Abstract:
With the continuous extension of mining activities to the depths, the difficulty of gas extraction in soft and low-permeability coal seams has increased, and the mine gas control is facing greater challenges. Conventional hydraulic fracturing coal seam permeability enhancement measures have some shortcomings such as uneven fracturing, low fracturing displacement, small fracturing coverage and easy fracture closure. Based on this, a technical scheme of advance gas pre-extraction in coal seam anti-penetration area fracturing by hydraulic sanding and staged fracturing with directional long drilling holes in the roof of crushed soft coal seam was proposed, and the composite technology of “directional sandblasting perforation + staged sand fracturing” was developed. and a fracturing effect evaluation method integrating multi-method and multi-angle inspection of the influence range of staged sand fracturing and analysis of coal seam gas drainage effect after fracturing has been formed. Industrial tests were carried out on the North sixth and North seventh working faces of the security area of Xinjing Coal Mine in Yangquan Mining Area, Shanxi Province, and two fracturing boreholes (both with a hole depth of 609 m) were completed, including directional sand blasting perforation and hydraulic sand adding staged fracturing engineering tests. A total of 80 times of directional sandblasting perforations have been carried out, including 30 times of directional sandblasting perforations for No.1 fracturing hole and 50 times of directional sandblasting perforations for No. 2 fracturing hole. The sand ratio is 2%～3%, and the directional sandblasting perforating pressure is 22.6～28.6 MPa. A total of 1 072 m3 of perforating fluid was injected, and 19.84 t of quartz sand was used. 16 stages of hydraulic sand fracturing were carried out. The No. 1 fracturing hole was divided into 6 stages, and the No. 2 fracturing hole was divided into 10 stages. Fracturing fluid 153.76～235.11 m3 was injected into a single stage, with an average sand ratio of 2.02%～2.56%. A total of 36.48 tons of walnut shell sand and 2 808.57 m3 of fracturing fluid were injected. The influence radius of fracturing is 20～38 m by means of borehole transient electromagnetic method and trace tracer method. The 100-day gas drainage data of Drilling Site 1 and Drilling Site 2 after fracturing were counted. The average gas extraction volume fraction of No. 1 drilling site is 43.97%, the average gas extraction mixed flow is 1.61 m3·min-1, and the daily average net gas extraction volume is 1 025.11 m3; The average gas extraction volume fraction of No. 2 drilling site is 23.17%, the average gas extraction mixed flow is 8.56 m3·min-1, and the daily average net gas extraction volume is 2 810.6 m3. Compared with the adjacent area, the gas drainage fraction of the bedding boreholes in the No. 1 and No. 2 drilling sites has increased by 3.36 times and 6.38 times, and the pure volume of gas drainage in the 100 meter boreholes has increased by 16.44 times and 45.14 times; The volume fraction of gas drainage increased by 10.53 times and 19.99 times compared with that of kilometer boreholes, and the net amount of gas drainage increased by 5.61 times and 15.42 times. The results show that this technology realizes large displacement, high sand ratio and continuous hydraulic fracturing operation in the coal mine, and the permeability of the coal seam after fracturing is significantly improved, which provides a technical reference for gas control in broken soft coal seam areas.
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