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Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut-in. Understanding the extent and duration of the post-injection microseismicity is critical to limit the risk of fluid-induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut-in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone towards induced seismicity mitigation strategies.

Equilibrium thermodynamics has been of fundamental importance to many branches of engineering including cyclical mechanical applications. However, in geomechanics and geological applications it has not yet reached a consensus in the community. Reason for the failure of establishing thermodynamic laws as a ground principle is the far from equilibrium nature of geomechanical problems which prevent the local equilibrium assumption. Problems including rate-dependence and poromechanical complexity, where deformation often occurs in a highly localized manner, were therefore thought to be not amenable to a thermodynamic approach. Here we show that the theory of thermomechanics, originally proposed for quasi-static hyperplastic deformation problems can be extended to include rate-dependent critical state-line models for porous rocks. The development therefore makes thermodynamic-consistent modeling available for civil engineering, geological and even geodynamic problems. In this two-part contribution, we present extensions of the thermomechanics theory to account for the poromechanics of path- and rate-dependent critical state line models and we cover the relevance of this thermodynamic-consistent model for civil engineering, geological and geodynamic applications. In this first part, we review the concepts behind the thermomechanics theory and present a thermodynamic extension of generic critical state line models for visco-plasticity and damage mechanics and analyze the model prediction for strain localization.

We propose a multiscale approach for coupling multi-physics processes across the scales. The physics is based on discrete phenomena, triggered by local thermo-hydro-mechano-chemical (THMC) instabilities, that cause cross-diffusion (quasi-soliton) acceleration waves. These waves nucleate when the overall stress field is incompatible with accelerations from local feedbacks of generalized THMC thermodynamic forces that trigger generalized thermodynamic fluxes of another kind. Cross-diffusion terms in the 4×4 THMC diffusion matrix are shown to lead to multiple diffusional P and S wave equations as coupled THMC solutions. Uncertainties in the location of meso-scale material instabilities are captured by a wave-scale correlation of probability amplitudes. Cross-diffusional waves have unusual dispersion patterns and, although they assume a solitary state, do not behave like solitons but show complex interactions when they collide. Their characteristic wavenumber and constant speed define mesoscopic internal material time–space relations entirely defined by the coefficients of the coupled THMC reaction–cross-diffusion equations. A companion paper proposes an application of the theory to earthquakes showing that excitation waves triggered by local reactions can, through an extreme effect of a cross-diffusional wave operator, lead to an energy cascade connecting large and small scales and cause solid-state turbulence.

Theoretical approaches to earthquake instabilities propose shear-dominated source mechanisms. Here we take a fresh look at the role of possible volumetric instabilities preceding a shear instability. We investigate the phenomena that may prepare earthquake instabilities using the coupling of thermo-hydro-mechano-chemical reaction–diffusion equations in a THMC diffusion matrix. We show that the off-diagonal cross-diffusivities can give rise to a new class of waves known as cross-diffusion or quasi-soliton waves. Their unique property is that for critical conditions cross-diffusion waves can funnel wave energy into a stationary wave focus from large to small scale. We show that the rich solution space of the reaction–cross-diffusion approach to earthquake instabilities can recover classical Turing instabilities (periodic in space instabilities), Hopf bifurcations (spring-slider-like earthquake models), and a new class of quasi-soliton waves. Only the quasi-soliton waves can lead to extreme focussing of the wave energy into short-wavelength instabilities of short duration. The equivalent extreme event in ocean waves and optical fibres leads to the appearance of “rogue waves” and high energy pulses of light in photonics. In the context of hydromechanical coupling, a rogue wave would appear as a sudden fluid pressure spike. This spike is likely to cause unstable slip on a pre-existing (near-critically stressed) fault acting as a trigger for the ultimate (shear) seismic moment release.

