Rock Mechanics Bulletin最新文献

The failure of rocks is a complicated process as the mechanical properties of the rock are governed by loading history and cumulative ruptures. The geometric aspects of fractures, such as the size and shape of the fractures, the spatial distribution of the fracture networks, and the relations among these aspects also depend on the loads acting on rock mass. In general, the fractures are randomly generated in space which is difficult to be described using mathematical methods. In this paper, the failure processes of rock have been analyzed using the percolation theory. The results indicate that the failure process of rock is a transition from a stable state to an unstable state. This phenomenon is essentially consistent with the phase transition in the percolation theory. Based on this consistency, a theoretical model of percolation for earthquake prediction is proposed. A large number of seismic data provided strong evidence in support of the reliability and applicability of this model.

The Longmaxi shale is an extensive, prolific unconventional play in southwestern China. Its development in the Changning area is affected by ineffective hydraulic fracturing (HF) stimulation, fault reactivation and casing damage. It is suspected that the stress contrast within and between the shale reservoirs and the formations above and below matters to hydraulic fracture propagation and reservoir stimulation. To this end, the Longmaxi shale in the Changning area deserves a dedicated quantification of the in situ stress state and its variations. In this study, we re-visit the available data from one of the play’s first appraisal wellbores (X01) for an integrated geomechanics study, focusing on profiling the stress across the Longmaxi and its adjacent formations. Combining geophysical logs and other stress indicators, we re-interpret its stress profile in the context of lithological variations. The resulting stress variations are modeled primarily through a viscoplastic stress relaxation framework, compared with the results via the frictional equilibrium and an elastic theory (the Extended Eaton model). We offer some discussions on the differences and similarities of these stress profiling methods, and examine their applicability to Longmaxi shale in the Changning area. Our objective is to connect the lithology-controlled stress variations to the first-order complexities (HF ineffectiveness and fault reactivation) that have been observed in the area to date.

The focus of this study is to analyze a parametric space for the problem of a constant height hydraulic fracture driven by a power-law fluid. The interplay of physical mechanisms related to toughness, fluid resistance, and leak-off is considered, but the model is restricted to local elasticity for simplicity. The problem of a semi-infinite constant height fracture is first analyzed: limiting solutions are obtained analytically and their locations inside the dimensionless parametric space are obtained. Then, the problem of a finite constant height fracture is investigated. Similarly, limiting vertex solutions are first outlined and then their locations in the parametric space are quantified. Results demonstrate that the effect of the power-law factor is relatively mild, as it does not significantly distort the parametric spaces. At the same time, there are quantitative differences, which are also determined by the obtained results. Numerical examples highlighting the effect of fracture regime on morphology of multiple fractures are presented at the end.

Earthquake prediction is a common scientific challenge for academics worldwide. This dilemma originates from the lack of precursory indicators that meet the sufficient and necessary conditions of earthquake occurrence, which may be the root cause of the failure of earthquake prediction. In light of this, a double-block catastrophic mechanics theory for earthquakes based on cross-fault Newton force measurement is proposed herein. Based on this theory and laboratory physical model tests of seismic Newton force monitoring, a new academic thought is envisioned “the sufficient and necessary condition for earthquake occurrence is the change of Newton force, and the sudden drop of Newton force on the fault surface can be used as a predictor of earthquake disaster.” Several equipment systems have been independently developed, and the technology has been successfully applied to engineering practice. This concept has currently been proven in small-scale double-block catastrophic events such as landslides. Based on the double-block catastrophic mechanics theory, landslides and earthquakes have the similar nature but different scales. According to the on-site monitoring of landslides, it is verified that the sudden drop of Newton force can be used as a predictor of landslide disaster which successfully solves the problem of short-term landslide prediction. The introduction of cross-fault Newton force measurement technology and idea has laid a foundation for improving the method and level of international earthquake monitoring and solving the world-class scientific problem of short-term earthquake prediction.

Understanding the mechanical and hydraulic properties of fractured rocks and their coupled processes is of great significance for the exploration, design, construction, operation, and maintenance of many rock engineering projects such as hydropower development, oil and gas extraction, and underground waste disposal. With the rapid advancement of global and national strategies such as the “Paris Agreement” and the “Belt and Road Initiative”, more and more projects are developed in the complex geological environment with varying geological structures. Shear failure and rock instability are prone to occur in fractured rock masses under the coupled effects of high stress, high pore pressure, and engineering disturbance, which are main sources for engineering disasters such as roof collapse and caving, water and mud inrushes, and induced earthquakes. To solve these problems, extensive research on the coupled shear-flow behavior of fractures has been conducted. However, due to the complex mechanical, hydraulic and geometrical characteristics of single fractures and fracture networks, a large number of outstanding issues related to the impact of the coupled processes on the engineering characteristics of rock masses are still unsolved. The relevant experimental apparatuses and methods remain to be further developed. Therefore, in this review, we analyze and summarize the existing shear-flow experimental apparatuses, classify apparatus configurations, specimen shapes, and testing principles, and compare their advantages and disadvantages. We also summarize the main scientific findings obtained from various experimental apparatuses, aiming to provide a reference for developing new shear-flow experimental apparatuses and conducting related scientific research in the future.

