Xuehua Li

Numerical Investigation of the Effect of the Location of Critical Rock Block Fracture on Crack Evolution in a Gob-side Filling Wall

Generation, propagation, and coalescence of the shear and tensile cracks in the gob-side filling wall are significantly affected by the location of the fracture of the critical rock block. The Universal Discrete Element Code software was used to investigate crack evolution characteristics in a gob-side filling wall and the parameter calibration process for various strata and the filling wall was clearly illustrated. The cracks in both the filling wall and the coal wall propagate inward in a V-shape pattern with dominant shear cracks generated initially. As the distance between the fracture and the filling wall decreases, the number of cracks in the filling wall decreases, and the stability of the filling wall gradually improves; thus, by splitting the roof rock at the optimal location, the filling wall can be maintained in a stable state. Additionally, we conducted a sensitivity analysis that demonstrated that the higher the coal seam strength, the fewer cracks occur in both the filling wall and the coal wall, and the less failure they experience. With the main roof fracturing into a cantilever structure, the higher the immediate roof strength, the fewer cracks are in the filling wall. With the critical rock block fracturing above the roadway, an optimal strength of the immediate roof can be found that will stabilize the filling wall. This study presents a theoretical investigation into stabilization of the filling wall, demonstrating the significance of pre-splitting the roof rock at a desirable location.

Numerical Investigation of Bedding Plane Parameters of Transversely Isotropic Shale

Determination of the physical properties of shale is receiving more attention as the numbers of shale gas exploration projects are initiated, and as hydraulic fracturing becomes an integral exploitation method. In particular, anisotropy caused by the bedding structure of shale needs specific attention. In this paper, an anisotropic mineral brittleness-based model (AMBBM) is proposed that makes use of the discrete element method (DEM) to study shale properties, such as anisotropy of non-penetrating bedding planes and separating brittle and non-brittle minerals. Micro-parameters of the AMBBM are calibrated using uniaxial compressive strength tests and by studying the parameter gradient of smooth joints (SJ), such that the strength of SJ mainly affects the failure load in Brazilian tests (FLBT). It is found that the ratio of cohesion to tensile strength of SJ mainly affects the number of cracks formed, which further leads to different failure modes. Normal stiffness and shear stiffness of SJ exerts different effects on FLBT and stiffness in the model. However, the percentage of cracks of various minerals is less affected. The degree of anisotropy is affected by the angle range of parallel bond replaced by bedding plane. Based on the results, a new validation method for AMBBM is proposed, given that the numerical results show good agreement with experimental results, such as FLBT, splitting modulus, and failure mode. The model can thus be used to study seepage properties of shale gas exploitation and hydraulic fracturing by DEM.

Numerical Investigation of Acoustic Emission Events of Argillaceous Sandstones under Confining Pressure

At the laboratory scale, locating acoustic emission (AE) events is a comparatively mature method for evaluating cracks in rock materials, and the method plays an important role in numerical simulations. This study is aimed at developing a quantitative method for the measurement of acoustic emission (AE) events in numerical simulations. Furthermore, this method was applied to estimate the crack initiation, propagation, and coalescence in rock materials. The discrete element method-acoustic emission model (DEM-AE model) was developed using an independent subprogram. This model was designed to calculate the scalar seismic tensor of particles in the process of movement and further to determine the magnitude of AE events. An algorithm for identifying the same spatiotemporal AE event is being presented. To validate the model, a systematic physical experiment and numerical simulation for argillaceous sandstones were performed to present a quantitative comparison of the results with confining pressure. The results showed good agreement in terms of magnitude and spatiotemporal evolution between the simulation and the physical experiment. Finally, the magnitude of AE events was analyzed, and the relationship between AE events and microcracks was discussed. This model can provide the research basis for preventing seismic hazards caused by underground coal mining.

Effect of Joint Geometrical Parameters on Hydraulic Fracture Network Propagation in Naturally Jointed Shale Reservoirs

The presence of a significant amount of discontinuous joints results in the inhomogeneous nature of the shale reservoirs. The geometrical parameters of these joints exert effects on the propagation of a hydraulic fracture network in the hydraulic fracturing process. Therefore, mechanisms of fluid injection-induced fracture initiation and propagation in jointed reservoirs should be well understood to unleash the full potential of hydraulic fracturing. In this paper, a coupled hydromechanical model based on the discrete element method is developed to explore the effect of the geometrical parameters of the joints on the breakdown pressure, the number and proportion of hydraulic fractures, and the hydraulic fracture network pattern generated in shale reservoirs. The microparameters of the matrix and joint used in the shale reservoir model are calibrated through the physical experiment. The hydraulic parameters used in the model are validated through comparing the breakdown pressure derived from numerical modeling against that calculated from the theoretical equation. Sensitivity analysis is performed on the geometrical parameters of the joints. Results demonstrate that the HFN pattern resulting from hydraulic fracturing can be roughly divided into four types, i.e., crossing mode, tip-to-tip mode, step path mode, and opening mode. As (joint orientation with respect to horizontal principal stress in plane) increases from 0° to 15° or 30°, the hydraulic fracture network pattern changes from tip-to-tip mode to crossing mode, followed by a gradual decrease in the breakdown pressure and the number of cracks. In this case, the hydraulic fracture network pattern is controlled by both (joint step angle) and . When is 45° or 60°, the crossing mode gains dominance, and the breakdown pressure and the number of cracks reach the lowest level. In this case, the HFN pattern is essentially dependent on and (joint spacing). As reaches 75° or 90°, the step path mode is ubiquitous in all shale reservoirs, and the breakdown pressure and the number of the cracks both increase. In this case, has a direct effect on the HFN pattern. In shale reservoirs with the same , either decrease in (joint persistency) and (joint aperture) or increase in leads to the increase in the breakdown pressure and the number of cracks. It is also found that changes in and result in the variation in the proportion of different types of hydraulic fractures. The opening mode of the hydraulic fracture network pattern is observed when increases to 1.2 × 10−2 m.