Guest Editorial to the CouFrac 2022 Special Issue
Fundamental Coupled Processes in Fractured Earth Systems
Mengsu Hu1*, Xuhai Tang 2, Jonny Rutqvist1
1Lawrence Berkeley National Laboratory, Berkeley, CA, USA
2Wuhan University, Wuhan, China
Guest Editorial to the CouFrac 2022 Special Issue
Fundamental Coupled Processes in Fractured Earth Systems
Mengsu Hu1*, Xuhai Tang 2, Jonny Rutqvist1
1Lawrence Berkeley National Laboratory, Berkeley, CA, USA
2Wuhan University, Wuhan, China
The third International Conference on Coupled Processes in Fractured Geological Media: Observation, Modeling, and Application (CouFrac 2022) took place at the Lawrence Berkeley National Laboratory in Berkeley California and online everywhere else on November 14-16, 2022. The conference included 24 technical sessions with 118 talks (including 7 invited lectures), 44 virtual posters, and 26 onsite posters. The conference program covered a wide range of classic topics relevant to coupled processes in fractured geological media, including numerical modeling, lab tests, field tests, hydraulic fracturing, flow and transport, induced seismicity, geothermal energy, nuclear waste disposal, and carbon sequestration. In addition, several new sessions were developed, including machine learning, hydrogen storage, Earth’s critical zone, shearing of fractures, reactive transport, salt mechanics and science, and clay and shale.
In this special issue, presentations focused on contributions in fundamental research that advance the understanding of coupled processes in Earth science and engineering were invited to develop full-length peer-reviewed research manuscripts.
Coupled thermo-hydro-mechanical (THM) processes in fractured rocks have been studied extensively for more than 30 years. Tsang (2024) reviewed the past achievements that were considered “out-of-the-box” and discussed possible future research directions. The three “out-of-the-box” insights obtained from the previous studies were: the importance of simultaneously studying coupled processes using multiple conceptual models and numerical techniques; the iterative interplay between model objectives, site characterization plans, and predictions with uncertainty assessment; and the importance of parallel comparative studies of the processes in different rock types and even in different geo-energy projects.
Three examples that may be worth pursuing or extending for future research directions are presented. The first example points to effort towards identifying and characterizing parameters of a fracture network that play a direct controlling role in major coupled THM phenomena (such as induced seismicity and flow channeling), rather than parameters of stochastic distributions of fractures in the network. The second example accounts for the heterogeneity and hierarchy of fractures in a fault or fracture zone that have been associated with major THM events in a number of energy geoscience systems. The third example is based on the understanding that coupled THM processes in fractured rocks may be controlled primarily by only a few key bridges in some cases. Thus, identification, characterization, and evaluation of these key bridges will be critically important for future research.
Macroscale properties and behavior of geomaterials are controlled by microscale processes. Thus, quantifying and understanding microscale coupled processes are crucial for understanding and obtaining macroscale properties and behavior. Four papers in this special issue cover such topics, from understanding microscale behavior to obtaining macroscopic material properties.
Prakash et al. (2023) examined the effects of chemo-mechanical loading on microstructural features and mechanical alteration of individual components within silicate-rich shale rock after exposure to CO2-rich brine at high temperature and pressure. The results show dissolution of clay and quartz-rich phases followed by precipitation of clay and quartz from transformation of feldspar grains in CO2 condition, and the modulus of clay- and quartz-rich zones decreases in reacted areas of the CO2 exposed samples but increases near the sample surfaces. They show that pore size-controlled solubility plays an important role in the evolution of porosity in the context of rock-fluid interaction. In addition, experiments conducted in which samples were exposed to CO2-saturated solution indicate a weakened clay–quartz interface at a distance from the reacted surface.
Wang et al. (2023) investigated the evolution of pore structure and adsorption capacity as functions of lithology and burial depth. They used multiple methods to characterize the pore system evolution and gas storage capacity of the Longmaxi and Wufeng shale reservoirs (in China) during burial and uplift as a function of lithologies. They found that mineral composition and fabric play major roles in the evolution of pore systems during mechanical compaction. Pores at micro, meso and macro scales may be preserved due to different mechanisms that are controlled by temperature, overlying pressure and mineral structures during sedimentation and uplift processes. In addition, a geological model for shale reservoirs at shallower depths was proposed and can be used to reconstruct the evolution of methane adsorption capacity under both overpressure and hydrostatic pressure conditions.
Qiao et al. (2023) investigated the mechanical behavior of four granitic samples with different weathering degrees at multiple scales using microscale rock mechanics experiments (micro-RME), macroscale rock mechanics experiments (macro-RME) and accurate grain-based model (AGBM). They found that quartz is erosion-resistant compared to feldspar and biotite. The Young’s modulus of quartz and feldspar remain almost constant; however, the Young’s modulus of biotite decreases significantly due to weathering. With the increasing weathering degree, larger volume fractions of quartz and weathering products were found. The volume of microdefects (including pores and microcracks) is significantly increased due to weathering. From the perspective of weathering effects, they observed a negative correlation between quartz content and the macroscale Young's modulus of granites.
