Geological media are a strategic resource for the forthcoming energy transition and constitute an important ally in the fight to mitigate the adverse effects of climate change. Several energy and environmental processes in the subsurface involve multi-physical interactions between the porous and fractured rock and the fluids filling the voids: changes in pore pressure, temperature, and strain of the solid skeleton are superposed to chemical reactions. This characteristic has profound implications on the production of energy and the storage of waste. Forecasts are bounded to the adequate understanding of field data associated with thermo-hydro-mechano-chemical processes and predictive capabilities heavily rely on the quality of the integration between the input data (laboratory and field evidence) and the mathematical models describing the evolution of the multi-physical system. This session is dedicated to studies covering applications of carbon capture and storage (CCS), geothermal systems, gas storage, energy storage, mining, reservoir management, reservoir stimulation, fluid injection-induced seismicity and radioactive waste storage. Welcomed contributions include approaches based on analytical, numerical, multi-scale, data-driven and artificial intelligence methods as well as studies focused on laboratory characterization and on gathering and interpreting in-situ geological and geophysical evidence of the multi-physical behavior of rocks.
vPICO presentations: Wed, 28 Apr
A finite-element based, quasi-static growth algorithm models mixed mode concurrent fracture growth in three dimensions, leading to the formation of non-planar arrays and networks. To model the fully coupled THM model, equations describing mechanical deformation as well as heat transfer in the matrix and in the fractures are introduced in the formulation, simultaneously accounting for the effect of fluid flow and stress-strain response. This results in five separate, but two-way coupled model equations: a thermoporoelastic mechanical model; two fluid flow equations, one for the rock matrix and one for the fractures; two heat transfer equations, similarly for both the matrix and fractures. Fractures are represented explicitly as discrete surfaces embedded within a volumetric domain . Growth is computed as a set of vectors that modify the geometry of a fracture by accruing new fracture surfaces in response to brittle deformation. Fracture tip stress intensity factors drive fracture growth. This growth methodology is validated against analytical solutions for fractures under compression and tension . Thermal effects on the apertures and growth patterns will be presented. Isolated fracture geometries are compared with selected experimental results on brittle media. Accurate growth is demonstrated for domains discretised by refined and coarse volumetric meshes. Fracture and volume-based growth rates are shown to modify fracture interaction patterns. Two-dimensional cut-plane views of fracture networks show how fractures would appear on the surface of the studied volume.
 N. Thomas, A. Paluszny and R. W. Zimmerman. Growth of three-dimensional fractures, arrays, and networks in brittle rocks under tension and compression. Computers and Geotechnics, 2020. doi: 10.1016/j.compgeo.2020.103447
 Paluszny and R. W. Zimmerman. Numerical fracture growth modeling using smooth surface geometric deformation. Eng. Fract. Mech., 108, 19-36, 2013. doi: 10.1016/j.engfracmech.2013.04.012
How to cite: Paluszny, A., Thomas, R. N., Saceanu, M. C., and Zimmerman, R. W.: Thermo-mechanical Effects on Fracture Growth and Apertures in Three-Dimensional Subsurface Fractured Rocks , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14418, https://doi.org/10.5194/egusphere-egu21-14418, 2021.
In this talk, I give an overview of our software ResFrac, which fully integrates a ‘true’ hydraulic fracturing simulator and a multiphase reservoir simulator (McClure et al., 2020a). Conventionally, these processes have been described with separate codes, using separate meshes, and with different physics. Integrating these two categories of software is advantageous because it enables seamless description of the entire lifecycle of a well. It is possible to seamlessly integrate wells with complex histories such as frac hits from offset wells, refracs, and huff and puff EOR injection.
