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This research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance. Highlights: Numerical modeling study accurately replicates the thermal-hydromechanical behavior of rock salt surrounding a heated borehole. Jumps in brine flow rate during transitions in heater power are observed, with the highest jump during cooling. Acoustic emission patterns suggest pronounced damage to rock salt during the cooling phase. Thermal pressurization and tensile-induced damage are identified as the primary contributors to brine inflow spikes. Findings improve predictions of brine availability, aiding the development of robust containment strategies for radioactive waste disposal. [ABSTRACT FROM AUTHOR]

Shearing of fractures and faults is important because it can result in permeability change or even induce seismicity—both are keys for efficient and safe energy recovery and storage in Earth systems. Quantitative analysis of shearing of intersecting fractures and faults is challenging because it can involve dynamic frictional contacts that are complicated by deformation of the rock matrix. To predict the shearing of intersecting fractures/faults, we attempt to answer the question of how intersections impact the shearing of a fracture network and whether we can simplify the description as compared to classical discrete fracture network (DFN) models. To answer these questions, we conducted a series of numerical simulations on scenarios for variable numbers of intersecting fractures. All these examples yield consistent results: the results of using DFNs are consistent with those of using hypothetical major paths. This leads to a new model, which we name simplified discrete fracture network model, to analyze shearing of intersecting fractures/faults using major path(s). We found that the intersections of fractures do not fundamentally change the shearing of two intersecting fractures if the intersecting angles are small. Furthermore, increasing the number of fractures/faults may relax the stress as more fractures/faults become available for shearing and distributing the stress. The simplified DFN model, which can capture efficiently the shearing behavior of each major paths from a large number of intersecting fractures/faults, will be a promising conceptual model that is complementary to existing equivalent continuum and discrete fracture models to analyze shearing of intersecting fractures/faults. Highlights: A new model—simplified discrete fracture network (DFN) model—that analyzes the shearing of intersecting fractures/faults using major path(s) was proposed and verified A step-by-step calculation and benchmarking to test the hypothesis of simplifying DFNs with major paths Quantitative understanding of how intersections affect shearing of intersecting fractures/faults Demonstration of the possibility of stress relaxation by increasing the number of fractures/faults in the DFN Demonstration of the simplified DFN model as a new model complementary to existing models, i.e., the equivalent continuum and discrete fracture models [ABSTRACT FROM AUTHOR]

Pressure solution, a mechanism that involves tight coupling between the geometry and thermal-hydro-mechanical-chemical (THMC) processes, plays an important role in diagenesis. In this study, we make the first attempt to conduct microscale THMC modeling to understand and quantify the impacts of geometry and temperature on pressure solution, taking natural salt rock as an example. This modeling capability is achieved by expanding a novel MC code that we developed previously (Hu et al. J Geophys Res: Solid Earth 126:e2021JB023112, 2021) to include temperature effects. We first conduct a simulation of an example that involves a single brine inclusion within a single halite grain and find that the temperature impact is limited for that case. We then extract geometry from an image of a natural salt rock and conduct simulations with different cases: (A) only temperature and no stress, (B) only stress and no temperature, and (C) with both stress and temperature. These different cases result in quite different phenomena. In case A, dissolution and precipitation occur across the entire system due to isolated pore space reaching a localized mass balance between dissolution, precipitation, and diffusion. In case B, intense geometric features (e.g., major asperities, inclusions) in one area undergo stress concentration, thus dominating pressure solution in that area. In case C, pressure solution is spread out at contacting highly stressed geometric features close to the hotter side. We conclude that geometric features dominate stress distribution, thus dominating pressure solution in a natural salt rock that may be affected by the temperature if a sufficient temperature gradient is applied. Highlights: First microscale THMC model for analyzing pressure solution that considers realistic geometry and complex multiphysics. Geometric features dominate pressure solution in a natural sedimentary rock because they dominate the stress distribution. Pressure solution increases the connectivity in a system, thus potentially increasing dissolution over a large area where stress is sufficiently high. Temperature gradients may have a limited impact on pressure solution, but can change the system behavior by changing the magnitudes of pressure solution in localized areas. [ABSTRACT FROM AUTHOR]

