Geological adaptability analysis and operational parameter optimization for staged fracturing horizontal wells in coal seam roof

The technique of staged fracturing horizontal wells in coal seam roof is effective for efficient extraction of coalbed methane (CBM) in broken soft and low-permeability coal seams. With the help of the major national science and technology projects of the 13th Five Year Plan for National Economic and Social Development, the efficient extraction technology of CBM by staged fracturing horizontal wells in coal seam roof was studied. By using theoretical analysis and numerical simulation, the fracture propagation patterns in different in-situ stress states were compared, and the influence of the angle between the horizontal well layout orientation and the minimum horizontal principal stress on the fracture deflection and propagation was studied. The competitive propagation phenomenon of fractures under the condition of multi-cluster perforation was also analyzed. The results show that: (1) the staged fracturing horizontal well in coal seam roof applied to CBM development in broken, soft and low-permeability coal seams can avoid contamination of reservoirs by drilling fluid, improve the safety in horizontal well drilling, cementing, and the effect of fracturing transformation, and controlling the output of coal fine. (2) In-situ stress is the key controlling factor for fracture propagation. In order to ensure the crack propagation through the bedding plane interface, the vertical stress must be greater than the minimum horizontal stress, and the minimum horizontal stress of the roof must be greater than that of the coal seam. The interlayer stress difference of 1-3 MPa can not only ensure the effect of through-layer propagation of fractures, but also avoid excessive fracture initiation and extension pressure. When the stress profile of “overlying rock-roof-coal seam” is “low-high-medium” type, the distance between the horizontal well and the top surface of the coal seam has a great influence on fracture propagation. It is recommended that the distance between the horizontal section of the horizontal well and the top surface of the coal seam should be less than 2.0 m. (3) The larger the angle between the horizontal well and the minimum horizontal stress, the greater the fracture turning radius and turning distance. With the same fracturing spacing, the fracture interference is stronger at an angle of 45° than at an angle of 0°, which is not conducive to the propagation of subsequent fractures. It is recommended that the angle between the horizontal well and the minimum horizontal stress should be within ± 15°. (4) For staged multi-cluster fracturing, the fracture stress interference, fracturing fluid flow friction, and perforation friction jointly lead to non-uniform propagation of fractures for each perforation cluster, which can be controlled by using limited entry fracturing, temporary plugging, etc., to promote the uniform propagation of fractures. (5) The pilot test has achieved good gas production, and the fracture extension characteristics are in good agreement with the theoretical research. The research results can provide reference for the optimization design and application of operational parameters of horizontal wells in the coal seam roof.