A three-dimensional thermo-hydro-mechanical numerical model has recently been enhanced with thermal capabilities to study the response of geothermal reservoirs to stimulation and production. In this paper, we present an effort to consider three relevant thermal mechanisms in an existing lattice code initially designed for hydraulic fracturing: a) thermal advection in the fluid; b) heat transfer by forced convection from the rock to the fluid; and c) accurate thermal conduction in the rock matrix considering the thermal boundary layer effect. A numerical implementation of the new coupled advection-forced convection logic as well as the coupling with the existing conduction logic in the commercial code XSite is summarized. The numerical solution is compared to analytical solutions for simple simulation cases. The new simulation capability is applied in a large-scale geothermal example to illustrate its performance.

In actual rock engineering, fissures play an important role in determining the mechanical parameters of rock mass, whereas it is very difficult to construct fissures in cylindrical specimens. Therefore, the pre-fissured rectangular rock specimens were constructed innovatively. Moreover, a series of triaxial compression experimental results on the failure mechanical behavior of rectangular solid sandstone specimens containing a single fissure were reported. The lateral strain in different directions was monitored and the experimental results show that elastic modulus and axial strain increase non-linearly with confining pressure, and the average Poisson’ s ratio parallel to fissure (μ₂) is larger than that vertical to fissure (μ₃). The cohesion, Hoek-Brown parameters of peak strength show similar trends with that of crack damage threshold to the fissure angle (α), and the parameters of the peak strength are larger than those of crack damage threshold. However, the internal friction angles of the peak strength and crack damage threshold are almost equal. Based on the geometries and properties of cracks, ten typical crack types are identified. Cracks vertical to pre-existing fissures occur in specimens under uniaxial compression, whereas cracks parallel to pre-existing fissures occur under triaxial compression. Finally, X-ray micro-computed tomography (CT) observations are conducted to analyze the internal damage mechanism of sandstone specimens with respect to various fissure angles. Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of sandstone specimens. This research elucidates the fundamental nature of rock failure under triaxial compression.

Although rock mechanical behaviour has a long record of study, attempts to understand the role of fractures on rock deformation still have unresolved issues. Due to technical and/or economic challenges, natural rock fractures are often dealt with crudely, without detailed consideration of fracture geometry and heterogeneity in many geoscience applications. Veined rocks that are ubiquitous in the upper Earth crust fall in that category where sustained efforts are needed to offer key information for rock mechanics and geomechanics applications. Following on from a recent study on the rupture of veined rocks (DOI: 10.1029/2019JB019052), we further examine stress path constraints on the deformation of veined rocks (i.e., stress-path-dependent behaviour of veined rocks) under polyaxial conditions. The Discrete Element Method is used to establish a calcite veined model where constant mean stress (σ_m) and constant least principal stress (σ₃) paths that are representative in the subsurface activities are considered. The results reveal the stress-path dependency of brittleness for models under different loading paths. Models tested under constant-σ_m conditions exhibit no brittleness, compared to cases where constant-σ₃ is applied. Sliding along the strike of an inclined vein is evident under constant-σ_m deformation, irrespective of the level of stress. Shear bands along the dominated (inclined) veins exhibit apparent particle trajectory anisotropy for the constant-σ_m deformations which is demonstrated by the evident colour contrast of the adjacent rock matrix and the displacement dispersion of the particles forming the shear bands. We envisage that the reactivation of veins is of relevance to Enhanced Geothermal Systems (EGS) development in terms of seismicity mitigation and multiphysics control of fracture and reservoir permeability.

Flow-through experiments were conducted with three permeants to determine the effect of pH, temperature, and cation concentrations on changes in permeability. Granite with a single fracture was used for each sample. Changes in the permeant concentrations due to pressure dissolution, free-face dissolution, and precipitation were identified by measuring the element concentrations before and after the experiments. In addition, the mineral transformation was analyzed by SEM-EDX. The results of the flow-through experiments showed a reduction in permeability in almost all the samples. This decrease in permeability may have been caused by the interaction between pressure dissolution and free-face dissolution, which occurred in the high pH water experiment, or between pressure dissolution and precipitation, which occurred in the saturated mineral water and simulated seawater experiments. When pressure dissolution and free-face dissolution occurred in the samples, the pH and temperature were seen to greatly affect the decrease in permeability, namely, the permeability decreased significantly with increasing pH and temperature. This remarkable decrease in permeability could have taken place because the dissolution rate constant of the mineral increased with the increasing pH and temperature. Moreover, when pressure dissolution and precipitation occurred in the samples, the cation concentrations and temperature were seen to greatly affect the changes in permeability, namely, the permeability decreased significantly with increasing cation concentrations and decreasing temperature.