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Numerical simulation is a commonly used method for investigating rock failure. However, the numerical model is usually insufficient to predict real rock damage and failure because of rock microstructural heterogeneity. In fact, rock damage can be quantified using acoustic emission (AE) data. The aim of this study is to simulate and predict the failure of Brazilian and uniaxial compression specimens using AE data recorded during experiments. An AE data-driven model, in which cracks are assumed to be tensile in nature, is developed. AE data recorded from the test start up to a fraction of the peak stress (e.g., 20%, 40%, and 60%) are input into the data-driven model to predict the evolution of failure pattern beyond that stress level up to failure. First, we quantified stress-induced rock damage with AE data based on the tensile model. The results indicate that most of damage source radii are less than one millimeter, and the corresponding damage degree is close to one. Then, the inversed damage is input as the initial conditions for the numerical simulation to predict the future damage and failure of rock. With the increase of damage elements driven by AE data, the inversed damage zone develops from diffuse to localized, and the dominant factor for rock failure transits from microstructural heterogeneity into stress-induced rock damage. The damage and failure pattern of rock is well predicted when sufficient AE data are taken into account as known conditions. [ABSTRACT FROM AUTHOR]

Dynamic disturbance such as blasting may significantly affect the creep of rock. Quantification of the influence of dynamic disturbance on the creep behavior of rock is a prerequisite to the understanding of the long-term behavior of rock around excavations. In this study, a new creep-impact test machine that is capable of testing the rock failure behavior under combined creep loading and dynamic disturbance was introduced. We performed creep experiments on sandstone while a dynamic disturbance was applied. Axial strain, volumetric strain and acoustic emission (AE) events were measured. The results from the creep-impact experiment show that the rock creep was greatly affected by the dynamic disturbance. Dynamic disturbance may introduce further damage on rock and shorten the time-to-failure of creeping rock specimens. Combination of creep stress and dynamic disturbance resulted in two failure conditions: failure along with accelerating creep and failure during dynamic disturbance. If the dynamic disturbance was not followed by failure, the axial strain consisted of an instantaneous response as the dynamic disturbance was applied, followed by a primary phase of decelerating creep and a steady-state creep phase. This pattern was repeated after the next dynamic disturbance until the last dynamic disturbance that led to the failure. The creep resulted from micro-fracturing in the rock, which can be characterized by the cumulative AE energy if the AE events were monitored during the creep-impact test of rock. The creep behavior was more sensitive to dynamic disturbance under higher creep stress. Under the same creep stress, a dynamic disturbance with higher impact energy resulted in a higher axial strain rate, absolute volumetric strain rate and AE energy rate. Dynamic disturbance not only increased the axial strain rate but also promoted the dilatancy of the rock specimens. The failure of the rock specimens was mostly in the shear mode, and this failure pattern was merely affected by the dynamic disturbance, even though the specimens became more fragmented under the higher creep stress and higher impact energy. [ABSTRACT FROM AUTHOR]

Sudden inelastic deformations in rock are associated with acoustic emission (AE). Therefore, AE monitoring technique can be used to study the fracture processes of rock. In this paper, AE tests were conducted on the granitic gneiss specimens under the uniaxial compressive loading conditions. The temporal changes in AE hit parameters and spatial-temporal evolution of AE events during the failure process of the granitic gneiss specimens were studied, and several characteristic AE phenomena (i.e., dramatic increase in dominant frequency, AE energy, and hit rate, the AE event with a high energy level, and the through-going distribution of the AE events with intermediate energy levels) were statistically analyzed before the failure occurred. It was found that the chronological order of the characteristic AE phenomena was relatively certain and correspondingly had a close relationship with the crack development stage. Because of the difference of the stress level at each crack development stage, the stability at different crack development stages is different. Therefore, a rock stability assessment approach was established based on the chronological order of the characteristic AE phenomena, and then the rock stability was assessed using the proposed approach. [ABSTRACT FROM AUTHOR]

