A Model For Hydraulic Fracturing In Soft Coal Seams By Elastic–plastic Softening Damage

In recent years,hydraulic fracturing technology has been used to increase the permeability of coalbed methane,and has achieved certain results.Using numerical simulation method to optimize fracturing parameters has become an effective method for hydraulic fracturing research.However,the existing hydraulic fracture calculation models and numerical methods are mostly based on linear elastic fracture mechanics or meso scale and micro scale.For the soft coal seam,there are insufficient.Soft coal seam has such characteristics as low elastic modulus,low tensile strength,high Poisson ratio,high pore compressibility and easy plastic deformation.It is difficult to form large-scale single connected fracture during fracture development.At the same time,the mechanical properties such as cohesive force and elastic modulus of soft coal seam will be changed remarkably,and it is necessary to consider the softening and damage of coal mass.Therefore,it is necessary to establish a hydraulic fracturing calculation model applicable to soft coal seams.From the view of porous media,a model for hydraulic fracturing in soft coal seams by elastic–plastic softening damage is established;the numerical solution is carried out by FLAC3 D and verified by field test;the fracturing radius of hydraulic fracturing calculation model of soft coal seam has been compared with that of the traditional hydraulic fracturing model of hard coal seam.The main research results are as follows:(1)Based on the theory of multiphase porous media,the dynamic evolution model of porosity,damage variable and bulk plastic tensile strain and plastic shear strain of coal mass are established,and an elastoplastic softening damage model for coupled seepage stress of hydraulic fracturing in soft coal seam is proposed.(2)The calculation model is solved numerically through the development of FLAC3 D.Taking the no.2709 working face of a coal mine as an example,the numerical simulation of 3D hydraulic fracturing in soft coal seam has been carried out,results show that the fracture radius is basically consistent with the measured fracture radius,which verifies the correctness and rationality of the model.The hydraulic fracturing calculation model can be used to optimize the hydraulic fracturing parameters of soft coal seam in underground coal mine.(3)In order to obtain the difference law of fracture effects evolution between soft coal seam and hard coal seam,the evolution law of fracturing radius of hydraulic fracturing calculation model of soft coal seam has been compared with that of the traditional hydraulic fracturing model of hard coal seam.The result shows: when the water injection amount is constant,the radius of hard coal seam fracturing increases with water injection rate,but the increase rate decreases gradually,while the soft coal seam fracturing radius with increasing water injection rate increased first and then slightly decreased and finally remained unchanged,and fracturing radius of soft coal seam was smaller than that of the hard coal seam in same water injection rate;when the water injection rate is constant,the fracturing radius of soft coal and hard coal both increased with the growth of time rapidly,after a period of time is approximately linear growth,but the growth rate of the fracturing radius of soft coal seam was significantly lower than that of hard coal seam.