ResFrac has been applied on 25+ studies for operators optimizing development of oil and gas resources in shale and has been commercially licensed by 15+ companies (https://www.resfrac.com/case-studies; https://www.resfrac.com/publications; https://www.resfrac.com/about-us/our-team). The simulator has a modern user-interface with embedded help-documentation, wizards to help set up simulations, automated validators to identify issues with the setup prior to submitting, and plotting capabilities to preview 3D and tabular inputs. Simulations are run on the cloud and results are continuously downloaded to the user’s computer. This allows a user to easily run a large number of simultaneous simulations from their personal computer. The user-interface includes a custom-built and fully-featured visualization tool for 3D visualization and 2D plotting.
Hydraulic fracturing simulators must handle a diverse set of coupled physics: mechanics of crack propagation and stress shadowing, fluid flow in the fractures, leakoff, transport of fluid additives that impart non-Newtonian flow characteristics, and proppant transport. Proppant transport is particularly complex because proppant settles out into an immobile bed and may screen out at the tip. Many fracturing simulators approximate wellbore flow effects. However, because these effects are closely coupled to fracturing processes (especially in horizontal wells that have multiple simultaneously propagating fractures), we include a fully meshed, detailed wellbore model in the code, along with treatment of perforation pressure drop and near-wellbore tortuosity.
In the literature, separate constitutive relations are available to describe transport in open cracks, closed unpropped cracks, and closed propped cracks. However, there were not relations in the literature designed to describe transport under conditions transitional between these end-member states. A general numerical simulator must be able to describe all conditions (and avoid discontinuous changes between equations). To address this limitation, we developed a new set of constitutive equations that can smoothly transition between these end-member states – smoothly handling any general combination of aperture, effective normal stress, saturation, proppant volume fraction, and non-Newtonian fluid rheology (McClure et al., 2020).
The code solves all equations in a fully coupled way, using an adaptive implicit method. The fully coupled approach is chosen because of the tight coupling between many of the key physical processes. Iterative coupling converges very slowly and/or forces excessively small timesteps when tightly coupled processes are handled with iterative or explicit coupling.
McClure, Kang, Hewson, and Medam. 2020. ResFrac Technical Writeup (v5). arXiv.
How to cite: McClure, M.: Fully Integrating a Hydraulic Fracturing, Reservoir, and Wellbore Simulator into a Practical Engineering Tool, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9770, https://doi.org/10.5194/egusphere-egu21-9770, 2021.
Aquifer Thermal Energy Storage (ATES) can help to balance energy demand and supply to make better use of infrastructures and resources. ATES consists of a pair or more wells that simultaneously inject or extract thermal energy into aquifers. The aim of ATES is to store the excess of energy during summer and to reuse it during winter, when there is an energy deficit. High-temperature Aquifer Thermal Energy Storage (HT-ATES) provides a good option to store water over 50°C, but it requires facing some problems, such as low efficiency recoveries and the uplift of the surface. Coupled thermo-hydro-mechanical (THM) modelling is a good tool to analyze the viability and cost effectiveness of the HT-ATES systems and understand the interaction of processes, such as heat flux, groundwater flow and ground deformation. We present the 3D THM modelling of a pilot HT-ATES system, inspired by one of the projects of HEATSTORE, which is a GEOTHERMICA ERA-NET co-funded project. The model aims to simulate the injection of hot water of 90°C in a central well and the extraction of water in four auxiliary wells during summer. In winter, the auxiliary wells inject water of 50°C and the central well extract water. The loading lasts longer than the unloading (8 months versus 4 months) and overall more heat is injected than extracted. We found that the system is more efficient in terms of energy recovery, the more years the system is operating. In the aquifer, both thermal loads and hydraulic loads have an important role in terms of displacements. At the surface, the vertical displacements are only a consequence of the hydraulic strains generated by the injection of water in the aquifer.
How to cite: Vidal, R., Saaltink, M. W., and Olivella, S.: A Geothermal Energy Concept based on Heat Storage in Geological Media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7928, https://doi.org/10.5194/egusphere-egu21-7928, 2021.