Supercritical CO 2 (SC–CO 2) would be a potential substitute for water-based fracturing fluid widely used in the present hydraulic fracturing of shale reservoirs. However, the field application of SC-CO 2 fracturing is still in its infancy. Laboratory experiments are crucial for understanding the effects of various fracturing parameters and validating numerical simulations before conducting costly field trials. This paper first introduces the design of a self-developed experimental apparatus for fracturing cylindrical rock specimens and outlines the working principles of six functional modules. The entire experimental system is flexible because its functional modules can be used interchangeably. Subsequently, some key designs are emphasized. (1) A packer with the sealing of O and Y rings exhibits a good seal efficiency for a relatively small wellbore; moreover, this packer can be conveniently moved out from the fractured shale without damaging the specimen. (2) Downhole temperature measurement structure integrated into the packer can sensitively monitor the CO 2 temperature variation at a high sampling rate during the entire SC-CO 2 fracturing process. (3) The design of the triaxial core holder contributes to the realization of withstanding high axial-confining and fluid injection pressures and customized installation of acoustic emission (AE) sensors. Ultimately, some fracturing experiments with water and SC-CO 2 as fracturing fluids and seepage tests with nitrogen (N 2) under various test parameters are successfully conducted. The results indicate that, after the specimen fractures, the injection pressure would be approximately decreased to the confining pressure due to the confining chamber sealing design; there is a simultaneous sharp variation in the temperature at the bottom of the wellbore. SC-CO 2 fracturing generates more fractures with greater bifurcation and higher tortuosity. The gas flow rate decreases with increasing confining pressure, whereas it increases significantly with increasing injection pressure. In general, these experimental results can validate the performance and feasibility of the self-developed experimental apparatus. • A self-designed apparatus for fracturing cylindrical rock specimens with SC-CO 2 was introduced. • The experimental apparatus was flexible because its functional modules could be used interchangeably. • The performances of the designed packer and down-hole temperature measurement structure were emphasized. • The experimental apparatus was used for fracturing and seepage tests on shale specimens. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (SC–CO 2) is a promising fracturing fluid with unique properties, making it well-suited for unconventional reservoir stimulation. Laboratory experiments are essential for determining hydraulic parameters prior to field trials. The fracturing apparatus thus plays a primary and indispensable role in evaluating SC-CO 2 fracturing potential. In this review article, a simple overview of water-based and waterless fracturing fluids is first provided from the perspective of their application effects and limitations. It is concluded that, compared with hydraulic fracturing, CO 2 fracturing shows tremendous potential, the research of which has the need to conduct laboratory experiments with new or improved fracturing apparatuses. Second, many multifunctional apparatuses have been designed to investigate the SC-CO 2 fracturing of reservoir rocks. In this regard, the currently developed SC-CO 2 fracturing apparatuses are reviewed with respect to their function units and related existing shortcomings. Different apparatuses correspond to various test schemes and results, among which the influences of the apparatus design on the experimental results are discussed. In particular, some remarks are made regarding the control of the initial injected CO 2 phase and the temperature–pressure measurement of CO 2 during fracturing. Third, inspired by the literature review, a self-designed experimental apparatus is developed to conduct SC-CO 2 fracturing tests. In particular, an integrated structure for the temperature–pressure measurement associated with CO 2 injection is proposed. Moreover, a triaxial core holder is customized to assist in measuring specimen strains and acoustic emission (AE) signals while ensuring the sealing performance of loading and injection systems. Ultimately, the challenges faced in industrial applications, such as fracturing equipment, CO 2 –rock interactions, and CO 2 phase control, have impeded the field-scale application of SC-CO 2 fracturing for shale gas development. Hereby, future apparatus design concepts are given for comprehensive investigations of SC-CO 2 fracturing mechanism. • State-of-the-art and limitation of water-based and waterless fracturing fluids are reviewed. • Current development of SC-CO 2 fracturing apparatuses with respect to function units is briefly introduced. • A self-designed experimental apparatus is customized to maintain the super-critical state of CO 2 during CO 2 injection. • Obstacles remaining for field-scale SC-CO 2 fracturing are discussed. [ABSTRACT FROM AUTHOR]

Uniaxial compressive tests were performed on Y-jointed granite specimens to explore how intersecting rough joints affect the strength, failure mode, and precursory damage characteristics of the jointed rock mass. The mechanical properties, joint dip angles, and joint roughness of the specimens were measured prior to the tests, then the loading stress, surface displacement fields, acoustic emission (AE) signals, and failure modes were recorded during the loading process. As the uniaxial compressive strength (σ c) of the specimens increased, three different failure modes appeared: shear failure along the persistent joint with rock wedge falling, compressive failure with rock wedge ejection, and overall crushing accompanied with rock fragment splashing, the σ c thresholds of the three failure modes are about 50 MPa and 90 MPa, respectively. The deformability of the specimens weakened gradually as σ c increased. AE events spread from the joint surface to the interior of the rock block until final failure. For specimens that failed in shear mode, multiple sudden stress drops along with sharp displacement increases occurred during the tests but no entire failure occurred. Other specimens tended to present one eventual unstable failure. When the peak shear strength of the non-persistent joint was higher than or near to that of the persistent joint, the σ c of a given Y-jointed specimen was mainly determined by the persistent joint. Otherwise, it was simultaneously controlled by the two joints. Both the joint dip angle and joint roughness were found to significantly influence the σ c of the specimens. Each stress drop was accompanied by significant AE signal variations, though the AE indicators are difficult to effectively utilize for early warning of specimen failure. • The effects of joint dip angle and joint roughness on the strength of Y-jointed granite specimens were investigated. • Three different failure modes emerged as the peak compressive strength of the Y-jointed specimens increased. • Shear failure is not the main failure mode in the jointed rock mass under high geo-stress. • Unlike the damage location, the early warning time for specimen failure differs substantially in different cases. [ABSTRACT FROM AUTHOR]

The process whereby a brittle fault was activated by mining is not only significant for disaster prediction, but also for earthquake prediction. Thus, it was necessary to study their activation process. In this study, stress state, AE (acoustic emission)/MS (microseismic) activities during brittle fault activation, stages of brittle fault activation, and the relation between AE/MS activities and brittle fault parameters were analysed and it was feasible to identify buried brittle faults, determine their parameters and activation process based on MS monitoring. The No. 22517 working face in the Dongjiahe Coalmine was taken as an example. Firstly, a buried brittle fault was identified in No. 22517 working face by the MS monitoring method. Secondly, the buried brittle fault parameters were determined. Its azimuth angle was 307.5° in the orbital roadway and 357.5° on the haulage roadway. Its dip angle was 73° and it was a reverse fault. Thirdly, the activation process of the buried brittle fault was determined. The buried brittle fault was not affected by the mining activities when the distance (from mining activities to the buried brittle fault) was over 300 m, the 'disturbance stage' was from 300 m to 80 m, and the 'local activation stage' from 100 m to 30 m, and the 'whole activation stage' around 30 m. The b value decreased during the process of brittle fault activation and reached a minimum value at the whole activation stage; it was concluded that these changes are related to the stress distribution around the brittle fault. The results demonstrated that MS monitoring technology can detect small buried brittle faults ahead of the working face, and is also an effective method to study the process and mechanism of brittle fault activation. [ABSTRACT FROM AUTHOR]