The Caney shale is an emerging hydrocarbon play located in southwest Oklahoma, USA. Within the Caney shale exist facies which were initially dubbed "reservoir" and "ductile" based on evaluation of well logging data. While past work has shown the distinction of "brittle" and "ductile" is not mechanically justifiable according to formal definitions, the current work shows some important differences between nominally ductile and reservoir zones. First, the "ductile" zones are more clay rich and have textural differences which can be expected to lead to differences in mechanical properties. One important impact of these differences is observed in triaxial creep experiments showing the "ductile" zones are more prone to creep deformation. Numerical simulations predict the "reservoir" zones will experience very little proppant embedment due to creep deformation of hydraulic fractures around proppant particles. On the other hand, "ductile" zones can be expected to undergo creep-driven proppant embedment leading to loss of fracture aperture ranging up to 100% loss, depending upon the spatial density of the proppant distribution. Hence, this research shows the identification of nominally "ductile" zones from well logs, while a misnomer, can be useful in finding clay-rich, creep-prone zones which will be the most prone to proppant embedment and hence vulnerable to greater production decline over time. Highlights: The Caney shale has previously been identified as a ductile formation which would be difficult to produce. The terms "ductile" and "brittle" are not sufficient to describe the behavior of the Caney shale and this is supported through laboratory characterization. "Brittle" reservoir zones contain less clay and are mechanically stronger than "ductile" zones. "Ductile" zones are more prone to creep and proppant embedment than the "brittle" reservoir zones. While not sufficient to describe the mechanical behavior of formations, "ductile" and "brittle" identification can be used to identify zones which are more or less prone to long-term deformation and which are mechanically weaker. [ABSTRACT FROM AUTHOR]

The greatest challenges of rigorously modeling coupled hydro-mechanical processes in fractured rocks at different scales are associated with computational geometry. In addition, selections of continuous or discontinuous models, physical laws, and coupling priorities at different scales based on different geometric features determine the applicability of a numerical model for a certain type of problem. In this study, we present our multi-scale modeling capabilities that have been developed based on the numerical manifold method for analyzing coupled hydro-mechanical processes in fractured rocks. Based on their geometric features, the fractures are modeled as continua—finite-thickness porous zones, and discontinua—discontinuous interfaces and microscale asperities and granular systems. Different governing equations, physical laws, coupling priorities, and approaches for addressing fracture intersections and shearing are then applied to describe these. We applied these models to simulate coupled processes in fractured rocks using realistic geometry obtained from rock images at different scales. We first calculated shearing of a single fracture with different models and demonstrated the impacts of asperities on shearing. We then applied the continuous and discontinuous models to simulate a network of rough fractures, demonstrating that contact dynamics contribute significantly to the geometric, multi-physical evolution of systems where rough fractures are not mineral filled. For a discrete fracture network, our coupled processes modeling demonstrates that shearing of the discrete fractures can have a major impact on stress and pore pressure distribution. Lastly, we applied the discontinuous granular model to simulate evolution of a complex granular system with a deformation band, demonstrating that the deformation band can dominate contact dynamics, the structural and the stress evolution of the granular system. [ABSTRACT FROM AUTHOR]

In this paper, a new numerical simulation tool named TOUGH-FEMM is presented and applied to model three-dimensional (3D) hydraulic fracturing in porous rock. The fluid flow in both fractures and porous rock is modeled using TOUGH2, which is a well-established code for analysis of multiphase and multi-component fluid flow. Rock deformations associated with fracture propagation are modeled using finite element-meshfree method (FEMM). FEMM is an approach to simulate fracture propagation without remeshing, in which the fracture path does not need to be predetermined. Fracture mechanics with mixed-mode stress intensity factors are employed to detect fracture instability and determine the direction of fracture propagation. TOUGH-FEMM is verified for modeling fluid-driven fracture propagation in 3D through a number of simulation examples, including modeling of hydraulic fracturing laboratory experiments and by comparison to independent numerical simulation results for multiple interacting hydraulic fractures at ten to hundred meter scale. [ABSTRACT FROM AUTHOR]

