The Mechanism And Method Of Directional Hydraulic Fracturing In Coal Seam

Hydraulic fracturing is an important means to strengthen coalbed methane(CBM)drainage and prevent coal and gas disasters.For hydraulic fracturing is controlled by the original ground stress,the cracks show problems of monotonous shape and disordered expansion,and the"Blank zone"on both sides of the fracture becomes a hidden safety issue for subsequent coal mining.Coal reservoirs are thin and complex in shape,which makes it easier for hydraulic fractures to damage the roof and floor,and increase the cost of subsequent roadway support.How to control the effective stress of the coal seam and the direction of the principal stress,so that the hydraulic cracks expand orderly along the seam,forming a complex seam network is a key problem that needs to be solved urgently.Therefore,it is of great significance to study the mechanism and method of controlling the hydraulic crack propagation in coal seams to promote the efficient development of coalbed methane.In this paper,by studying the influence law of coal seam occurrence on the initiation and propagation of hydraulic fracturing,the pore pressure gradient and hydraulic slotting induced fracture propagation mechanics model are established.The double-acting fracturing method of pore pressure gradient and hydraulic slotting is proposed.Based on the physics experiments and numerical analysis of directional hydraulic fracturing under true triaxial conditions,the control mechanism of crack propagation in the coal seam was revealed.The main achievements are as below:(1)An analysis model of hydraulic fracture initiation and expansion with coal seam occurrence factors was constructed,and the mechanism of influence of factors such as the occurrence of coal seam and original ground stress on the initiation and propagation of hydraulic fractures was revealed.The analysis model shows that the hydraulic fracture crack initiation and expansion of the coal seam is mainly limited by the occurrence and stress of the coal seam.The results show that the hydraulic fracturing crack propagation direction is significantly constrained by the in-situ stress and the occurrence of coal seams.With coal seam inclination increasing,hydraulic fracture tends to initiate and propagate at the coal-rock interface when?_v>?_H>?_h.Hydraulic fracture tends to initiate and propagate in the coal seam when?_H>?_h>?_v.Hydraulic fracture initiation and propagation appears more random with the change of coal seam inclination angle when?_H>?_v>?_h.With the increasing of the angle between the coal seam inclination and the maximum horizontal principal stress,the crack initiation and propagation is overall manifested as the trend of"in coalbed?at coal-rock interface?in coalbed".(2)The directional hydraulic fracturing method based on the induction by pore pressure gradients is proposed.A directional hydraulic fracturing mechanical model was established based on pore pressure gradient,and the induced mechanism of pore pressure gradient field and hydraulic slotting were revealed.The mechanical model shows that high pore pressure will reduce the energy required for crack initiation and propagation.The crack initiation pressure(p_f)decrement has a positive correlation with injection pressure(p_w)of the controlling borehole,and it has a negative correlation with the distance(d)between the fracturing borehole and the controlling borehole.The cracks expand to the direction where the pore pressure is higher.Besides,the hydraulic slotting will lead to redistribution of local stress in the coalbed,where the maximum principal stress zone is formed in the slotting direction.The direction of the maximum principal stress in this region produces a distorted change compared to the original maximum principal stress,forming an"S"-type crack induced control zone around the slotting.On this basis,the directional hydraulic fracturing method based on the induction by pore pressure gradients is proposed to break through the control of coal seam occurrence and in-situ stresses,leading hydraulic crack to propagate in the coal seam.(3)The directional propagation conditions of directional hydraulic fracturing are clarified,and the influence of the hydraulic slotting deviation angle and the horizontal stress difference coefficient on crack initiation and orientation propagation are revealed.Through the physical similarity simulation experiment and numerical analysis of directional hydraulic fracturing,it was found that there was a"secondary fracturing"phenomenon in the fracturing process.The smaller the hydraulic slotting deviation angle and the horizontal stress difference coefficient,the more favorable the hydraulic fractures orientation extensions.When hydraulic slotting deviation angle(?)is less than 30°or the horizontal stress difference coefficient(K_h)is less than 0.5,the hydraulic fractures can be oriented and expanded along the direction of the slotting arrangement.When the hydraulic slotting deviation angle(?)is greater than 30°and the horizontal stress difference coefficient(K_h)is greater than 0.5,the cracks begins to deflect in the direction of the maximum principal stress.And the greater the K_h,the greater the deflection angle.(4)Field experiments were used to compare and analyze the effects of directional hydraulic fracturing and conventional hydraulic fracturing.The results of field tests show that both directional hydraulic fracturing and conventional hydraulic fracturing can enhance coalbed permeability efficiently.However,the directional hydraulic fracturing method is superior to the conventional hydraulic fracturing method.The directional hydraulic fracturing method increased the initial extraction concentration from single borehole by 24 to 30 percent over the conventional hydraulic fracturing method,and increased the initial extraction concentration from summary boreholes by 18 percentage points,and increased initial average amount of gas extraction from single borehole by0.75 times.The decay trend of summary extraction concentration by directional hydraulic fracturing method was slower,with an attenuation rate of 8.9%,while that of the conventional fracturing technology was 34.5%.Field tests demonstrated the reliability of directional hydraulic fracturing method.