Superhot geothermal environments with temperatures of approximately 400-500︒C at depth of approximately 2-4 km are expected as a new geothermal energy frontier. In order to efficiently exploit the superhot geothermal resources, fracture systems are necessary as flow path of working fluid. Hydraulic fracturing is a promising technique because it is able to create a new fracture system or enhance the permeability of preexisting fracture system. Laboratory-scale hydraulic fracturing experiments of granite have demonstrated the formation of densely distributed network of permeable fractures throughout the entire rock body at or near the supercritical temperature for water. Though the process has been presumed to involve continuous infiltration of low-viscosity water into preexisting microfractures followed by creation and merger of the subsequent fractures, plausible criterion for the fracturing is yet to be clarified. The possibility that the Griffith failure criterion is available to predict the occurrence of fracturing was shown by hydraulic fracturing experiments with acoustic emission measurements of granite at 400︒C under true triaxial stress. The present study provides a theoretical basis required to establish the procedure for hydraulic fracturing in superhot geothermal environment.
How to cite: Goto, R., Watanabe, N., Sakaguchi, K., Chen, Y., Ishibashi, T., Pramudyo, E., Parisio, F., Yoshioka, K., Nakamura, K., Komai, T., and Tsuchiya, N.: Flow-induced microfracturing of granite in superhot geothermal environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8561, https://doi.org/10.5194/egusphere-egu21-8561, 2021.
Wellbore instability is one of the most serious drilling problems increasing well cost in well construction processes. It is widely known that many wellbore instability problems are reported in shale formations where water sensitive clay mineral exist. The problems become further complicated when the shale exhibits variation in strength properties along and across bedding planes. In this study, a coupled thermal-hydro-mechanical-chemical (THMC) model was developed for time-dependent anisotropic wellbore stability analysis considering chemical interactions between swelling shale and drilling fluids, thermal effects, and poro-elastoplastic stress-strain behaviors.
The THMC simulator developed in this work assumes that the shale formation behaves as an ion exchange membrane where swelling depends on chemical potential of drilling fluids invading from the wellbore to the pore spaces. The time-dependent chemical potential changes of water within the shale are evaluated using an analytical diffusion equation resulting in the evolution of swelling strain around the wellbore. On the other hand, the thermal and pressure diffusion equations are evaluated numerically by finite differences. The stress changes associated with thermal, hydro, and chemical effects are coupled to the 3D poroelastoplastic finite element model. The effects of bedding planes are also taken into account in the FEM model through the crack tensor method in which the normal and tangential stiffnesses of the bedding planes have stress dependency. The failure of the formation rock is judged based on the critical plastic strain limit.
The numerical analysis results indicate that the rock strength anisotropy induced by the existence of bedding planes is the most important factor influencing the stability of the wellbore among various THMC process parameters investigated in this work. The numerical results also reveal that an established theory to orient the wellbore in the direction of the minimum principal stress is not always a favorable option when the effect of the anisotropy of in-situ stresses and the distribution angle of bedding planes cancel out each other. Depending on both the distribution angle of bedding plane and ratio of the vertical to the horizontal stress, the trend of minimum mud pressure showed a great variation as predicted by the yield and failure criterion implemented in the model. Furthermore, the analysis results reveal that the distribution and evolution of plastic strains caused by the THMC processes have the time dependency, which can be controlled by the temperature and salinity of the drilling fluids.
The numerical wellbore stability analysis model considering shale swelling and bedding plane effects provides an effective tool for designing optimum well trajectories and determining safe mud weight windows for drilling complex shale formations. The time-dependent margins of safe mud weight window of drilling can be fine-tuned when the interaction among various parameters is fully considered as the THMC processes.
How to cite: Xue, J. and Furui, K.: Coupled Thermal-Hydro-Mechanical-Chemical Modeling for Time-Dependent Anisotropic Wellbore Stability Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9138, https://doi.org/10.5194/egusphere-egu21-9138, 2021.
Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems. The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs. A series of three-dimensional intersected fracture models are constructed to perform the flow-through heat transfer simulations. The geometries effects of dead-end fractures on the heat transfer are evaluated in terms of intersected angles, apertures, lengths, and the connectivity. The results indicate that annular streamlines appear in the rough dead-end fracture and cause an ellipsoidal distribution of the cold front. Compared to the steady flow in plate dead-end fractures, the fluid flow formed in the rough dead-end fracture enhances the heat transfer. Both the outlet water temperature Tout and heat production Q present the largest when the intersected angle is 90°. A larger intersected angle and longer length extension of the intersected dead-end fracture, raising Tout and Q, are beneficial to the heat production, while increasing the aperture is ineffective. Solely increasing numbers of dead-end fractures poses a little increase on Tout and Q. More significant heat extraction is obtained through connecting these dead-end fractures with the main flow fracture forming the flow network.
How to cite: chen, Y.: Convective heat transfer of water flowing in intersected rock fractures for enhanced geothermal extraction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9347, https://doi.org/10.5194/egusphere-egu21-9347, 2021.
Creation of fractured reservoir for enhanced geothermal system (EGS) in granitic rock at unconventional superhot geothermal environments (>400°C) has been found possible by injection of low viscosity supercritical water (Watanabe et al., 2017, Geoph. Res. Lett.; Watanabe et al., 2019, Sci. Rep.). Accordingly, the complex cloud-fracture network is formed through stimulation of pre-existing microfractures by the low viscosity (<50 μPa∙s) water. Nonetheless, water reactivity to rock forming minerals (Brown, 2000, Proc. 25th wksh. Resv. Engr.) and its high water footprint (Wilkins et al., 2016, Environ. Sci. Technol.) hinder water application. Therefore, CO2 is proposed to replace water, since it less reactive to rock forming minerals (Brown, 2000), and can reduce water footprint (Wilkins et al., 2016). CO2 has also low viscosity at conventional (c.a. 150 – 300°C) and superhot geothermal temperatures (based on Heidaryan et al., 2011, J. Supercrit. Fluid), which motivates to create similarly complex fracture pattern in those geothermal environments. A set of traditional-triaxial stress fracturing experiments was performed on cylindrical granite samples subjected to 200 – 450-°C temperature and varying differential stress to determine characteristics and mechanism of CO2-based fracturing in conventional and superhot geothermal environments. The experiments demonstrated that complex fracture pattern was formed at low pressure in all experimental-stress state and temperature. Breakdown pressure was also found to decrease with increasing differential stress. Hence, it was hypothesized that fracturing mechanism by injection of CO2 is governed by Griffith fracture theory. To unveil the fracturing process in detail, a CO2-based fracturing experiment was conducted on cubical granite sample with a realistic true-triaxial stress state and 300-°C temperature. The later experiment confirmed the above-mentioned hypothesis and showed that the fracturing occurs gradually.
How to cite: Pramudyo, E., Watanabe, N., Goto, R., Sakaguchi, K., Nakamura, K., and Komai, T.: Characteristics and mechanism of CO2-based fracturing of granite in conventional and superhot geothermal environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10538, https://doi.org/10.5194/egusphere-egu21-10538, 2021.
Many natural and engineered geomaterials have double-porosity structure where two dominant pore systems coexist. Examples include structured soils where the two pore systems are inter-aggregate pores and intra-aggregate pores, and fissured rocks where the two pore systems are fissures and matrix pores. Although such double-porosity materials are frequently observed in geosciences and geoengineering applications, it remains mostly unclear how fluid flow and solid deformation interact differently in single- and double-porosity materials. The presentation explores this question through numerical simulation of consolidation – a paradigmatic problem in poromechanics – based on a recently developed modelling framework for fluid-infiltrated, inelastic materials with double porosity. Built on a combination of continuum principles of thermodynamics and standard plasticity theory, the framework can capture deformation, flow, and their coupling that occur individually in each pore system. Simulation results using this framework suggest that double-porosity structure gives rise to a two-staged consolidation behaviour, where the second stage appears similar to secondary compression in clays. It is also found that the simulated two-staged behaviour bears a striking semblance to experimentally observed consolidation processes in shales. These findings suggest that double-porosity structure may exert dominant control over the long-term hydro-mechanical behaviour of geomaterials.
