Study On Fracture Propagation Mechanism Of Low Permeability Coal Seam With Network Fracturing

For the low permeable coal seam fracturing,it is difficult to achieve the expected yield by traditional single main fracture,though the length and conductivity of fracture is very high.It is also difficult to maintain high production and long time even if the early stage is very high.Network fracturing technology is a new type of fracturing method.Its purpose is to improve the permeability of coal by fracturing network formation.It has been successfully applied in the natural gas and shale gas.This thesis summarized the characteristics of low permeability coal seam in Lu’an mining area.It studied the intersection mechanism between hydraulic fracture and cleat.It analyzed the influence of different geological conditions on the fracture by numerical simulation method by ANSYS.The low stress ratio is helpful to produce a plurality of main seam fracturing but it causes the hydraulic fracture directly through the cleat.The high stress ratio is helpful to hydraulic fracture open cleat and steer along the direction of cleat,but it produces only one single main fracturing.Through simulation experiments,it was found that it is easy to generate network when the main stress ratio(λ)is 1.5~2.The angle between maximum principal compressive stress direction and cleat is θ.In the conventional hydraulic fracturing,when the θ<45 degrees,hydraulic fracture maybe turn to the cleat direction,the smaller the easier.When the θ≥ 45 degrees,the hydraulic fracture directly will cross the cleat and continue developing.During the hydraulic fracturing of low permeability coal seam,it is optimal to select the low viscosity fracturing fluid and it is should to add temporary plugging agent or increase sand ratio to increase the crack pressure in the fracture after the main fracture is produced.The hydraulic fracture communicates with the cleat system.The fracture network is realized in the process of hydraulic fracture.