Reorientation Model For Hydraulic Fracture After Directional Initiation In Rock

Fully mechanized top coal caving with hard thick coal seam,the control of hard roof,gas extraction in coal seam with low permeability and coal and gas outburst are issues threating the safety mining at present.Directional hydraulic fracturing is an effective way to solve the above problems.In directional hydraulic fracturing,the propagation of hydraulic fracture is an important issue to be investigated.In this thesis,the reorientation law of hydraulic fracture after initiating from the prefabricated slot is studied by laboratory experiments,numerical simulation and theoretical analysis.Laboratory experiments are performed by large-size true triaxial hydraulic fracturing experimental system and numerical simulation with RFPA3D-Flow software are carried out to study the reorientation law of hydraulic fractures.The results show that in directional hydraulic fracturing,hydraulic fractures usually initiate along the edge of the prefabricated slot.However,the angle between the initiated hydraulic fracture and the prefabricated slot is not 0°.During the propagation of hydraulic fracture,the propagation direction will gradually turn towards the direction parallel to the maximum principal stress.When the propagation direction of hydraulic fractures have been parallel to the maximum principal stress,it will not change anymore.The reorientation speed and distance has great impact on the effective propagation range of hydraulic fracture and the transformation of coal and rock mass.Based on the laboratory experiments and numerical simulation,macro and micro conceptual models for the reorientation of hydraulic fracture tip are proposed.Then,a mechanical model for hydraulic fracture reorientation is established on the basis of Maximum Tensile-Stress Criterion.The reorientation process of hydraulic fracture can be regarded as follows: under the effect of local stress field,tensile stress will occur around the hydraulic fracture tip.The magnitude of tensile stress changes with the direction around the hydraulic fracture tip.The tensile stress will reaches the maximum at a certain direction around the hydraulic fracture tip,which is the reorientation direction for hydraulic fracture.The mechanical model shows that the reorientation of hydraulic fracture is affected by the fracture water pressure,the differences of principal stresses and the initial azimuth angle of the hydraulic fracture.Physical experiments and numerical simulation are carried out to study the influence of the length of the prefabricated slot,the differences of principal stresses,the water injection rate and the number of prefabricated slot on hydraulic fracture reorientation.In directional hydraulic fracturing with one prefabricated slot,the differences of principal stresses has the greatest impact on the hydraulic fracture reorientation among the three factors,while the influence of the water injection rate on hydraulic fracture reorientation is the least important.If two prefabricated slot exist,the reoriented hydraulic fractures initiated from different prefabricated slots are not connected.The reorientation processes of hydraulic fractures initiating along different prefabricated slot are relative independent.In field application of directional hydraulic fracturing,characteristics of the engineering problem must be taken into consideration.The location,dip angle and spacing of borehole,the length and number of the prefabricated slot and other technological parameters should be determined reasonably to achieve the best effect.