The temporal and spatial distribution of fluid pressure and temperature within a fault core are key determinants of the onset and nature (seismic or aseismic) of fault slip. Laboratory and field observations indicate that transient localization of fluid pressure and temperature often go hand in hand with strain localization upon seismic rupture: as slip occurs on a fault plane, temperature increases due to dissipated energy and fluid pressure decreases due to dilatant strengthening. An accurate description of this thermo-hydro-mechanical multiphysics coupling controlling slip mechanisms is therefore essential to characterize the stability of fault slip.Here, we present results from analytical and numerical analyses of the stability of fault slip adopting a thermo-hydro-mechanical coupling scheme together with a rate-dependent plasticity formulation. In particular, we focus on the relevance of dilatant strengthening competing with energy dissipation as driving processes for stick-slip events and aseismic slip. We analyze the multiple steady states of the system and their respective stability by means of a numerical continuation technique, and we describe the dynamic evolution of deformation, fluid pressure and temperature fields by considering an associated transient problem.The results presented here provide insights into the stability criterion for aseismic slip and the dynamic evolution of slip instability as a function of the physical (thermal and hydraulic) properties of the fault material and the boundary conditions (tectonic stresses and off-fault fluid pressure and temperature conditions). We identify two mechanisms for periodic slip, one driven by elastic loading and the other by multiphysics oscillations. We discuss the implications of these results for characterizing the transition from stable aseismic slip to unstable seismic slip in the context of natural and induced seismicity.

Observations of the mechanical behavior of porous rocks subject to external loading indicate the existence of complex dependencies on the level of confining pressure, fluid pressure and rate of deformation. Due to the heterogeneous nature of porous rocks, their macroscopic response is the result of underlying microscopic processes which can alter the microstructural organization of the grain–pore network. The impacts of the multiscale and poromechanical behavior of geomaterials are relevant for a number of applications ranging from civil engineering, reservoir engineering, geological and geodynamic. The use of thermodynamic-consistent approaches to construct constitutive laws which span a large range of time scales is particularly relevant in this context. In this two-part contribution, we present extensions of the thermomechanics theory to account for the poromechanics of path- and rate-dependent critical state line models and we cover the relevance of this thermodynamic-consistent model for civil engineering, geological and geodynamic applications. In this second paper, we extend the thermomechanics theory to account for the poromechanics of geomaterials in agreement with the theory of poroelasticity and considering in addition dissipative inelastic processes. We illustrate using experimental data how the thermodynamic-consistent model derived can account for the macroscopic mechanical and porous responses in triaxial loading experiments. We particularly focus on the transition from dilation to compression regime with confining pressure and the resulting localization styles ranging from shear dilation to compaction bands.

Theoretical approaches to earthquake instabilities propose shear-dominated instabilities as a source mechanism. Here we take a fresh look at the role of possible volumetric instabilities preceding a shear instability. We investigate the phenomena that may prepare earthquake instabilities using the coupling of Thermo-Hydro-Mechano-Chemical reaction-diffusion equations in a THMC diffusion matrix. We show that the off-diagonal cross-diffusivities can give rise to a new class of waves known as cross-diffusion waves. Their unique property is that for critical conditions cross-diffusion waves can funnel wave energy into a quasi-stationary wave focus from large to small-scale. The equivalent extreme event in ocean waves and optical fibres leads to the appearance of rogue waves and high energy pulses of light in lasers. In the context of hydromechanical coupling, a rogue wave would appear as a sudden fluid pressure spike on the future fault plane. This is here interpreted as a trigger for the ultimate (shear) seismic moment release.

We propose a non-local, meso-scale approach for coupling multiphysics processes across scale. The physics is based on discrete phenomena, triggered by local Thermo-Hydro-Mechano-Chemical (THMC) instabilities, that cause cross-diffusion (quasi-soliton) acceleration waves. These waves nucleate when the overall stress field is incompatible with accelerations from local feedbacks of generalized THMC thermodynamic forces that trigger generalized thermodynamic fluxes of another kind. Cross-diffusion terms in the 4 × 4 THMC diffusion matrix are shown to lead to multiple diffusional P- and S-wave equations as coupled THMC solutions. Uncertainties in the location of meso-scale material instabilities are captured by a wave-scale correlation of probability amplitudes. Cross-diffusional waves have unusual dispersion patterns and, although they assume a solitary state, do not behave like solitons but show complex interactions when they collide. Their characteristic wavenumber and constant speed define mesoscopic internal material time-space relations entirely defined by the coefficients of the coupled THMC reaction-cross-diffusion equations. For extreme conditions, cross-diffusion waves can lead to an energy cascade connecting large and small-scales and cause solid-state turbulence.

