Thomas Poulet

Thermo-poro-mechanics of chemically active creeping faults. 1: Theory and steady state considerations

In this paper, we study the behavior of a fluid-saturated fault under shear, based on the assumption that the material inside exhibits rate- and temperature-dependent frictional behavior. A creeping fault of this type can produce excess heat due to shear heating, reaching temperatures which are high enough to trigger endothermic chemical reactions. We focus on fluid-release reactions and incorporate excess pore pressure generation and porosity variations due to the chemical effects (a process called chemical pressurization). We provide the mathematical formulation for coupled thermo-hydro-chemo-mechanical processes and study the influence of the frictional, hydraulic, and chemical properties of the material, along with the boundary conditions of the problem on the behavior of the fault. Regimes of stable-frictional sliding and pressurization emerge, and the conditions for the appearance of periodic creep-to-pressurization instabilities are then derived. The model thus extends the classical mechanical stick-slip instabilities by identifying chemical pressurization as the process governing the slip phase. The different stability regimes identified match the geological observations about subduction zones. The model presented was specifically tested in the Episodic Tremor and Slip sequence of the Cascadia megathrust, reproducing the displacement data available from the GPS network installed. Through this process, we identify that the slow slip events in Cascadia could be due to the in situ dehydration of serpentinite minerals. During this process, the fluid pressures increase to sublithostatic values and lead to the weakening of the creeping slab.

Thermo-poro-mechanics of chemically active creeping faults: 3. the role of serpentinite in episodic tremor and slip sequences, and transition to chaos

During the last decade, knowledge over episodic tremor and slip (ETS) events has increased dramatically owing to the widespread installation of GPS and seismic networks. The most puzzling observations are (i) the periodic nature of slow seismic events, (ii) their localization at intermediate depths (estimated 15–40 km), and (iii) the origin of the nonvolcanic fluids that are responsible for the tremor activity. We reconcile these observations using a first principles approach relying on physics, continuum mechanics, and chemistry of serpentinite in the megathrust interface. The approach reproduces the GPS sequences of 17 years of recording in Cascadia, North America, as well as over 10 years in the Hikurangi Trench of New Zealand. We show that strongly endothermic reactions, such as serpentinite dehydration, are required for ETS events. We report that in this tectonic setting, it is its chemical reaction kinetics, not the low friction, that marks serpentinite as a key mineral for stable, self-sustained oscillations. We find that the subduction zone instabilities are driven from the ductile realm rather than the brittle cover. Even when earthquakes in the cover perturb the oscillator, it relaxes to its fundamental mode. Such a transition from stable oscillations to chaos is witnessed in the ETS signal of NZ following the M6.8, 2007 seismic event, which triggered a secondary mode of oscillations lasting for a few years. We consequently suggest that the rich dynamics of ductile modes of failure may be used to decipher the chaotic time sequences underpinning seismic events.

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.

Thermo-poro-mechanics of chemically active creeping faults: 2. Transient considerations

This work studies the transient behavior of a chemically active, fluid-saturated fault zone under shear. These fault zones are displaying a plethora of responses spanning from ultrafast instabilities, like thermal pressurization, to extremely slow creep localization events on geological timescales. These instabilities can be described by a single model of a rate-dependent and thermally dependent fault, prone to fluid release reactions at critical temperatures which was introduced in our companion work. In this study we integrate it in time to provide regimes of stable creep, nonperiodic and periodic seismic slip events due to chemical pressurization, depending on the physical properties of the fault material. It is shown that this chemically induced seismic slip takes place in an extremely localized band, several orders of magnitude narrower than the initial shear zone, which is indeed the signature field observation. Furthermore, in the field and in laboratory experiments the ultralocalized deformation is embedded in a chemical process zone that forms part of the shear zone. The width of this zone is shown here to depend on the net activation energy of the chemical reaction. The larger the difference in forward and backward activation energies, the narrower is the chemical process zone. We apply the novel findings to invert the physical parameters from a 16year GPS observation of the Cascadia episodic tremor and slip events and show that this sequence is the fundamental mode of a serpentinite oscillator defined by slow strain localization accompanying shear heating and chemical dehydration reaction at the critical point, followed by diffusion and backward reaction leading the system back to slow slip.

