Study On De-stressing Mechanism Of Directional Hydraulic Fracturing To Control Deformation Of Reused Roadway In Longwall Mining With Two Gateroad Layout And Its Onsite Practices

For longwall panel with multiple entries,reused roadway generally suffers from mining-induced stresses caused by the extraction of both the current panel and subsequent panel,resulting in serious deformation and support challenge of the reused roadway.To overcome this problem,hydraulic fracturing is proposed as a measure to de-stress mining-induced stresses.A case study was carried out at a reused roadway at the Yuwu Coal Mine of the Lu'an Group.The associated mechanisms were examined through a combination of theoretical analysis,numerical modeling,laboratory test and field test.This thesis consists of three aspects:(1)Theory analyzed associated de-stressing mechanisms and application conditions through directional hydraulic fracturing for retaining roadway;(2)Numerical Simulation studied extending disciplines of directional hydraulic fractures;(3)The true-triaxial hydraulic fracturing test in the laboratory studied the effect of in-situ stress and bedding direction on the direction and morphology of directional hydraulic fracturing crack propagation;(4)case study of the application of directional hydraulic fracturing for de-stressing mining-induced stress and comprehensive monitoring.The main outcomes of this thesis are as follows:(1)The large deformation of reused roadway is due to high abutment pressure caused the extraction of longwall panel.The competent massive roof in the gob cannot cave easily behind the longwall support,causing a high abutment pressure applied on the chain pillar between the entries and subsequent deterioration of the reused roadway.The measure of directional hydraulic fracturing is able to eliminate or weaken the bridge effect of the competent massive roof,change stress transfer and distribution ratio,and improve the state of the reused roadway.(2)The major factor affecting the effectiveness of de-stressing is the suspended effect of the competent massive roof above the pillar.The greater the thickness of the hard roof or cantilever area,the stronger the suspended roof effect,the greater the deformation and the more serious the damage.In addition,the stronger the effect of overhanging,the greater the reduction of deformation,the greater the de-stressing effect.(3)The difference of principal stress and the tensile strength of rock mass are the two main factors controlling the fracture pressure of hydraulic fracturing.The minimum principal stress increases or the difference of principal stress decreases,the required fracturing pressure increases correspondingly;As the tensile strength of rock mass increases,the required fracturing pressure increases accordingly.(4)The direction of hydraulic fracture propagation depends on the direction of the minimum principal stress.Transverse grooving is an effective method to create directional hydraulic fracture.Under certain condition of limited difference between the major and minor principal stress,hydraulic fractures can overcome the constrain of the minor principal stress and propagate in the direction of the transverse groove.Increase in the angle between the minimum principal stress and transverse groove,and increase in the difference of the principal stress can promote the effort of transverse groove on the hydraulic fracture propagation.(5)Large-scale sandstone specimens with prefabricated grooving have high main stress difference and horizontal layering scheme.The cracks near the grooving groove extend along the direction of the grooving to form a very obvious main crack.The crack size and extension distance are large,and multiple cracks occur.The primary structural planes meet to form a more complex network of cracks,and the effect of grooving is obvious.There are a large number of micro-cracks under the low ground stress difference and vertical bedding scheme,there is no obvious main crack,and the direction is various,and the effect of grooving is not Too obvious.It shows that the direction of the main crack formed in the specimen space is always consistent with the direction of the maximum horizontal principal stress,Furthermore,regardless of the test scheme.Grooving can control the direction of crack propagation near the slot,and the direction of crack propagation far from the slot is perpendicular to the minimum horizontal principal stress.(6)The field test of hydraulic fracturing at the Yuwu Coal Mine and the integrated monitoring data show that with the treatment,the roof-to-floor convergence was reduced by 50%,and the wall-to-wall convergence was reduced by 30%.This result suggested an effective relief of the high abutment pressure and verified the correctness of the theoretical analysis and numerical simulation results.