We provide details on a novel formulation derived to describe the multiphysics controlling the deformation of porous rock under lithospheric conditions. The theory is developed consistent with the principles of thermodynamics and enables to capture the behaviour of porous rocks at the transition from frictional brittle behaviour to ductile viscous behaviour. It also accounts for the nonlinear feedback mechanisms derived from energetic consideration for the bi-phasic fluid-rock matrix system.The formulation depicts a consistent, implicit visco-elasto-(visco)plastic rheology accounting for both a volumetric and a deviatoric response to applied loads, thereby avoiding the use of, the commonly assumed, plasticity limiter concept. The overstress plastic formulation introduces rate dependent mechanical behavior, an aspect that is consistent with experimental rock mechanics evidence and is also demonstrated to improve numerical stability when addressing problems related to plastic strain accumulation even in the absence of energetic feedbacks.The introduction of a damage rheology permits to account for microstructural processes responsible for brittle-like material weakening and rate-dependent dissipative material behavior. The presence of a fluid phase is considered via a dynamic porosity, the evolution of which is demonstrated to primarily control the volumetric mechanical response of the stressed rock during incremental loading.The above formulation has been integrated in a massively parallel, open source numerical framework with interfaces to state of the art HPC clusters. The results of a scalability and profile performance analysis on multi-core supercomputer are presented alongside with dedicated applications describing lithospheric rock deformation under different confining conditions as well as the bulk macroscopic material response recorded by laboratory experiments under triaxial conditions.

Material instabilities are critical phenomena which can occur in geomaterials at high stress and temperature conditions. They generally result in the degradation of the microstructure organisation, ultimately leading to material failure. These phenomena are relevant to a large variety of geoscientific and geotechnical applications including earthquake physics, fault mechanics, successful targeting of unconventional georesources and mitigation of induced seismicity. Quantifying and predicting the onset of material degradation upon instability remains a major challenge due to our lack of understanding of the physics controlling the behaviour of porous rocks subject to high temperature and pressure conditions.In the laboratory, rocks gradually transition from a time-independent brittle behaviour to a transient semi-brittle, semi-ductile behaviour upon an increase in pressure and/or temperature. Furthermore, even when subject to constant subcritical stress conditions rocks have been observed to macroscopically fail due to growth of subcritical processes such as stress corrosion. Brittle creep is a phenomenon observed on a variety of rock types (volcanic and sedimentary) and shows a high sensitivity to temperature and stress conditions. In the field, such subcritical transient processes are difficult to detect and can jeopardise the safety of geothermal projects. Transient failure mechanisms in the reservoir have set back geotechnical projects through induced seismicity occurring days or even weeks after stimulation shut in as observed at the Basel geothermal site in Switzerland or at the Pohang geothermal project in South Korea. These observations demonstrate how conventional techniques fail at describing the physics responsible for fault reactivation, which is controlled by dynamic processes resulting from transient multiphysics coupling.In this contribution, we detail the theory and procedure to develop a constitutive model for rate-dependent damage poro-elasto-plastic material behaviour suitable for porous rocks. To allow for a generic framework for assessing geomaterials instabilities, this model incorporates the potential for microstructure degradation and a path- and rate-dependence. To that purpose, we rely on thermodynamic principles to derive in the frame of the hyperplasticity theory a coupled hydro-mechanical rate-dependent plasticity and damage rheology. We present numerical examples of this new constitutive model at the laboratory scale using experimental data on brittle creep in sandstones and discuss the implications upon upscaling at the reservoir and lithosphere scale.