How to cite: Choo, J.: On the consolidation behaviour of double-porosity geomaterials, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9616, https://doi.org/10.5194/egusphere-egu21-9616, 2021.
This work presents the advancement of a self-developed Cellular Automata Software for engineering Rockmass fracturing processes (CASRock, http://en.casrock.cn/) in the applications of thermo-hydro-mechanical-chemical (THMC) processes in fractured geological media. It contains a series of previous developed numerical systems, namely EPCA for simulation of heterogeneous rock failure process, VEPCA for visco elastoplastic analysis, D-EPCA for rock dynamic response simulation, THMC-EPCA for coupled THMC processes in geological media and RDCA for simulation of rock cracking process from continuity to discontinuity. In CASRock, the non-isothermal, unsaturated fluid flow, mechanical process and chemical reaction are sequentially coupled by updating all the state variables using cellular automaton technique and finite difference method on spatial and temporal scale, respectively. The Lagrangian method is used to simulate the particle transport. The control equations, coupling scheme and numerical implementation are briefly introduced. Several applications, including in the background of high level nuclear waste disposal are provided to show the abilities of CASRock in the simulation of coupling processes between physical fields. These applications include, (1) stability analysis of engineering rockmass under mechanical loading, (2) numerical study on coupled TM processes in hard rock pillar, (3) study on coupled THM processes in engineering barrier, (4) simulation of the THMC process in fractured rock.
How to cite: Pan, P.: CASRock—a code to simulate THMC processes in fractured geological media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12203, https://doi.org/10.5194/egusphere-egu21-12203, 2021.
This talk will present a machine learning framework that builds interpretable macroscopic surrogate elasto-plasticity models inferred from sub-scale direction numerical simulations (DNS) or experiments with limited data. To circumvent the lack of interpretability of the classical black-box neural network, we introduce a higher-order supervised machine learning technique that generates components of elasto-plastic models such as elasticity functional, yield function, hardening mechanisms, and plastic flow. The geometrical interpretation in the principal stress space allows us to use convexity and smoothness to ensure thermodynamic consistency. The speed function from the Hamilton-Jacobi equation is deduced from the DNS data to formulate hardening and non-associative plastic flow rules governed by the evolution of the low-dimensional descriptors. By incorporating a non-cooperative game that determines the necessary data to calibrate material models, the machine learning generated model is continuously tested, calibrated, and improved as new data guided by the adversarial agents are generated. A graph convolutional neural network is used to deduce low-dimensional descriptors that encodes the evolutional of particle topology under path-dependent deformation and are used to replace internal variables. The resultant constitutive laws can be used in a finite element solver or incorporated as a loss function for the physical-informed neural network run physical simulations.
How to cite: Sun, W. and Vlassis, N.: Thermodynamic-informed machine learning for pressure-sensitive plasticity model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16373, https://doi.org/10.5194/egusphere-egu21-16373, 2021.
Dissolution of porous media introduces a positive feedback between fluid transport and chemical reactions at mineral surfaces leading to the formation of pronounced wormhole-like channels. While the impact of flow rate and reaction rate on the shapes of the wormholes is now well understood, much less is known about the dynamics of their propagation. In this study we capture the evolution of wormholes and their effects on flow patterns by in-situ X-ray microCT imaging of dissolving limestone cores. 4D tomography allows us in particular to correlate the permeability changes in a dissolving core with the advancement of the tip position of the wormhole. Surprisingly, we find that the relation between the two is highly nonlinear, with extensive periods of relatively fast growth of the wormhole which is nevertheless not reflected in any significant change in the overall permeability. We hypothesize that this is caused by the presence of highly cemented regions in the core which act as permeability barriers for the flow. The presence of such regions is confirmed by a detailed analysis of the pore geometry based on the tomographic data. The results demonstrate that the analysis of the wormhole dynamics in 4d tomography can be used to probe the internal structure of the rock.
