Enhanced geothermal systems rely on increasing permeability and pore surface area in rock. Cyclic thermal shocking can achieve both by inducing thermal cracks through repeated rapid cooling. Laboratory experiments subjected micritic limestone, granodiorite, and trachybasalt to up to 10 thermal shock cycles, while tracking crack evolution qualitatively using time‐lapse electron microscopy and quantifying pressure‐dependent permeability and elastic wave velocities. This work advances prior efforts focused primarily on crack initiation by demonstrating how lithology‐specific microstructures govern the cyclic evolution, persistence, and efficiency of pressure‐dependent permeability enhancement during cyclic thermal shocking. This reframes microstructure as a key design variable controlling permeability enhancement and monitoring during geothermal stimulation. Contrasting mineral thermal properties, large mineral grains, and irregular vugs promote the greatest permeability enhancement. Velocity reductions were most pronounced <10 MPa effective pressure (Peff) and diminished with increasing cycle number, indicating that velocity‐based monitoring in geothermal systems must account for Peff and cycle number.
The periodic fluctuation of reservoir water levels induces dry‐wet cycles, deteriorating sandy rock slope stability and potentially triggering collapses. This study investigates the degradation mechanism of Three Gorges Reservoir fine sandstone through uniaxial compression tests and PFC2D simulations under varying dry‐wet cycles. Key findings include: (1) Increasing cycles reduce uniaxial compressive strength and elastic modulus (showing “V‐shaped” and “N‐shaped” degradation trends), while permeability and porosity rise, and P‐wave velocity declines. (2) Crack growth is nonlinear, dominated by shear cracks and high‐angle microcracks, with force chains aligning with the loading direction. (3) Dissipation energy rate follows a “W‐shaped” trend, while elastic energy rate exhibits an “M‐shaped” pattern; both energies at crack initiation, damage, and peak stress correlate exponentially with cycle count (
K
sd
being more sensitive). (4) Dry‐wet cycles weaken intergranular bonds, reduce elastic energy storage (
U
e
), increase dissipated energy (
U
d
), lower
M
‐value stability, and shift failure from brittle to ductile. (5) A Weibull‐based segmented damage model effectively simulates sandstone behavior under cyclic conditions. These insights enhance understanding of reservoir slope stability under hydrological fluctuations.
Senlin Gao, Qingyang Ren, Bin Chen et al.· International journal for nu...· 0 citations
In high‐altitude cold regions, ice–water phase change during freezing expands ice and compresses trapped gas in unsaturated rock cracks, whereas temperature gradients induce thermal stress, jointly driving crack initiation. This study derives governing equations for ice–gas mechanics, establishes stress intensity factors induced by heat flux, and formulates an MTS‐based initiation criterion. The effects of freezing temperature, saturation, and heat flux on key crack initiation parameters are analyzed. Within the considered parameter range, freezing pressure, ice–rock interfacial friction, and fissure gas pressure increase as freezing temperature decreases, whereas fissure gas pressure rises sharply under high saturation, making crack initiation mainly tensile. When heat flux intensity exceeds 1000 mW/m
2
, shear effects intensify, and the initiation mode evolves from tensile‐dominated to tensile–shear‐mixed or shear‐dominated. As
q
increases, the tangential stress changes from cosine‐ to sine‐type distribution, whereas
θ
0
decreases in the negative angle range and increases in the positive angle range, with the peak generally shifting rightward.
Li Wei, Chenghu Wang, Guiyun Gao· Fatigue & Fracture of En...· 0 citations
Tight reservoirs commonly exhibit low permeability and pronounced lithological heterogeneity, resulting in complex interactions among far-field stress, local structural weakness, and fluid-driven fracture propagation. In this study, four non-replicated 2 m × 2 m × 1 m physical-model specimens representing tight glutenite, tight sandstone, and No. 3 coal rock from the Huabei Oilfield were investigated using an ultra-large true-triaxial hydraulic-fracturing system. Surface-fracture observations, microseismic monitoring, and high-frequency wellhead-pressure measurements were integrated to compare fracture responses under lithology-specific combinations of injection rate, fluid viscosity, perforation configuration, and stress state. The tested glutenite cases exhibited branched or localized fracture patterns depending on the combined treatment configuration; the sandstone case was dominated by a throughgoing main fracture approximately aligned with the principal-stress direction; and the coal-rock case showed extensive participation of bedding and cleat systems. These morphological differences were accompanied by distinct pressure and microseismic signatures, indicating different pathways of hydraulic-energy redistribution and fracture activation. For the two glutenite cases, the combined change from a single-perforation configuration at 0.5 m3/min to three helical perforations at 120° and 0.7 m3/min was associated with a 42.2% larger microseismic-derived stimulated reservoir volume (SRV). Taken together, these responses indicate a shift from stronger far-field-stress-controlled localization in the comparatively uniform sandstone to progressively greater local structural control by heterogeneous interfaces in glutenite and by bedding/cleat discontinuities in coal rock. Because each configuration was represented by a single specimen and several experimental variables changed simultaneously among cases, the observed differences are interpreted as case-specific mechanistic trends rather than statistically established universal relationships. The results show the value of combining fracture morphology, microseismic spatial evolution, and pressure dynamics for interpreting lithology-dependent fracture propagation in ultra-large physical models and for developing qualitative, lithology-adapted hydraulic-fracturing concepts.