Strain localization is an instability phenomenon occurring in deformable solid materials which undergo dissipative deformation mechanisms. Such instability is characterized by the localization of the displacement or velocity fields in a zone of finite thickness and is generally associated with the failure of materials. In several fields of material engineering and natural sciences, estimating the thickness of localized deformation is required to make accurate predictions of the evolution of the physical properties within localized strain regions and of the material strength. In this context, scientists and engineers often rely on numerical modeling techniques to study strain localization in solid materials. However, classical continuum theory for elasto-plastic materials fails at estimating strain localization thicknesses due to the lack of an internal length in the model constitutive laws. In this study, we investigate at which conditions multiphysics coupling enables to regularize the problem of strain localization using rate-dependent plasticity. We show that coupling the constitutive laws for deformation to a single generic diffusion–reaction equation representing a dissipative state variable can be sufficient to regularize the ill-posed problem under some conditions on the softening parameters in the plastic potential. We demonstrate in these cases how rate-dependent plasticity and multiphysics coupling can lead to material instabilities depicting one or several internal length scales controlled by the physical parameters resulting in the formation of regular or erratic patterns. As we consider a general form of the equations, the results presented in this study can be applied to a large panel of examples in the material engineering and geosciences communities.

Enhanced geothermal systems (EGS) are engineered reservoirs developed to extract heat from low permeability and low porosity geological formations. Cyclic hydraulic stimulation treatments can be used in some cases to create hydraulic fractures and gain access to the target formation fluids, drain the geothermal fluid and increase the overall productivity of the reservoir. During these operations, successive cycles of injection at high flow rates are conducted to decrease the effective minimum principal stress to the tensile strength of the material, developing a mode I fracture. Opening of the newly developed fractures induces additional deformation of the reservoir rocks whose impact on the far field reservoir hydraulics has not been addressed so far for real case applications. These rates of compressive deformation can be significantly high in the context of cyclic stimulation treatments and can lead to a poroelastic pore pressure increase which spans quasi-instantaneously at greater distances than diffusive processes. In this study, such a poroelastic response resulting from cyclic deformation during hydraulic stimulation treatment of a well is investigated using hydromechanical coupling between pore pressure variations and rock deformation. The effects of this poroelastic response on the hydromechanical state of a reservoir is illustrated with field measurements of a cyclic hydraulic stimulation treatment conducted at the Gros Schonebeck geothermal research site in August 2007. This study demonstrates that the pore pressure increase monitored in a neighbor well located approximately 475 m away from the stimulated well at reservoir depth is controlled by diffusive processes responsible for the long-term increase of pore pressure, but also by poroelastic effects responsible for the quasi-instantaneous local peaks in pore pressure. The results from this applied study helped to quantify the relevance of the poroelastic behavior of stimulated reservoir rocks in a way than can improve current understanding of such hydraulic stimulation settings.

