Mechanism Of Hydraulic Fracture Initiation And Propagation In Coal Seam Induced By Artificial Fracture

Extraction of coalbed methane(gas)can alleviate the shortage of natural gas,prevent the occurrence of coal and gas disasters,and ensure the safe and efficient mining of coal.Hydraulic fracturing technology has been widely used as the main method of extraction coalbed methane,but the traditional underground hydraulic fracturing theory does not consider the variability of borehole morphology and the diversity of fracture initiation failure modes in multi weak coal seams under in-situ stress,which makes it difficult to accurately predict the fracture initiation pressure,and the construction pump pressure can not be reasonably determined,and effective fractures can not be formed in the coal seam.The slotted composite fracturing technology proposed by the author's research team has achieved good permeability enhancement effect.However,the hydraulic fracture propagation mechanism of the technology under complex in-situ stress after artificial fracturing is not clear,which can not be explained by the existing fracture induced propagation theory based on pore water pressure field or fracture area,and can not provide accurate theoretical guidance for the determination of construction parameters of slotted composite fracturing technology.The application of this technology is limited.How to accurately predict the fracture initiation pressure of multi weak coal seams to form effective fractures and induce hydraulic fractures to spread uniformly and orderly along the coal seam is a key problem of hydraulic fracturing technology to be solved.Therefore,by using the methods of theoretical derivation,physical similarity simulation experiment and field test,the mechanism of hydraulic fracture initiation and propagation induced by artificial fracture is studied in this paper,and the key factors affecting the initiation and propagation of hydraulic fracture induced by hydraulic fracture are explored.The main achievements are as follows:(1)The fracture initiation criteria of three-dimensional multi weak coal seam hydraulic fracturing are established,and the fracture mechanism of hydraulic fracture in multi weak coal seam is revealed.Based on the theory of complex variable function and maximum tensile stress failure,the criterion of water pressure crack initiation under the condition of coal seam drilling pressure deformation is established.The fracture initiation and failure law of multi weak face coal seam under different geological parameters and construction parameters are calculated and analyzed by MATLAB software With the increase of coal seam dip angle,positive shear failure occurs most easily in coal seam(?z plane).(2)The prediction model of hydraulic fracture propagation induced by artificial fracture(slotted hole)is established,and the mechanism of hydraulic fracture propagation induced by artificial fracture is revealed.This paper analyzes the principle of stress-induced hydraulic fracture propagation in coal seam with artificial seam making,establishes the calculation model of induced fracture propagation distance,and obtains the influence law of horizontal stress difference and azimuth angle of slotting hole on induced propagation distance of hydraulic fracture.With the decrease of horizontal stress difference,the induced propagation distance increases exponentially.With the increase of drilling azimuth angle,the induced propagation distance of hydraulic fracture decreases rapidly.When the azimuth angle of drilling hole increases to 70 °,the induced propagation distance of hydraulic fracture remains unchanged.(3)The mechanism of hydraulic fracture propagation induced by double slotted holes is revealed.Based on the theories of Cauchy integral and residue theorem,the stress distribution around the slotted hole under different slotting spacing is studied,and the calculation model of double slotted holes induced fracture propagation distance is established.The mechanism of stress interference and non-uniform pore water pressure on induced propagation of hydraulic fracture is revealed.The stress interference hinders the induced propagation of hydraulic fractures.The non-uniform pore water pressure promotes the induced propagation of hydraulic fractures,and the promotion of non-uniform pore water pressure is dominant.