friction approach to rate and temperature-dependent ductile rocks, and has been successfully applied to explain spatial features of exposed thrusts as well as temporal evolutions of current subduction zones. In this contribution we implement that model in REDBACK, a parallel open-source multi-physics simulator developed to solve such geological instabilities in three dimensions. The resolution of the underlying system of equations in a tightly coupled manner allows REDBACK to capture appropriately the various theoretical regimes of the system, including the periodic and non-periodic instabilities. REDBACK can then be used to simulate the drastic permeability evolution in time of such systems, where nominally impermeable faults can sporadically become fluid pathways, with permeability increases of several orders of magnitude.

Multiscale, multiphysics geomechanics for geodynamics applied to buckling instabilities in the middle of the Australian craton

We propose a new multi-physics, multi-scale Integrated Computational Materials Engineering framework for ‘predictive’ geodynamic simulations. A first multiscale application is presented that allows linking our existing advanced material characterization methods from nanoscale through laboratory-, field and geodynamic scales into a new rock simulation framework. The outcome of our example simulation is that the diachronous Australian intraplate orogenic events are found to be caused by one and the same process. This is the non-linear progression of a fundamental buckling instability of the Australian intraplate lithosphere subject to long-term compressive forces. We identify four major stages of the instability: (1) a long wavelength elasto-visco-plastic flexure of the lithosphere without localized failure (first 50 Myrs of loading); (2) an incipient thrust on the central hinge of the model (50–90 Myrs); (3) followed by a secondary and tertiary thrust (90–100 Myrs) 200 km away to either side of the central thrust; (4) a progression of subsidiary thrusts advancing towards the central thrust ( Myrs). The model is corroborated by multiscale observations which are: nano–micro CT analysis of deformed samples in the central thrust giving evidence of cavitation and creep fractures in the thrust; mm–cm size veins of melts (pseudotachylite) that are evidence of intermittent shear heating events in the thrust; and 1–10 km width of the thrust – known as the mylonitic Redbank shear zone – corresponding to the width of the steady state solution, where shear heating on the thrust exactly balances heat diffusion.

A Framework for Fracture Network Formation in Overpressurised Impermeable Shale: Deformability Versus Diagenesis

Abstract Understanding the formation, geometry and fluid connectivity of nominally impermeable unconventional shale gas and oil reservoirs is crucial for safe unlocking of these vast energy resources. We present a recent discovery of volumetric instabilities of ductile materials that may explain why impermeable formations become permeable. Here, we present the fundamental mechanisms, the critical parameters and the applicability of the novel theory to unconventional reservoirs. We show that for a reservoir under compaction, there exist certain ambient and permeability conditions at which diagenetic (fluid-release) reactions may provoke channelling localisation instabilities. These channels are periodically interspersed in the matrix and represent areas where the excess fluid from the reaction is segregated at high velocity. We find that channelling instabilities are favoured from pore collapse features for extremely low-permeability formations and fluid-release diagenetic reactions, therefore providing a natural, periodic network of efficient fluid pathways in an otherwise impermeable matrix (i.e. fractures). Such an outcome is of extreme importance the for exploration and extraction phases of unconventional reservoirs.

Deep geothermal: The ‘Moon Landing’ mission in the unconventional energy and minerals space

Deep geothermal from the hot crystalline basement has remained an unsolved frontier for the geothermal industry for the past 30 years. This poses the challenge for developing a new unconventional geomechanics approach to stimulate such reservoirs. While a number of new unconventional brittle techniques are still available to improve stimulation on short time scales, the astonishing richness of failure modes of longer time scales in hot rocks has so far been overlooked. These failure modes represent a series of microscopic processes: brittle microfracturing prevails at low temperatures and fairly high deviatoric stresses, while upon increasing temperature and decreasing applied stress or longer time scales, the failure modes switch to transgranular and intergranular creep fractures. Accordingly, fluids play an active role and create their own pathways through facilitating shear localization by a process of time-dependent dissolution and precipitation creep, rather than being a passive constituent by simply following brittle fractures that are generated inside a shear zone caused by other localization mechanisms. We lay out a new theoretical approach for the design of new strategies to utilize, enhance and maintain the natural permeability in the deeper and hotter domain of geothermal reservoirs. The advantage of the approach is that, rather than engineering an entirely new EGS reservoir, we acknowledge a suite of creep-assisted geological processes that are driven by the current tectonic stress field. Such processes are particularly supported by higher temperatures potentially allowing in the future to target commercially viable combinations of temperatures and flow rates.

Zebra rocks: compaction waves create ore deposits.