Targeting deep unconventional geothermal reservoirs at or nearby the brittle-to-ductile boundary has become re- cently the subject of several experimental and modelling studies. In this context, understanding the non-linear dynamics controlling the brittle-to-ductile transition at depth under different tectonothermal conditions and their modifications by operational activities is crucial not only to assess the strength and long-term behaviour of the lithosphere, but also to characterize emplacement conditions and the evolution in space and time of geothermal resources. Published results from laboratory studies, have clarified how porosity evolution in time and space exerts a critical control on the different deformation modes (from diffusive to localization), dynamically buffering the transition from one mode to the other. However, few numerical models have succeeded to account for such porosity feedbacks on the deformation of porous rocks. In this study, we present a new modelling framework together with its implementation in the numerical simulator LYNX (Lithosphere dYnamic Numerical toolboX) which relies on implicit multiphysics coupling of the physics describing the deformation modes as they occur in the rigid portion of lithosphere plates including thermal, mechanical and hydraulic feedback processes. In particular we address, as based on a new theoretical understanding of the role that porosity and its evolution through time within porous rock compartments, the long-term dynamic re- sponses of geological systems to external forcing of diverse nature (from natural, tectonic driven to anthropogenic ones). The LYNX simulator in its core compartments relies on the Multiphysics Object-Oriented Simulation Environment (MOOSE), which provides a powerful and flexible platform to solve for multiphysics problems implicitly and in a tightly coupled manner on unstructured meshes, thus providing an efficient computational base to tackle the non-linear, multiphysics problem at hands. In a first part, we present a thermodynamically consistent physical framework describing deformation of porous rocks accounting for coupled thermal, hydraulic and damage visco-plastic rheology together with key aspects of its numerical implementation. In a second part, we present two applications to illustrate the robustness and novelty of the physical framework. In particular, we address (1) how the modelling framework consistently integrate observa- tions at the laboratory scale as derived on tri-axial experiments. These experiments nicely illustrate the influence of porosity and pressure on the transition between brittle and ductile deformation modes and therefore provide useful to text both the theoretical and numerical framework. (2) Insights into how permeable structures can be generated and evolve in time and space under varying external forces, and under which poro-thermo-mechanical conditions such structures can be maintained over human up to geological time scales.

This study aims at evaluating the spatial and temporal distribution of 26 micro-seismic events which were triggered by hydraulic stimulation at the geothermal site of Gros Schonebeck (Germany). For this purpose, the alteration of the in-situ stress state and the related change of slip tendency for existing fault zones due to stimulation treatments and reservoir operations is numerical simulated. Changes in slip tendency can potentially lead to reactivation of fault zones, the related movement can lead to the occurrence of seismic events. In the current numerical study, results obtained based on the thermal–hydraulic–mechanical coupled simulation are compared to field observations. In particular, the study focuses on describing the fault reactivation potential: (1) under in-situ stress conditions; (2) during a waterfrac stimulation treatment; and (3) during a projected 30 years production and injection period at the in-situ geothermal test-site Gros Schonebeck. The in-situ stress state indicates no potential for fault reactivation. During a waterfrac stimulation treatment, micro-seismic events were recorded. Our current evaluation shows an increase of slip tendency during the treatment above the failure level in the direct vicinity of the micro-seismic events. During the projected production and injection period, despite increased thermal stress, the values for slip tendency are below the threshold for fault reactivation. Based on these results, and to prove the applied method to evaluate the observed micro-seismic events, a final discussion is opened. This includes the in-situ stress state, the role of pre-existing fault zones, the adopted criterion for fault reactivation, and a 3D rock failure criterion based on true triaxial measurements.

European Geosciences Union General Assembly 2015 - Division Energy, Resources and Environment, EGU 2015 / Edited by Maria Ask, Suzanne Hangx, Viktor Bruckman, Michael Kühn European Geosciences Union General Assembly 2015, EGU 2015, Vienna, Austria, 12 Apr 2015 - 17 Apr 2015; Amsterdam [u.a.] : Elsevier, 311-320 (2015). doi:10.1016/j.egypro.2015.07.868, Published by Elsevier, Amsterdam [u.a.]

European Geosciences Union General Assembly 2017, EGU Division Energy, Resources & Environment (ERE) / Edited by Sonja Martens, Christopher Juhlin, Suzanne Hangx, Michael Kühn European Geosciences Union General Assembly 2017, EGU 2017, Vienna, Austria, 23 Apr 2017 - 28 Apr 2017; Amsterdam [u.a.] : Elsevier, Energy procedia, 125, 612-621 (2017). doi:10.1016/j.egypro.2017.08.227, Published by Elsevier, Amsterdam [u.a.]

