Stress Disturbance Effect Induced By Hydraulic Fracturing In Coal And Rock Seams

Hydraulic fracturing technology has been widely used in coal industry.Achievements have been made in preventing and controlling the coal and gas outburst and rock burst by hydraulic fracturing.At present,the variation law of stress disturbance during hydraulic fracturing has not been studied sufficiently.When hydraulic fracturing is performed to eliminate dynamic disasters in coal mine,whether the stress disturbance will lead to coal and gas outburst and rock burst is not clear.Therefore,it is necessary and urgent to investigate the stress disturbance law during and after hydraulic fracturing.In this thesis,laboratory experiments,numerical simulation,theoretical analysis and field analysis are carried out to study the stress disturbance law during and after hydraulic fracturing in coal and rock mass.To study the stress disturbance law during hydraulic fracturing,physical experiments are performed using large size true triaxial hydraulic fracturing experimental system.Frame stress sensors and pore pressure sensors are embedded in the test block in advance.During the experiments,the dynamic variation of water pressure and frame stress are monitored and recorded in real-time.The change law of stress gradient around hydraulic fractures is investigated.Experiment results show that: along the three principal stresses,the larger the water injection volume,the greater the stress disturbance,the shorter the distance away from the borehole,the greater the stress disturbance.The stress disturbance in the direction perpendicular to the main hydraulic fracture is the most obvious.In other directions,the stress disturbance is relatively small.Investigation of stress disturbance law during hydraulic fracturing is carried out using RFPA2D-FLOW numerical simulation software.The simulation results indicate that: before the initiation of hydraulic fracture,stress concentration occurs around the borehole.With the increase of the borehole water pressure,the stress concentration in the direction perpendicular to the minimum principal stress increases gradually.At the same time,stress concentration in the direction parallel to the minimum principal stress decreases.The range of stress disturbance correspondingly increases.After the initiation of hydraulic fracture,with the opening and propagation of hydraulic fracture,the stress concentration factor and the range of stress-increased area increases gradually in the direction perpendicular to the minimum principal stress.As a result,obvious stress concentration occurs at the hydraulic fracture tip,leading to the tensile failure of the fracture tip.The further the distance to the borehole,the less the impact of water pressure on the principal stresses decreases is,and correspondingly the smaller the stress gradient variation is.Theoretical analysis is performed on the basis of elastic mechanics,hydraulics,seepage mechanics and fracture mechanics.Stress field around the borehole in different stages of before water injection,before the initiation of the hydraulic fracture and after the initiation of hydraulic fracture are calculated separately.Based on the hydraulic fracturing data measured in 8403 working face in Yanzishan coal mine,hydraulic fracturing data of two fracture boreholes are analyzed.The results show that the sounds of roof fracture were heard many times during hydraulic fracturing,indicating that the stress disturbance caused by the initiation and propagation of hydraulic fractures is obvious and great stress concentration occur at local area.When the stress concentration exceeds the plasticity limitation of the coal and rock mass,the strain energy is released and stress is transferred into the deep area of the coal and rock mass.