De Simone et al. (2023) used a quantitative and analytical approach to estimate equivalent Biot and Skempton coefficients for saturated fractured rock masses. They found that the coefficients are highly anisotropic and vary substantially with fracture orientation with respect to the applied stress tensor. In addition, they found that both coefficients increase with fracture density, which directly impacts the deformation caused by a load in undrained conditions. The results show that considering the presence of fractures is key for accurate evaluation of the hydro-mechanical response in a fractured system where a poro-elastic estimation is used.
The shearing and friction of fractures and faults control the seismic stability of a fractured/faulted system. Five papers in this Special Issues focus on analyzing shearing and friction of single and intersecting fractures, their effects on permeability changes and their interplay with fluid injection using laboratory and/or modeling approaches.
Miao et al. (2024) investigated different shear behavior of tensile- and shear-induced fractures in sandstone specimens using the acoustic emission (AE) technique. It was shown that the AE sequences of tensile fractures follow a power law, while a significant deviation from the power law was observed in the AE sequence of shear fractures in direct shear tests. It was suggested that the power law evolution of the AE sequence before and after the mainshock, together with anomalous b-values, can be used as indicators to distinguish young faults from mature faults.
Cui et al. (2024) used 3D scanning and carving techniques to reconstruct natural shale fractures recovered from the Longmaxi shale reservoir and conducted experiments to study the relationship between friction and permeability. During shearing, the combination of roughness decreases and the effects of fluid leads to reduction in the frictional coefficient. It was shown that fluid injection leads to a transition of frictional stability from velocity strengthening to velocity neutral. In addition, they found that velocity step shear has a more obvious effect on permeability enhancement than does a constant velocity shear.
Ishibashi and Asanuma (2024) conducted an experimental study on the hydraulic–mechanical–seismic coupled behavior of rough granite fractures under various stress conditions and fluid pressurization during hydraulic shearing. They show the “self-propping shear slip concept” as the primary mechanism for maintaining an fracture permeability increase, even under stress conditions over 50 MPa. It was found that the Gutenberg–Richter b-value gradually decreases during shear dilation and accompanying increase in fracture permeability. These observations can be explained by spontaneous formation of preferential flow paths during the injection of pressurized fluid into the rock fracture and the subsequent detachment of the small contacting asperities due to localized shear slips, which lead to the creation of porosity and irreversible increase in fracture permeability. However, other information (such as the evolution amplitude and the occurrence timing of the maximum amplitude of AEs, and the classification of tensile/shear modes) did not correlate with the fracture permeability changes during hydraulic shearing.
Bianchi et al. (2024) combined novel laboratory techniques (i.e., distributed strain sensing array, and both passive and active AE) and numerical modeling to investigate (a)seismic preparatory processes associated with deformation localization during a triaxial failure test on a dry sample of Berea sandstone. Three distinct stages of preparatory processes were identified from the simulations and were consistent with laboratory observations: i) highly dissipative fronts propagated towards the middle of the sample correlating with the observed AE locations; ii) dissipative regions were individuated in the middle of the sample and could be linked to a discernible decrease of the P-wave velocities; and iii) a system of conjugate bands formed and coalesced into a single band that grew from the center towards the sample surface that was interpreted to be representative of the preparation for the development of a weak plane. Dilatative lobes at the process zones of the weak plane extended outwards and grew to the surface, causing strain localization and an acceleration of the simulated deformation prior to failure. The combined approach of such laboratory and numerical techniques provides an enriched view of (a)seismic preparatory processes preceding the mainshock.
To quantify how intersections of fractures affect shearing of fracture networks and to simplify the quantitative analysis, Hu et al. (2024) developed a new model, referred to as simplified discrete fracture network (DFN) model, to analyze shearing of intersecting fractures/faults using major path(s). They 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. It was demonstrated that the simplified DFN model can capture efficiently the shearing behavior of each major path from a large number of intersecting fractures/faults. Thus, this approach offers a promising conceptual model that is complementary to existing equivalent continuum and discrete fracture models to analyze shearing of intersecting fractures/faults.
In subsurface energy geoscience systems, the strategy of fluid injection may affect both the permeability via creation, propagation and shearing of fractures and seismic stability of the systems. Cyclic injection can reduce the monotonic breakdown pressure. However, compared with monotonic injection, cyclic injection needs a significantly longer time to lead to failure, and it creates complex fracture patterns that can be challenging to predict and control.