Ning Li, Xinfang Ma, Guohua Liu et al.· Processes· 0 citations
Sequential injection of fluids with contrasting mobility in porous media causes the leading fluid's pressure history to reshape the permeability field, which then governs the seepage and fracture propagation of the following fluid. Using a cross-scale model that couples seepage and discrete element fracturing and distinguishes the flow behaviors of CO2 and water, this study investigates CO2 pre-injection enhanced hydraulic fracturing in coal, capturing permeability evolution driven by CO2 pressure diffusion and subsequent hydraulic fracture propagation. The CO2 pressurization rate controls the time for pressure diffusion, thereby determining permeability uniformity; an intermediate rate creates a moderately heterogeneous field, maximizing fracture length, area, and fractal dimension, while lower and higher rates, respectively, over-homogenize and over-localize the field, reducing complexity. CO2 pre-injection pressure governs the magnitude and heterogeneity of permeability enhancement, transitioning fracture propagation from length-dominated to complexity-dominated. Constant pressure duration regulates the spatial extent of CO2 diffusion, longer durations homogenizing the field and favoring primary fracture extension over complexity. Water injection pressure governs the driving pressure gradient and the shift from branching networks to localized channel flow. Below this threshold, the gradient cannot fully penetrate the CO2-treated zone, limiting fracture complexity; above it, faster flow suppresses branching, widens fractures, and reduces the fractal dimension. Orthogonal analysis reveals a hierarchical control: water pressure has the greatest influence, followed by CO2 pressure, water pressurization rate, constant pressure duration, and CO2 pressurization rate. These findings link pore pressure diffusion and permeability evolution to fracture complexity, providing a theoretical basis for the optimization of process parameters.
Shuo Dai, Xinwei Zhang, Zhaolong Ge et al.· The Physics of Fluids· 0 citations
The injection of CO₂ into deep geological formations triggers a complex network of coupled geochemical and geomechanical processes. While reservoir pressure assessments often treat rock strength as a static parameter, the introduction of CO₂ alters the subsurface environment by acidifying the resident pore fluids. This acidic brine promotes the dissolution of carbonate and silicate mineral cements that maintain structural integrity. Simultaneously, high fluid injection pressures reduce the effective confining stress of the formation. This interaction drives time-dependent subcritical crack propagation, allowing micro-fractures to grow slowly over decades or centuries at stresses below the instantaneous failure threshold. If these fractures extend into the low-permeability caprock seal, containment security is compromised. This review synthesizes current advancements in laboratory triaxial testing, micro-computed tomography, and reactive transport modeling (RTM) to map these multi-physics interactions. By evaluating data from key field cases—including Sleipner, In Salah, and Weyburn-Midale, we demonstrate that isolated hydraulic or chemical assessments are insufficient. Ensuring permanent carbon storage requires an integrated thermo-hydro-chemo-mechanical (THMC) framework that treats the subsurface as a dynamic, fully coupled system.
Fatima Arshad· Rock Mechanics Letters· 0 citations
Internal erosion has been widely observed in geomaterials, resulting in many instability problems. However, there is little experimental research concerning mechanisms of internal erosion under oscillating hydraulic gradient resulting from storm surges and waves, which can be induced by extreme events and climate change. A series of laboratory seepage experiments on gap-graded sand specimens was conducted under a compound seepage mode including monotonic and sinusoidal cyclic hydraulic gradients, which were characterized by real-time permeability measurements and particle image velocimetry (PIV). The results show that a cyclic hydraulic gradient can initiate unclogging in previously clogged specimens and erosion in unstable specimens without reaching the critical hydraulic gradient. Increasing the inflow loading frequency from 0.005 to 0.02 Hz resulted in an increase in the final permeability, with all changing patterns affected by cyclic pore-throat narrowing and pore-throat reopening. The permeability evolution, fine-particle PIV, and fine-particle distribution showed good agreement, indicating that the effects of unclogging and erosion decrease with the progression of inflow loading cycles. Recent developments in laboratory testing and imaging analysis were combined to offer a comprehensive understanding of permeability evolution in gap-graded sand, which is relevant to climate change mitigation and adaptation of geotechnical structures.
Yuliang Guo, Budi Zhao, Xueyu Geng et al.· Journal of Geotechnical and...· 0 citations