Search

Fractures have significant impact on the stability of underground projects such as coal mining. The identification of geological features based on drilling parameters is a promising option for intelligent detection of rock formations. A series of studies conducted to identify fractures under unstable drilling conditions are presented in this paper. First, the forces acting around the drill bit are analyzed to further improve the force model of the two-wing PDC drill bit. Then, based on the self-developed borehole drilling device, the drilling parameters are collected in real time while drilling in concrete of different strengths and different fracture widths. Finally, the response characteristics of drilling parameters are analyzed, and the multi-parameter voting method is proposed for fracture identification. The experimental results show that when fracture width increases, the crushing area, which forms around the borehole when the drill bit encounters the fracture, tends to increase. The rate of penetration (ROP) increases suddenly, the revolution per minute (RPM) decreases, and the torque increases at fracture encounter. The sudden changes are recovered after passing the fracture. The multi-parameter voting method has a high recognition rate for fractures of width greater than 2 mm, but a relatively low recognition rate for fractures of 1 mm width. In addition to detecting fractures, the method performs well in predicting fracture location and fracture width. Highlights: The response characteristics of multiple drilling parameters at the fracture were analyzed. As the thrust (torque) increases, the amount of change in penetration rate caused by the unit change in thrust (torque) decreases. Results show that the proposed multi-parameter voting method has a high recognition rate for fractures with width greater than 2 mm. [ABSTRACT FROM AUTHOR]

While large-scale rock strata affected by underground coal mining have been widely studied through numerical modeling, there are still some aspects that can be better understood. Specifically, researchers do not fully utilize borehole logs and test results of rock specimens to reveal rock mass property variation along horizontal directions and strata lateral thickness variation at bed level. In this paper, we address this knowledge gap by proposing a data-intensive numerical modeling method (DINMM) that can make full use of these data with consideration of the modeling limitations of BlockRanger, a grid generation tool. Both the proposed method and the conventional numerical modeling method (CNMM) are applied to the Ying-Pan-Hao coal mine via the FLAC3D (Fast Lagrangian Analysis of a Continua in 3 Dimensions) package, and their predictions are then calibrated and compared to discuss the validity. Results show that, compared to the CNMM-based predictions, the root mean square error of 70 monitoring points is decreased at least by 27.4% in the DINMM-based prediction, and the relative error of maximum subsidence is reduced by 5.1% with a reduction rate of 66.5% on average, even though the CNMM-based model was originally better calibrated. We also find that a DINMM-based model is more in line with field observations and theoretical understanding in terms of displacement, stress, and failure propagation. The notion of data-intensive modeling seems to be quite promising and the DINMM should be useful for a better understanding of strata movement and subsidence prediction. Highlights: • We propose a data-intensive modeling method (DINMM) to build numerical models based on multiple borehole logs, rather than on a single or generalized borehole log. • We realized rock mass property variation along horizontal directions and strata lateral thickness variation at bed level when modeling large-scale rock strata. • We find that the root mean square error of 70 monitoring points is decreased at least by 27.4% in the DINMM-based prediction, even though the control model was originally better calibrated. [ABSTRACT FROM AUTHOR]

The main constrain for effective gas drainage in coal mines is the low permeability nature of coal reservoirs. As coal mining activities are extending to deeper subsurface, the ever-increasing in situ stress conditions is anticipated to result in much lower permeability and more challenges for gas emission control in coal mines. In recent years, hydraulic slotting using high-pressure waterjet along underground gas drainage boreholes, as a general solution to stimulate low permeability coal seams, has become increasingly favourable. This paper presents a systematic investigation into the sensitivity of borehole slotting performance to a number of field and operational parameters. A wide range of geomechanical properties, in situ stress conditions, slot geometry and spacing of multiple slots were considered in a series of numerical simulations. The relations between these key parameters and the failure zone size/volume induced by slotting were quantified. The effect of different parameters in improving slotting performance has also been ranked, which provides theoretical base for mine operators to optimise slotting operations. [ABSTRACT FROM AUTHOR]

Grout injection into the horizontal fracture at the bed separation of an overburden is used as a surface subsidence control measure in coal-producing countries such as Poland, China, and Australia. However, the stiffness and distribution of the grouting mass in the post-injected overburden, as well as its effect on surface subsidence control, have not been investigated. To address these issues, two surface boreholes were drilled into the post-injected overburden above a longwall panel on the study site. It was found that the fly ash injected more than 7 months previously had become compacted. More than 90 % of the total fill was concentrated in the main injection section (MIS) of the overburden (thickness of 4.8-18.4 m). The MIS was located between the bottom of the injection borehole and the key stratum immediately above. Following injection, the central gob area received the maximum possible compression and served as the foundation of the support for the grout, allowing the grout to restrain the deformation of the strata above it, thus reducing surface subsidence. The results of this investigation may facilitate future injection design, thus improving the control of surface subsidence. [ABSTRACT FROM AUTHOR]

Methane extraction from in-seam boreholes is the main approach for recovering methane in China. However, the methane concentration for this method is generally lower than 30 %, which incurs a risk of methane outbursts during pipeline transportation. To increase the methane concentration, we first conducted permeability experiments to investigate the relationships between the permeability and the effective stress at different stages in the complete effective stress-strain process. We then adopted FLAC3D software to calculate the stress distributions around roadways and boreholes after their consecutive excavations and thereby divided the coal mass around the roadway and borehole according to different effective stress stages to understand the gas flow characteristics. The results show that the coal mass along the radial direction of the roadway and borehole can be sequentially divided into four zones, including the full flow zone (FFZ), the transitive flow zone (TFZ), the flow-shielding zone (FSZ), and the in situ rock flow zone (IRFZ), which have been proven correct by field experiments. The methane in the IRFZ was difficult to extract because of the low permeability of coal mass in this zone. The permeability of the FSZ was lower than that of the IRFZ. The permeability along the interface between the FSZ and TFZ was nearly one time as low as that of the IRFZ, while the permeability of the FFZ was two orders of magnitude higher than that of the IRFZ. This four-zone division demonstrates the decaying mechanism of methane extraction concentration and flow in the in-seam borehole and can provide theoretical guidance for improvement of methane extraction. [ABSTRACT FROM AUTHOR]