Underground compressed air energy storage (CAES) in lined rock caverns (LRCs) provides a promising solution for storing energy on a large scale. One of the essential issues facing underground CAES implementation is the risk of air leakage from the storage caverns. Compressed air may leak through an initial defect in the inner containment liner, such as imperfect welds and construction joints, or through structurally damaged points of the liner during CAES operation for repeated compression and decompression cycles. Detection of the air leakage and identification of the leakage location around the underground storage cavern are required. In this study, we analyzed the displacement (or strain) monitoring method to detect the mechanical failure of liners that provides major pathways of air leakage using a previously developed numerical technique simulating the coupled thermodynamic and geomechanical behavior of underground CAES in LRCs. We analyzed the use of pressure monitoring to detect air leakage and characterize the leakage location. From the simulation results, we demonstrated that tangential strain monitoring at the inner face of sealing liners could enable one to detect failure. We also demonstrated that the use of the cross-correlation method between pressure history data measured at various sensors could identify the air leak location. These results may help in the overall design of a monitoring and alarm system for the successful implementation and operation of CAES in LRCs. [ABSTRACT FROM AUTHOR]

We present a formulation of a discontinuous cellular automaton method for modeling of rock fluid pressure induced fracture propagation and coalescence without the need for remeshing. Using this method, modelers discretize a discontinuous rock-mass domain into a system composed of cell elements in which the numerical grid and crack geometry are independent of each other. The level set method, which defines the relationship between cracks and the numerical grid, is used for tracking the crack location and its propagation path. As a result, no explicit meshing for crack surfaces and no remeshing for crack growth are needed. Discontinuous displacement functions, i.e., the Heaviside functions for crack surfaces and asymptotic crack-tip displacement fields, are introduced to represent complex discontinuities. When two cracks intersect, the tip enrichment of the approaching crack is annihilated and is replaced by a Heaviside enrichment. We use the 'partition of unity' concept to improve the integral precision for elements, including crack surfaces and crack tips. From this, we develop a cellular automaton updating rule to calculate the stress field induced by fluid pressure. Then, the stress is substituted into a mixed-mode fracture criterion. The cracking direction is determined from the stress analysis around the crack tips, where fracture fluid is assumed to penetrate into the newly developed crack, leading to a continuous crack propagation. Finally, we performed verification against independent numerical models and analytic solutions and conducted a number of simulations with different crack geometries and crack arrangements to show the robustness and applicability of this method. [ABSTRACT FROM AUTHOR]

In this paper, the two computer codes TOUGH2 and RDCA (for 'rock discontinuous cellular automaton') are integrated for coupled hydromechanical analysis of multiphase fluid flow and discontinuous mechanical behavior in heterogeneous rock. TOUGH2 is a well-established code for geohydrological analysis involving multiphase, multicomponent fluid flow and heat transport; RDCA is a numerical model developed for simulating the nonlinear and discontinuous geomechanical behavior of rock. The RDCA incorporates the discontinuity of a fracture independently of the mesh, such that the fracture can be arbitrarily located within an element, while the fluid pressure calculated by TOUGH2 can be conveniently applied to fracture surfaces. We verify and demonstrate the coupled TOUGH-RDCA simulator by modeling a number of simulation examples related to coupled multiphase flow and geomechanical processes associated with the deep geological storage of carbon dioxide-including modeling of ground surface uplift, stress-dependent permeability, and the coupled multiphase flow and geomechanical behavior of fractures intersecting the caprock. [ABSTRACT FROM AUTHOR]

This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS-rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in horizontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geomechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our initial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low-permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribution. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabilization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a possible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model. [ABSTRACT FROM AUTHOR]

The article reports on method for step-rate injection method for fracture in-situ properties (SIMFIP) with the help of a 3-components borehole deformation sensor. Topics discussed include scope of suggested method, apparatus used in the determination of SIMFIP and estimation of rock stress. Also presents a schematic diagram of SIMFIP test comprising of pressure valve, fiber optic interferometer and pressure temperature sensors.