Dissertation, RWTH Aachen University, 2017; Aachen, 1 Online-Ressource (xiv, 179 Seiten) : Illustrationen, Diagramme (2017). = Dissertation, RWTH Aachen University, 2017, Motivated by the increasing need of carbon-free energy supply, development of concepts related to geothermal energy production has gained increasing attention in the last decades. Production of geothermal energy consists in the extraction of the heat stored in the Earth’s crust for direct use or electricity production. One or several borehole doublets are usually drilled to gain access to the targeted geological formations and the heat stored within the reservoir rocks is extracted by producing and injecting the geothermal fluid. It has also become a standard practice to use well stimulation or enhancement treatments in order to create secondary permeability and enhance the productivity of medium to low enthalpy reservoirs. The main challenges to guarantee a productive, sustainable and environmental safe geothermal operational campaign are to capture the details of the geology of the reservoir structures (geological formations, fractures and faults) and to quantify in time and space the dynamics of the relevant physical processes and their interactions with the geological environment where they occur. Continuous production and injection of geothermal fluids lead to changes in the thermodynamic conditions of a reservoir, where significant variations in pore pressure and temperature induce local gradients in the stress acting on the porous rock, thus affecting reservoir’s performance. The transport properties of a porous medium, which are porosity and permeability are indeed sensitive to the deformation of the bulk and pore volume and can alter if not control the overall reservoir productivity. Furthermore, changes in the stress acting on a geological discontinuity such as a fracture or a fault, as induced by geothermal operations can compromise the mechanical stability of such structures. This can result in the propagation or closing of existing fractures but it can also induce slip along fault planes which is usually accompanied by induced seismicity. This thesis aims at quantifying the impacts of the dynamics of coupled thermo-hydro-mechanical processes on the transport properties of the reservoir rocks. The goal is to provide a working framework which will assist field scientists in assessing the reservoir behaviour during operations in the light of increasing the productivity of such operations by limiting related hazards. For this purpose, a multiphysic and multiscale workflow has been developed which combines data obtained from laboratory-based rock deformation experiments and field operations and an accurate description of the main physical processes and their coupling into a novel numerical framework. A poroelastic framework capturing non-linear processes related to the closure of microcracks at low confining pressure during hydrostatic loading in drained conditions has been constrained and validated using published laboratory data on two different sandstones. It was therefore possible to capture the non-linear decrease of porosity during hydrostatic loading in numerical simulation as monitored in the laboratory. The resulting poroelastic framework was later extended to account for thermal feedbacks in a thermo-poroelastic model. The impacts of temperature changes on the mechanical behaviour and as well as on the porosity and permeability distributions were analysed and quantified by means of a generic model and a field case application, the Groß Schönebeck geothermal research facility, located north of Berlin, Germany. The results predicted a decrease of approximatively 14 % in the life time of the reservoir with respect to previous estimates based on studies neglecting such deformation feedbacks. This thermo-poroelastic framework has also been used to study the impacts of pore pressure and temperature changes on the slip tendency of major faults zones in the vicinity of an operating well. The results showed that gradients in temperature can contribute to the increase of slip movements along major fault zones, with the magnitude of such movement correlating with magnitudes of induced thermal gradients by fluid injection and faults geometry with respect to the in situ stress field. Finally, a complete hydromechanical model formulation has been used to provide an explanation on the causative process behind field observations which were left unexplained before. These consist in instantaneous increases in pore pressure propagating at large distances (∼500 m) from a stimulated well which has been observed during a stimulation campaign at the Groß Schönebeck geothermal field. These observations can be explained as the results from a poroelastic effect in relation to a solid to fluid hydromechanical coupling, similar to the Mandel-Cryer effect triggered by compressional deformation as induced by the opening of a hydraulic fracture. In summary, this thesis demonstrates the importance of mechanical processes and multiscale non-linear relations between transport properties and the microstructure of the reservoir rocks for assessing the hydrothermal state of a geothermal reservoir and ultimately the productivity, sustainability and safety of geothermal operations. By means of complementary laboratory- and field-based observations, the outcomes of this thesis provide a complete workflow including space discretisation of complex geometries of a geological model and a powerful scalable numerical simulator capable of integrating new information as available. The tool developed during the course of this thesis is intended to help at improving the predictive capabilities of geothermal operations and could be extended to any reservoir operation in the future., Published by Aachen

Solid earth 8(5), 921-941 (2017). doi:10.5194/se-8-921-2017, Published by Copernicus Publ., Göttingen

GOLEM is a numerical simulator for modelling coupled Thermo-Hydro-Mechanical processes in faulted geothermal reservoirs. The simulator is developed by Antoine B. Jacquey and Mauro Cacace at the GFZ German Research Centre for Geosciences from the section Basin Modelling.

