320 Figure 3 Technical model of underground directional borehole hydraulic fracturing in coal mines 3. Technology and Equipment for Underground Directional Borehole Hydraulic Fracturing Destressing 3.1 Underground 5 m³/min Displacement Fracturing Pump Set Based on equipment location, hydraulic fracturing can be classified as surface fracturing and underground fracturing. Underground fracturing equipment is placed in underground roadways or chambers to drill into and fracture target strata. Considering Caojiatan Mine's mining depth, strata occurrence characteristics, surface conditions, and underground situation, the underground fracturing method was selected. To meet the fracturing requirements for ultra-large mining height face roof strata, remote centralized control of multiple parallel pump stations and a sand mixing skid was adopted to achieve a large underground flow output of 5 m³/min. The intelligent remotely controlled fracturing pump set with 5 m³/min displacement consists of 10 injection pumps in parallel, integrated with core components including a control system, high-pressure manifold skid, sand mixing skid, flow meter manifold skid, high-pressure pipelines, power cables, control cables, and pipeline lines. The developed pump set flow rate reaches 5 m³/min, significantly higher than fracturing pump sets currently used in the coal industry. Simultaneously, due to borehole cluster arrangements, the water volume injected within the same rock volume can reach or exceed that of surface hydraulic fracturing, demonstrating a significant scale effect. Figure 4 Intelligent remotely controlled fracturing pump set with 5 m³/min displacement 3.2 Underground Directional Borehole Abrasive Jet System An underground directional borehole abrasive jet system was developed, including underground sand mixing skids and abrasive jet lances, with a jet displacement exceeding 1 m³/min and perforation depth reaching 0.5 m. This achieved directional fracturing in deep underground boreholes, enhancing fracture initiation capability. Its core principle is to use the impact, shear, and erosion of high-speed abrasive jets to form pre-designed slots in the rock, providing guidance for subsequent hydraulic fracture propagation. By combining abrasive jetting with segmented fracturing, precise weakening and regional destressing of hard roofs are achieved. A super-wear-resistant pendulum-type directional jet lance was developed with a jet velocity of 280–300 m/s and sand throughput exceeding 200 m³. An adaptive directional perforation device was developed with a positioning error of less than
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