How to cite: Szymczak, P., Cooper, M. P., Magni, S., Sharma, R. P., Blach, T. P., Radlinski, A. P., Dohnalik, M., and Tengattini, A.: Wormhole Growth in Dissolving Limestones: Insights from 4D Tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13883, https://doi.org/10.5194/egusphere-egu21-13883, 2021.
By 2080, Germany will have to store around 600 000 m3 of low and intermediate-level nuclear waste (L-ILW) with negligible heat generation. This kind of waste is largely made up of used parts of nuclear power stations such as pumps, pipelines, filters, etc. placed in various types of waste containers made from either steel, cast iron, or reinforced concrete in different designs and sizes (i.e. cylindrical or box shaped). It is already decided that a total of 303 000 of the 600 000 m3 L-ILW will be disposed in a final storage facility in the former iron ore mine Schacht Konrad which is under construction. However, it is still not clear where the L-ILW emplaced in in the old salt mine Asse (200 000 m3) will be stored in the future. The situation is particularly critical, as the waste have to be retrieved from the instable mine shafts partially flooded with groundwater, causing strong socio-political concerns as radioactive waste could contaminate the water nearby. For this reason, the new search for a nuclear waste repository for high-level waste (HLW), started in 2017, should also consider the possibility to accommodate the waste from Asse. Obviously, this is still subject to critics as this will make finding a final repository more difficult as storing HLW and L-ILW together requires different concepts and designs for each other and, above all, much more space.
In this context, in this contribution we have defined conceptual and numerical models to assess the hydro-chemical evolution of a L-ILW disposal cell in indurated clay rocks, involving the interaction of different components/materials and the expected hydraulic and/or chemical gradients over 100 000 years. The L-ILW disposal cell leverages a multi-barrier concept buried 400 m below the surface. The multi-barrier system is comprised of the waste matrix (i.e. backfilling the waste drums), the disposal container, the mortar backfill in the emplacement tunnel (where the disposal containers are located) and the clay host rock. The dimensions and design of the emplacement tunnel (e.g. 11 × 13 m) and disposal cells represent and consider some aspects taken into account in the designs of some European countries. In addition, tunnel walls reinforced with a shotcrete liner and the Excavation Damaged Zone is considered in the concept. The model is implemented in OpenGeoSys-6, an open-source version-controlled scientific software based on Finite Element Method which is capable of handling fully coupled hydro-chemical models by coupling OpenGeoSys to iPHREEQC. First calculation results, demonstrate that the most important processes affecting the near-field chemical evolution are i) the degradation of the concrete and cementitious grouts with porewater migrating inwards from the host rock and ii) the significant quantities of reactive and non-reactive gases (i.e. hydrogen, carbon dioxide and methane) that are generated as a result of: i) the anaerobic corrosion of metals present in the waste and containers and ii) the degradation of organic compounds by microbial and chemical processes. As a first approximation, some assumptions and simplifications have been considered, probably resulting in a wort case scenario.
How to cite: Montoya, V., Garibay-Rodriguez, J., and Kolditz, O.: Hydro-geochemical processes in the emplacement cavern of a low and intermediate-level waste repository in an indurated clay-rock, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12226, https://doi.org/10.5194/egusphere-egu21-12226, 2021.
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.
How to cite: Jacquey, A., Veveakis, M., and Juanes, R.: Impact of dilatant strengthening and energy dissipation on fault slip stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13313, https://doi.org/10.5194/egusphere-egu21-13313, 2021.