A porosity-gradient replacement approach for computational simulation of chemical-dissolution front propagation in fluid-saturated porous media including pore-fluid compressibility

In dealing with chemical-dissolution-front propagation problems in fluid-saturated porous media, the chemical dissolution front represented by the porosity of the medium may have a very steep slope (i.e., a very large porosity gradient) at the dissolution front, depending on the mineral dissolution ratio that is defined as the equilibrium concentration of the dissolved minerals in the pore-fluid to the solid molar density of the dissolvable minerals in the solid matrix. When the mineral dissolution ratio approaches zero, the theoretical value of the porosity gradient tends to infinity at the chemical dissolution front. Even for a very small value of the mineral dissolution ratio, which is very common in geochemical systems, the porosity gradient can be large enough to cause the solution hard to converge when the conventional finite element method is used to solve a chemical dissolution problem in a fluid-saturated porous medium where the pore-fluid is compressible. To improve the convergent speed of solution, a porosity-gradient replacement approach, in which the term involving porosity-gradient computation is replaced by a new term consisting of pore-fluid density-gradient and pressure-gradient computation, is first proposed and then incorporated into the finite element method in this study. Through comparing the numerical results obtained from the proposed approach with the theoretical solutions for a benchmark problem, it has been demonstrated that not only can the solution divergence be avoid, but also the accurate simulation results can be obtained when the proposed porosity-gradient replacement approach is used to solve chemical-dissolution-front propagation problems in fluid-saturated porous media including pore-fluid compressibility.

From transient to steady state deformation and grain size: A thermodynamic approach using elasto‐visco‐plastic numerical modeling

Numerical simulation experiments give insight into the evolving energy partitioning during high-strain torsion experiments of calcite. Our numerical experiments are designed to derive a generic macroscopic grain size sensitive flow law capable of describing the full evolution from the transient regime to steady state. The transient regime is crucial for understanding the importance of micro structural processes that may lead to strain localization phenomena in deforming materials. This is particularly important in geological and geodynamic applications where the phenomenon of strain localization happens outside the time frame that can be observed under controlled laboratory conditions. Ourmethod is based on an extension of the paleowattmeter approach to the transient regime. We add an empirical hardening law using the Ramberg-Osgood approximation and assess the experiments by an evolution test function of stored over dissipated energy (lambda factor). Parameter studies of, strain hardening, dislocation creep parameter, strain rates, temperature, and lambda factor as well asmesh sensitivity are presented to explore the sensitivity of the newly derived transient/steady state flow law. Our analysis can be seen as one of the first steps in a hybrid computational-laboratory-field modeling workflow. The analysis could be improved through independent verifications by thermographic analysis in physical laboratory experiments to independently assess lambda factor evolution under laboratory conditions.

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.

Interactive inverse methodology applied to stratigraphic forward modelling

Abstract An effective inverse scheme that can be applied to complex 3-D hydrodynamic forward models has so far proved elusive. In this paper we investigate an interactive inverse methodology that may offer a possible way forward. The scheme builds on previous work in linking expert review of alternate output to rapid modification of input variables. This was tested using the SEDSIM 3-D stratigraphic forward-modelling program, varying nine input variables in a synthetic example. Ten SEDSIM simulations were generated, with subtle differences in input, and five dip sections (fences) were displayed for each simulation. A geoscientist ranked the lithological distribution in order of similarity to the true sections (the true input values were not disclosed during the experiment). The two or three highest ranked simulations then acted as seed for the next round of ten simulations, which were compared in turn. After 90 simulations a satisfactory match between the target and the model was found and the experiment was terminated. Subsequent analysis showed that the estimated input values were ‘close’ to the true values.

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.