Application of CBM horizontal well development technology in the roof strata close to broken-soft coal seams

Broken coal structure and low permeability are the main reasons for the difficult production of coalbed methane (CBM) in broken-soft coal seams. In this paper, No.8 coal seam in the Luling Mine of the Huaibei Coal Field was taken as an example to study the way to improve CBM production of broken-soft coal reservoirs. First, drilling, fracturing and production of horizontal wells were considered comprehensively. Then, based on the idea of exploiting CBM through horizontal wells in roof strata close to the broken-soft coal seams, the propagation laws of fractures created by layer-penetrating fracturing of horizontal wells in roof strata were studied, and accordingly the location of horizontal well was optimized. Finally, the CBM horizontal well exploitation technology for the roof strata close to coal seams was developed and tested on site. And the following research results were obtained. First, the vertical fractures created during layer-penetrating fracturing of horizontal wells in roof strata can propagate from the roof strata with high stress values downward to the coal seam with low stress values. In addition, the location of horizontal well has an important influence on the effect of layer-penetrating fracturing. The closer the horizontal well is to the coal seam, the better the fracture propagation effect of layer-penetrating fracturing. Second, the horizontal well should be arranged in the roof strata 1.5 m away from the top of coal seam, so that fracturing stimulation requirements of horizontal wells in roof strata can be satisfied to the uttermost. Third, three key technologies are formed, including the “high-quality, fast and safe” drilling technology, the deep-penetration oriented perforating technology and the “large displacement, large scale, high pad ratio and moderate proppant concentration” active water fracturing technology. Fourth, the gas production with this technology in engineering practice is remarkable. In conclusion, the CBM development technology of horizontal wells in roof strata close to broken-soft coal seams is feasible. The research results provide a new technical method for the development of CBM in broken-soft coal seams. Keywords: Broken-soft coal seams, Coalbed methane (CBM), Roof strata, Horizontal well, Development technology, Layer-penetrating fracturing, Propagation laws of fractures, Physical simulation, Well location arrangemen