Study on Horizontal Well Indirect Fracturing Technology in the Gangue of Broken Soft Coal Seams for CBM Development

Crushed soft coal seams have the features of low mechanical strength and broken coal structure. Fracturing directly in broken soft coal seams, short and coarse fractures are formed and a large amount of coal powder is produced. In this study, horizontal well indirect fracturing technology in the gangue of crushed soft coal seams was proposed to improve the coalbed methane (CBM) development effect. Based on the No. 5 coal seam in the Daning-Jixian block of the Ordos Basin in China, the fracability of the gangue and coal seam were evaluated by logging interpretation, rock mechanical tests and immersion experiments. The technological process of horizontal well staged fracturing in the gangue of crushed soft coal seams was constructed by field test. A corresponding hydraulic fracturing physical simulation experiment was performed, and fracture propagation behaviors were investigated. The results show that compared with crushed soft coal seams, gangue has better brittleness and fracability. Horizontal well indirect fracturing in gangue can well stimulate upper and lower coal seams, significantly decrease the fracturing pressure and fracture extension pressure, and reduce the production of coal power. The desirable crack extension and considerably enhanced production effect demonstrated the remarkable engineering feasibility of this technology. The suitable thickness is 0.5~1 m for mudstone gangue, whereas a thickness of 2~3 m is appropriate for interbedded sandstone and mudstone gangue. The stage spacing should be larger than 80 m, and the cluster spacing should be greater than 24 m to avoid the stress shadow effect. Hydraulic fractures initiated from the gangue of coal seams, first spread vertically in the gangue layer and extend rapidly along the interfaces between the coal seams and gangue, then penetrated the lithologic interface and expanded into the coal seams. The upper and lower coal seams were torn open, and 3D complex fractures were formed in the coal seams, creating flow channels for CBM into the horizontal wellbore. This research presents a novel technical idea for the CBM extraction of crushed soft coal seams. The findings can offer guidance for the optimal design of horizontal well indirect fracturing in the gangue of crushed soft coal seams.