In this paper, we investigate the influence of the excavation damaged zone (EDZ) on the geomechanical performance of compressed air energy storage (CAES) in lined rock caverns. We conducted a detailed characterization of the EDZ in rock caverns that have been excavated for a Korean pilot test program on CAES in (concrete) lined rock caverns at shallow depth. The EDZ was characterized by measurements of P- and S-wave velocities and permeability across the EDZ and into undisturbed host rock. Moreover, we constructed an in situ concrete lining model and conducted permeability measurements in boreholes penetrating the concrete, through the EDZ and into the undisturbed host rock. Using the site-specific conditions and the results of the EDZ characterization, we carried out a model simulation to investigate the influence of the EDZ on the CAES performance, in particular related to geomechanical responses and stability. We used a modeling approach including coupled thermodynamic multiphase flow and geomechanics, which was proven to be useful in previous generic CAES studies. Our modeling results showed that the potential for inducing tensile fractures and air leakage through the concrete lining could be substantially reduced if the EDZ around the cavern could be minimized. Moreover, the results showed that the most favorable design for reducing the potential for tensile failure in the lining would be a relatively compliant concrete lining with a tight inner seal, and a relatively stiff (uncompliant) host rock with a minimized EDZ. Because EDZ compliance depends on its compressibility (or modulus) and thickness, care should be taken during drill and blast operations to minimize the damage to the cavern walls. [ABSTRACT FROM AUTHOR]

Fractured rock has often been conceptualized as a dual-continuum system for many practical applications. This study proposes a systematic approach to deal with multiphase flow in a dual-continuum system. Considering that fluid flow occurs in pore volumes (including fracture apertures), we first develop a so-called pore-space conservation equation for deformed fractured rock and then combine this equation with fluid mass balance to derive governing equations for multiphase flow associated with rock deformation. Constitutive relationships are also presented for describing stress dependence of hydraulic properties and effective mechanical parameters for bulk rock body (as a function of the corresponding parameters for fracture and matrix continua). Finally, we applied the developed approach to a CO geological sequestration problem to demonstrate the usefulness of the approach. [ABSTRACT FROM AUTHOR]

In the context of carbon capture and storage, deep underground injection of CO induces the geomechanical changes within and around the injection zone and their impact on CO storage security should be evaluated. In this study, we conduct coupled multiphase fluid flow and geomechanical modeling to investigate such geomechanical changes, focusing on probabilistic analysis of injection-induced fracture reactivation (such as shear slip) that could lead to enhanced permeability and CO migration across otherwise low-permeability caprock formations. Fracture reactivation in terms of shear slip was analyzed by implicitly considering the fracture orientations generated using the Latin hypercube sampling method, in one case using published fracture statistics from a CO storage site. The analysis was conducted by a coupled multiphase fluid flow and geomechanical simulation to first calculate the three-dimensional stress evolution during a hypothetical CO injection operation and then evaluate the probability of shear slip considering the statistical fracture distribution and a Coulomb failure analysis. We evaluate the probability of shear slip at different points within the injection zone and in the caprock just above the injection zone and relate this to the potential for opening of new flow paths through the caprock. Our analysis showed that a reverse faulting stress field would be most favorable for avoiding fracture shear reactivation, but site-specific analyses will be required because of strong dependency of the local stress field and fracture orientations. [ABSTRACT FROM AUTHOR]