Numerical simulation of interface dip angle on hydraulic fracture propagation path in composite coal rock

For deep coalbed methane (CBM) exploitation, it is very important to control hydraulic fractures in coal seam to avoid the decrease of fracturing energy caused by crossing coal-rock interface. To reveal the influence of interface dip angle on hydraulic fracture layer-crossing propagation, the test methods and engineering significance of physical parameters of coal-rock interface were analyzed. Numerical model of “roof - coal seam - floor” was established by RFPA2D-Flow numerical simulation software, hydraulic fracture propagation path and geometrical shape under different interface dip angles were studied. The results show that coal-rock interface involves geometric parameters, mechanical parameters, geological parameters, etc. Their acquisition methods and engineering significance are described. The coal-rock interface parameters and in-situ stress jointly affect hydraulic fracture. As hydraulic fracture passes through coal-rock interface, the proportion of rupture surface increases obviously, which can reflect the relationship of propagating energy, pumping pressure and fractured stage. When interface dip angles are different, hydraulic fractures propagate along coal-rock interface to varying degrees. The smaller interface dip angle makes hydraulic fracture more easily to propagate along coal-rock interface. With the increase of interface dip angle, the breakdown pressure increases, it is more difficult to control whole hydraulic fracture in coal seam. Therefore, directional perforations can be applied to control the initiation direction of hydraulic fracture, so as to avoid hydraulic fracture encountering coal-rock interface in advance and affecting fracturing effect. The research results can provide a basis for improving deep coalbed methane exploitation theory and optimizing field fracturing parameters.