To simulate CO2 fracturing under an isothermal condition, we propose a phase field model. We take advantage of the ability of the phase field approach to predict fracture initiation and branching, as well as to avoid tracking the fracture path. We model the CO2 as a compressible fluid by modifying Darcy's law. In particular, we assume that the permeability is correlated to the value of the phase field by the exponential function. The dependence of the CO2 density as a function of the pressure is captured by the Span-Wagner state equation. The computed pressure breakdown values show good agreement with analytical solutions and experimental results.
How to cite: Mollaali, M., Ziaei-Rad, V., and Shen, Y.: CO2 fracturing using the phase field approach for the brittle fracture, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12893, https://doi.org/10.5194/egusphere-egu21-12893, 2021.
Hydraulic stimulation of pre-existing fractures and faults plays a significant role in improving hydraulic conductivity of the fracture network around injection and production wells in geothermal reservoirs. It is therefore important to characterize the hydro-mechanical behavior of the faults against fluid injection. The Soultz-sous-Forêts site (France) has been an EGS pilot site where several major hydraulic stimulations have been performed and are well documented (https://cdgp.u-strasbg.fr/ and https://tcs.ah-epos.eu/).
Here we use the 3DEC numerical modeling tool (Itasca) to analyze the year 2000 stimulation of GPK2 well where large scale seismic anomalies have been evidenced during the different stages of the stimulation using 4D-P-wave tomography (Calo et al, 2011). The specificity of the approach is to combine two modeling at different scales. First, a small-scale model (100x100x100 m3) is built to analyze the effective mechanical response of a stochastic discrete fracture network (DFN) following the statistical features of the observed fracture network (Massart et al, 2010). Second, a large-scale numerical model of the Soultz-sous-Forêts reservoir (5000x5000x5000 m3) containing the largest faults of the reservoir defined by Sausse et al., 2010, is developed including regional stresses. The objective is to constrain the large-scale mechanical properties of the surrounding matrix around the fault from the small-scale model, in particular, its hydro-mechanical behavior in terms of non-linear elastic response related to the stochastic DFN. As a first step only the largest fault (GPK3-FZ4770) is considered. The first stage of the stimulation is modelled as a constant flow rate of 30 ls-1 of water injected into the fault at the depth of approximately 4.7 km. We explored the effect of the normal and shear stiffness of the fault on the dynamical response of pore pressure along the fracture and the onset of slip. It is found that the increase of the aperture of the fault during the injection shows a slow migration (~2 cm/s) owing to poro-elastic effects. Also generated fluid pressure throughout the fault shows a long period oscillating behavior (~5 hr) sensitive to the magnitude of the fracture normal stiffness.
How to cite: Javani, D., Schmittbuhl, J., and Cornet, F.: Hydro-Mechanical Modeling of the Year 2000 Hydraulic Stimulation of GPK2 Well, Soultz-sous-Forêts, France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15088, https://doi.org/10.5194/egusphere-egu21-15088, 2021.
Understanding fluid flow in rough fractures is of high importance to large scale geologic processes and to most anthropogenic geo-energy activities. Here, we conducted fluid transport experiments on Carrara marble fractures with a novel customized surface topography. Transmissivity measurements were conducted under normal stresses from 20 to 50 MPa and shear stresses from 0 to 30 MPa. An open-source numerical procedure was developed to simulate normal contact and fluid flow through fractures with complex geometries. It was validated towards experiments. Using it, we isolated the effects of roughness parameters on fracture fluid flow. Under normal loading, we find that i) the transmissivity decreases with normal loading and is strongly dependent on fault surface geometry ii) the standard deviation of heights (hrms) and macroscopic wavelength of the surface asperities control fracture transmissivity. Transmissivity evolution is non-monotonic, with more than 4 orders of magnitude difference for small variations of macroscopic wavelength and roughness. Reversible elastic shear loading has little effect on transmissivity, it can increase or decrease depending on contact geometry and overall stress state on the fault. Irreversible shear displacement (up to 1 mm offset) slightly decreases transmissivity and its variation with irreversible shear displacements can be predicted numerically and geometrically at low normal stress only. Finally, irreversible changes in surface roughness (plasticity and wear) due to shear displacement result in a permanent decrease of transmissivity when decreasing differential stress. Generally, reduction of a carbonate fault’s effective stress increases its transmissivity while inducing small shear displacements doesn’t.
