Influence Of Stress Disturbance On The Initiation And Propagation Of Hydraulic Fractures In Coal

After the coal seam is mined,a goaf is formed,and the stress environment and mechanical properties of the underlying coal seam in the goaf will be disturbed to a certain extent,which will affect the initiation and propagation law of coal seam hydraulic fracturing fractures.Gas drainage can effectively prevent coal and gas outburst accidents,improve gas utilization and reduce greenhouse gas emissions.As an important means of coal seam gas drainage,hydraulic fracturing is very important to study the law of hydraulic fracture initiation and expansion.In this paper,theoretical analysis,physical test and field measurement are used to study the initiation and propagation law of hydraulic fracture in coal seam disturbed by stress under goaf,and the damage factor representing the damage degree of disturbed coal body is proposed based on the research results.The main research results are as follows:Using the self-developed true triaxial stress loading and unloading and hydraulic fracturing test system,the true triaxial hydraulic fracturing test is carried out to study the initiation and propagation law of hydraulic fractures in the test block under stress disturbance action.There are three specific situations:(1)If the disturbance stress is too high,the test block will produce large strain and a large amount of damage inside the test block,but the damage expansion during unloading is very small,indicating that the test block has been maintained in the state of stress compaction,the hydraulic fracturing fracture pressure is higher and the fracture time is longer,which is not conducive to the initiation and expansion of hydraulic fracturing fracture;(2)If the disturbance stress is too small,the test block will undergo elastic deformation,that is,the strain generated in the loading process will be basically restored after unloading.Although there is stress disturbance and a certain amount of strain,there is virtually no damage in the test block,that is,the mechanical properties of the test block will not change under weak stress disturbance and remain in the original state;(3)If the disturbance stress is at an appropriate level,it will not cause permanent compaction of the test block,nor will it be too small to cause damage to the test block.In this state,the initial fracture pressure of hydraulic fracturing is lower,the time is shorter,the fracture expands faster to the surface of the test block,and the internal damage develops more fully.Considering the influence of the strain in the two processes of loading and unloading of the test block on the overall damage,the damage factor characterizing the damage degree of coal under uniaxial stress disturbance is proposed,the significance of the damage factor under two limit conditions is analyzed,and the expression form of the damage factor under biaxial stress disturbance is further proposed.It is explained that the damage factor does not represent the absolute damage of the test block,but the relative parameters under the initial state,When the initial reference stress state changes,the size of the damage factor will also change,and it is specifically explained that there are two limit cases when the damage factor is very small:(1)ΔR_-ΔR_+≈0,under this condition,the disturbance stress of the test block is too large,the strain in the unloading stage is very small,the test block is highly compacted,or the degree of unloading is too low,the damage has not been fully released,the size of the damage factor is almost 0,and the hydraulic fracture pressure of the test block is even higher than the initial state;(2)(ΔR_-ΔP_-)(ΔR_+ΔP_+)≈1,under this condition,the displacement changes with the stress at the same speed in the loading and unloading stages.The test block has elastic deformation and the internal damage is small,that is,the damage factor.The vertical well fracturing data of 30 coal seams in the non mining area and 30 coal seams in the mining area of Sihe mine field are collected,as well as the buried depth,minimum principal stress,vertical stress and other parameters of the coal seam.The analysis shows that the vertical stress of coal seam 9 and 15 decreases after the mining of coal seam 3.The hydraulic fracturing fracture pressure is lower than that before the mining disturbance,which is conducive to the initiation and expansion of hydraulic fracturing fractures,Improve the fracturing efficiency of coal seam;Under mining conditions,the hydraulic fracturing pressure of coal seam 9 is lower than that of coal seam 15,because the buried depth of coal seam 9 is shallow and greatly affected by the mining of coal seam 3,which is more conducive to the initiation and expansion of hydraulic fractures.The research results provide a reference for the hydraulic fracturing technology of the underlying coal seam in the goaf and improve the gas drainage efficiency.