Zhuang et al. (2024) explored a different injection scheme employing rock fatigue behavior on cylindrical granite cores. The injection scheme (referred to as creep injection) creates continuous pressurization under a constant borehole pressure with a pre-defined maximum value below the monotonic breakdown pressure. They compared creep, cyclic, and monotonic injection to understand fatigue hydraulic fracturing of granite cores. The results show that the injection scheme can modify hydraulic fracture patterns (i.e., fracture aperture, branching, and propagation). It was shown that the breakdown pressure was reduced, and fracture branching was created through fatigue hydraulic fracturing. For long-duration fluid injection, fluid infiltration significantly affects hydraulic fracture propagation.
Temperature-driven coupled processes may take place in high-level nuclear waste repositories where the nuclear waste generates heat and poses changes in both engineered (such as engineered barrier systems) and natural (such as host rocks) multi-barrier systems. Understanding these processes and managing the potential risks are important for the long-term safety of nuclear waste disposal in different types of host rocks. In this special issue, two papers present state-of-the-art studies on such topics.
Yoon et al. (2024) conducted coupled thermal-hydro-mechanical (THM) modeling to analyze high-temperature bench-scale laboratory experiments of bentonite samples under heating and hydration. The modeling effort focused on exploring the effect of the soil–water retention curve and the thermal conductivity function under high temperatures. The modeling results show overall good agreement with the laboratory tests. The model suggests that (1) a non-linear thermal conductivity function should be used to capture the complex THM behavior, (2) temperature-dependent soil–water retention affects the simulation results in all aspects, and (3) a linear swelling model underperforms a state surface approach in matching the displacement data.
Tounsi et al. (2024) conducted 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. The THM simulations successfully captured two important phenomena that were observed from the field test. First, water flow rate surges during heater power transitions, with the highest jump during cooling. Second, 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 excellent match between the THM simulations and field observations highlights the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. It was 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 the understanding and prediction of the complex coupled processes involved in salt repository performance.
[1] Tsang CF (2024) Coupled Thermo-Hydro-Mechanical Processes in Fractured Rocks: Some Past Scientific Highlights and Future Research Directions. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03676-7
[2] Prakash R, Mahgoub SA, Abedi S (2023) Chemo-mechanical Alteration of Silicate-Rich Shale Rock after Exposure to CO2-Rich Brine at High Temperature and Pressure. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03664-x
[3] Wang D, Li X, Li S, Li G, Mao T, Zheng B (2023) Evolution of Pore Structure and Methane Adsorption in Lower Silurian Longmaxi Shale: Implications for Uplifted Shale Gas Reservoirs. Rock Mech Rock Eng. https://doi.org/10.1007/s00603-023-03441-w
[4] Qiao J, Nie M, Zhao Q, Liu Q, Tang X (2023) The effect of weathering on the mineral grains and macroscale young’s modulus of granites. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03670-z
[5] De Simone S, Darcel C, Kasani HA, Mas Ivars D, Davy P (2023) Equivalent Biot and Skempton Poroelastic Coefficients for a Fractured Rock Mass from a DFN Approach. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03515-9
[6] Miao S, Pan PZ, Zang A, Zhang C, Hofmann H, Ji Y (2024) Laboratory Shear Behavior of Tensile-and Shear-Induced Fractures in Sandstone: Insights from Acoustic Emission. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-024-03780-2
[7] Cui L, Zhang F, An M, Zhong Z, Wang H (2024) Frictional Stability and Permeability Evolution of 3D Carved Longmaxi Shale Fractures and Its Implications for Shale Fault Stability in Sichuan Basin. Rock Mechanics and Rock Engineering.
[8] Ishibashi T, Asanuma H (2023) Investigating Geophysical Indicators of Permeability Change During Laboratory Hydraulic Shearing of Granitic Fractures with Surface Roughness. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03590-y
[9] Bianchi P, Selvadurai PA, Dal Zilio L, Salazar Vásquez A, Madonna C, Gerya T, Wiemer S (2024) Pre-Failure Strain Localization in Siliclastic Rocks: A Comparative Study of Laboratory and Numerical Approaches. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-024-04025-y
[10] Hu M, Sasaki T, Rutqvist J, Birkholzer J (2024) A New Simplified Discrete Fracture Model for Shearing of Intersecting Fractures and Faults. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-024-03889-4
[11] Zhuang L, Sun C, Hofmann H, Zang A, Zimmermann G, Xie L, Lu G, Bunger AP (2024) Comparison of Fatigue Hydraulic Fracturing of Granite Cores Subjected to Creep and Cyclic Injection. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-024-03870-1
[12] Yoon S, Zheng L, Chang C, Borglin S, Chou C (2024) Coupled Thermo-Hydro-Mechanical Modeling of Bentonite Under High Temperature Heating and Hydration for a Bench-Scale Laboratory Experiment. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-024-03927-1
[13] Tounsi H, Rutqvist J, Hu M, Kuhlman K (2023) Thermo-Hydro-Mechanical Modeling of Brine Migration in a Heated Borehole Test in Bedded Salt. Rock Mechanics and Rock Engineering. https://doi.org/10.1007/s00603-023-03632-5