How to cite: Acosta, M., Maye, R., and Violay, M.: Effect of normal and shear loading on the hydraulic transport properties of calcite bearing faults with customized roughness., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16294, https://doi.org/10.5194/egusphere-egu21-16294, 2021.
Chemical stimulation using high-concentration hydrofluoric and hydrochloric acids has been a classic method to enhance the permeability of a geothermal reservoir. Our research group has recently proposed a new chemical stimulation using a weakly acidic (moderate-reactivity) aqueous solution containing an environmentally friendly chelating agent to create voids, which are sustained under crustal stress, by selective mineral dissolution with preventing precipitation by chelation of metal ions. In the present study, we have conducted chelating agent flooding experiments using an aqueous solution of pH 4 containing readily biodegradable chelating agent (GLDA) on various types of fractured volcanic rocks at 200 oC and effective confining stress of 15 MPa. The experiments have revealed fast permeability enhancement of up to approximately four times, from the initial value, in two hours. Further analyses have revealed phenocrysts of Fe-bearing minerals (ex. Hematite) dissolved faster than the groundmass of the rocks to create the voids. These results show the possibility of the new chemical stimulation.
Keywords: Chemical stimulation, Chelating agents, Geothermal energy, EGS
How to cite: Salala, L., Watanabe, N., Takahashi, K., Erazo, J., and Tsuchiya, N.: Selective mineral dissolution and permeability enhancement of fractured volcanic rocks by chelating agent flooding in geothermal environments , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13536, https://doi.org/10.5194/egusphere-egu21-13536, 2021.
A geothermal well drilled into a reservoir at temperatures exceeding the critical point of pure water (>374 °C) could generate substantially greater quantities of energy than conventional geothermal wells. Although these temperatures can be found at shallow depths (<2-3 km) in high-grade geothermal resources located in volcanically active areas, similar temperatures are only found at depths >10 km beneath vast areas of continental crust with lower heat fluxes. Permeability decreases markedly with increasing depth below 2-3 km, so exploiting the tremendous heat resources of high temperature rock at such great depths will require permeability stimulation by the injection of high-pressure fluids. In this study, we use the CSMP++ platform to perform 3D simulations of transient permeability evolution around a geothermal doublet drilled to depths between 10-16 km. The simulations incorporate a well model initially devised by Peaceman (1978) to calculate well pressures and rates of fluid production/injection. The dynamic permeability model is based on Weis et al. (2012), initially developed to simulate the evolution of ore-forming magmatic-hydrothermal systems, and links a failure criterion for critically-stressed crust with depth-dependent permeability profiles characteristic for tectonically active crust as well as pressure- and temperature-dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. We investigate the permeability changes in response to high-pressure fluid injection in brittle and ductile rock, the timescales over which the zone of permeability stimulation migrates towards production wells, and dynamic permeability evolution in response to changes in injection and production parameters. These simulations aim to mitigate resource risks that could limit the ability to extract heat from geothermal resources in ductile upper crust and to help anticipate the conditions that would be required to make the exploitation of ultra-deep supercritical geothermal resources a reality.
Peaceman, D. W. (1978) Interpretation of Well-Block Pressures in Numerical Reservoir Simulation. SPE 6893, 183–194.
Weis, P., Driesner, T., & Heinrich, C. A. (2012). Porphyry-copper ore shells form at stable pressure-temperature fronts within dynamic fluid plumes. Science, 338(6114), 1613–1616.
How to cite: Scott, S., Yapparova, A., Weis, P., and Houde, M.: Dynamic permeability evolution during fluid injection and production in ultra-deep geothermal reservoirs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16079, https://doi.org/10.5194/egusphere-egu21-16079, 2021.
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