European Geosciences Union General Assembly 2016, EGU Division Energy, Resources & the Environment (ERE) / Edited by Maria Ask, Viktor Bruckman, Chris Juhlin, Thomas Kempka and Michael Kühn European Geosciences Union General Assembly 2016, EGU Division Energy, Resources & the Environment (ERE), Vienna, Austria, 16 Apr 2016 - 22 Apr 2016; Amsterdam [u.a.] : Elsevier, Energy procedia, 97, 302-309 (2016). doi:10.1016/j.egypro.2016.10.005, Published by Elsevier, Amsterdam [u.a.]

Significant pressure and temperature changes can occur within geothermal reservoirs caused by injection and production of fluid which can affect transport properties of the rocks and therefore alter reservoir performance and sustainability. To understand the coupling between transport properties evolution and state variable changes, a complete description of the mechanical behavior of the reservoir is required which should consider thermo- and poroelastic effects. This study aims to integrate transport properties evolution for coupled thermo-hydro-mechanical (THM) process modelling of fluid-bearing reservoirs. This approach is here applied to the geothermal research site of Gros Schonebeck (40 km north of Berlin, Germany) which consists of a doublet system at a target depth of about − 4100 m in which both injection and production wells have been hydraulically stimulated. A 3D reservoir model including the main geological units, major natural fault zones and hydraulic fractures is integrated in the finite-element method-based simulator OpenGeoSys for modelling coupled THM processes during geothermal activity. One challenge of this study is to integrate both hydro-geological and physical complexity to better describe the dynamic behavior of the geothermal reservoir. From the results of the simulation, thermal breakthrough is observed after 18 years of injection and life time of the system has been evaluated as 50 years. Furthermore, a 5.5% increase of porosity around the injection well is observed as well as an increase of the anisotropy ratio for permeability ( k z / k xy ) of about 2%. These transport properties enhancements lead to a decrease of the thermal breakthrough time (around − 8%) and life time of the system (− 14%) compared to classic thermo-hydro simulations with constant transport properties. The results presented here provide therefore valuable insights for understanding porosity and permeability distributions and evolutions during injection and production of geothermal fluids and related impacts on reservoir performance.

During geothermal operations, injection and production of fluid can induce significant pore pressure and temperature changes which impact the stress field thus affecting the reservoir performance. In this context, the transport and mechanical properties of the porous rock can be altered by deformation of the pore and bulk volumes. Different poroelastic formulations for describing the hydro-mechanical behavior of porous rocks are considered. Additionally, a formulation for the drained bulk modulus is introduced which takes into account the poroelastic behavior. This poroelastic formulation has been implemented in the 3D finite element method-based simulator OpenGeoSys to predict porosity changes. Based on the porosity changes, permeability variation under different loading conditions as main property controlling transport of mass and energy in porous media is determined by the Kozeny–Carman relation. Triaxial laboratory experiments conducted on two different kinds of sandstones (Flechtinger and Bentheimer) are used to constrain parameters which control porosity changes and thus permeability changes under drained conditions and to calibrate the numerical simulations. Hydrostatic loading from 1 to 70 MPa in confining stress has been simulated for Flechtinger and Bentheimer sandstones resulting in porosity and permeability decreases. Results are compared to experimental measurements to evaluate the precision of each poroelastic model. Reduction in porosity of 8.1 % with about 1.15 % error for the Flechtinger sandstone has been simulated and of 1.4 % with about 0.54 % error for the Bentheimer sandstone.