Nature has a range of distinct mechanisms that cause initially heterogeneous systems to break their symmetry and form patterns. One of these patterns is zebra dolomite that is frequently hosting economically important base metal mineralization. A consistent generic model for the genesis of these periodically banded rocks is still lacking. In this contribution, we present for the first time a fully consistent mathematical model for the genesis of the pattern by coupling the reactive fluid-solid system with hydromechanics. We show that visual banding develops at a given stress and host-rock permeability indicating that the wavelength and occurrence of the pattern may be predictable for natural settings. This finding offers the exciting possibility of estimating conditions of formation of known deposits as well as forecasting potential exploration targets.

Boudinage and folding as an energy instability in ductile deformation

We present a theory for the onset of localization in layered rate- and temperature-sensitive rocks, in which energy-related mechanical bifurcations lead to localized dissipation patterns in the transient deformation regime. The implementation of the coupled thermomechanical 2-D finite element model comprises an elastic and rate-dependent von Mises plastic rheology. The underlying system of equations is solved in a three-layer pure shear box, for constant velocity and isothermal boundary conditions. To examine the transition from stable to localized creep, we study how material instabilities are related to energy bifurcations, which arise independently of the sign of the stress conditions imposed on opposite boundaries, whether in compression or extension. The onset of localization is controlled by a critical amount of dissipation, termed Gruntfest number, when dissipative work by temperature-sensitive creep translated into heat overcomes the diffusive capacity of the layer. Through an additional mathematical bifurcation analysis using constant stress boundary conditions, we verify that boudinage and folding develop at the same critical Gruntfest number. Since the critical material parameters and boundary conditions for both structures to develop are found to coincide, the initiation of localized deformation in strong layered media within a weaker matrix can be captured by a unified theory for localization in ductile materials. In this energy framework, neither intrinsic nor extrinsic material weaknesses are required, because the nucleation process of strain localization arises out of steady state conditions. This finding allows us to describe boudinage and folding structures as the same energy attractor of ductile deformation.

Entropic Bounds for Multi-Scale and Multi-Physics Coupling in Earth Sciences

The ability to understand and predict how thermal, hydrological,mechanical and chemical (THMC) processes interact is fundamental to many research initiatives and industrial applications. We present (1) a new Thermal– Hydrological–Mechanical–Chemical (THMC) coupling formulation, based on non-equilibrium thermodynamics; (2) show how THMC feedback is incorporated in the thermodynamic approach; (3) suggest a unifying thermodynamic framework for multi-scaling; and (4) formulate a new rationale for assessing upper and lower bounds of dissipation for THMC processes. The technique is based on deducing time and length scales suitable for separating processes using a macroscopic finite time thermodynamic approach. We show that if the time and length scales are suitably chosen, the calculation of entropic bounds can be used to describe three different types of material and process uncertainties: geometric uncertainties,stemming from the microstructure; process uncertainty, stemming from the correct derivation of the constitutive behavior; and uncertainties in time evolution, stemming from the path dependence of the time integration of the irreversible entropy production. Although the approach is specifically formulated here for THMC coupling we suggest that it has a much broader applicability. In a general sense it consists of finding the entropic bounds of the dissipation defined by the product of thermodynamic force times thermodynamic flux which in material sciences corresponds to generalized stress and generalized strain rates, respectively.

The Role of Temperature in Shear Instability and Bifurcation of Internally Pressurized Deep Boreholes

This paper investigates localized shear deformation around a borehole due to internal pressure in the well such as by fluid injection. Using an elasto-visco-plastic formulation combined with damage mechanics for the effect of shear cracking, we first benchmark the model against analytical solutions and then provide bifurcation criteria for the onset of localized cracking at different temperature conditions. We report that at increased temperatures of the rock formation, hot fluid injection promotes shear stimulation, while cold fluid suppresses it. This counter-intuitive result can offer new pathways of effective stimulation in high-temperature environments, like those encountered in enhanced geothermal systems.

Localisation of Deformation for Shearing of a Fault Gouge with Cosserat Microstructure and Different Couplings

In this paper, we show the impact of Thermo-Hydro Mechanical couplings (THM) on the stability of a saturated fault gouge under shear. By resorting to Cosserat continuum mechanics, that allows to take into account rotational degrees of freedom, we regularize the problem of localisation and we predict the thickness of a shear band. A linear stability analysis of the homogeneous state is performed and then the system of equations is integrated using a Finite Element (FE) analysis. These analyses can be used for studying the evolution of the thickness of the principal slip zone in a fault under undrained adiabatic shear. Good agreement is found between theoretical predictions and field observations.