321 3°. An underground sand-liquid continuous mixing unit achieves precise control of sand concentration from 3% to 10%. (a) Underground abrasive jetting process (b) Underground sand mixing skid (c) Abrasive jet lance (d) Borehole camera result Figure 5 Underground directional borehole abrasive jet system 3.3 Underground Directional Borehole Segmented Fracturing Technology and Tools Segmented fracturing technology divides a directional long borehole into multiple independent intervals (typically 10–30 m per segment) for fracturing. Fracturing locations can be adjusted based on actual borehole conditions. Each segment is injected with highpressure water separately. By optimizing the fracturing sequence and finely controlling the process, strata fracturing efficiency is improved. Underground regional fracturing in coal mines typically employs open-hole segmented hydraulic fracturing technology. Accordingly, an open-hole segmented fracturing tool string was developed, as shown in the figure, mainly consisting of a conveyance guide, forward/reverse circulation controller, open-hole packer, pressure-activated release device, and separable safety device. It enables long-distance conveyance (over 1000 m). The open-hole sealing pressure is increased to 85 MPa, allowing forward/reverse circulation within the hole to improve safety. A single hole can achieve unlimited-stage segmented fracturing, pressure-controlled fracturing, and selfseparation retrieval upon obstruction. Figure 6 Schematic diagram of open-hole segmented fracturing tool string assembly 3.4 Underground Kilometer-Scale Directional Drilling Equipment Underground boreholes were drilled using kilometer-scale directional drilling equipment developed by Xi'an Research Institute of CCTEG. It mainly consists of a directional drill rig, mud pump, Measurement While Drilling (MWD) system, downhole Positive Displacement Motor (PDM), drill rods, water swivel, and bit. This equipment can strictly control the drilling trajectory based on the determined target fracturing horizons, ensuring accuracy in reaching the intended modification zones. MWD is one of the key technologies in underground directional drilling. By real-time measurement of the borehole trajectory and downhole PDM toolface orientation, it controls the trajectory to follow the design, improving spatial positioning accuracy. It enables trajectory control and timely correction in ultra-deep holes over 1000 m, with errors within ±0.5 m.
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