Hydraulic sand fracturing for gas extraction through directional long drilling in brittle and soft coal seam roofs

To enhance the prevention and control of coal bursts in brittle and soft coal seams, a novel approach involving directional long drilling in coal seam roofs, hydraulic fracturing with sand addition, and segmented fracturing for enhancing gas extraction was proposed. This approach includes the development of technology for continuous directional sandblasting and perforation in open-hole long drilling, as well as the technology for drag-type hydraulic sand fracturing in open-hole long drilling. Additionally, a comprehensive evaluation method that integrates multiple monitoring techniques was established for assessing fracture stimulation effectiveness. A regional gas pre-control model was established for coal seams using hydraulic sand fracturing through long roof boreholes. An industrial-scale trial was conducted at the Shanxi Xinjing coal mine, where six stages of hydraulic sand fracturing were successfully completed, and 30 directional sandblasting and perforation operations were performed. The sand ratio ranged from 2% to 3%, with a jetting pressure between 17 and 28 MPa, and a total of 8.93 t of quartz sand was used. In the segmented hydraulic sand fracturing process, each stage involved an injection volume of 153–235 m3, with a sand ratio ranging from 2.02% to 2.56%, and a total of 6.69 t of walnut shell sand was injected. The cumulative injection volume reached 963 m3. By employing trace substances as tracers and utilizing borehole transient electromagnetic method for monitoring, the comprehensive evaluation indicated that the fracture stimulation radius can reach 25–35 m. Analysis of gas extraction data from the wells over a period of 100 days following the fracturing revealed that the volume fraction of gas extracted from the fractured wells accounted for 50.99%. The average daily volume of pure gas extracted was 699.71 m3. Compared to the gas extraction volume per hundred meters of horizontal wells in the adjacent areas, the extraction volume was increased by 16.87 times. The gas extraction concentration was 23 times higher than that of the kilometer-long wells, indicating a significant improvement in the effectiveness of coal seam gas extraction.