Mechanism Research On The Propagation Into Barrier Layer Of Hydraulic Fractures For The Horizontal Well Staged Fracturing Technology In Coal Reservoirs

Coal reservoir always contains complex structure and has strong anisotropy,thus the hydraulic fracture can be easily disturbed during its propagation.Because of the different target formation chosen in the development,the requirement for the fracture morphology has differences during the hydraulic fracturing.In the normal hydraulic fracturing,hydraulic fracture was usually controlled in the target layer to acquire short and wide type fracture with high conductivity.However,during the horizontal well staged fracturing in the virtual production zones,the hydraulic fractures are required to have the ability of penetrating into the barrier layer to develop the seepage channel between the horizontal wellbore and the pay zones.Currently,the researches about the initiation and propagation of the hydraulic fracture in the coal reservoir is incomplete,which has become the bottleneck restricting the developing of the horizontal well staged fracturing technology in coal bed and needs further researching.In this paper,the ture triaxial hydraulic fracturing simulation experiment is conducted using the layered rock sample made by nature rock outcrops and concrete respectively.In this simulation experiment,the influence factor,including stress state,nature fractures,the properties of the rock layer interface and the inclined angle of the rock layer interface,was analyzed.The result shows that the stress state is the main factor controlling the propagation of the hydraulic fracture,uniform and compact rock layer interface is conductive to the form and propagation of the hydraulic fracture with the ability of penetrating into the barrier layer,the angle of the rock layer interface has little influence on the form and propagation of the hydraulic fracture with the ability of penetrating into the barrier layer.According to the geological condition of Qinshui Basin in Shanxi Province,the situation of stress distribution around the horizontal wellbore,which is mainly caused by in-situ stress,wellbore pressure and filtration of fracture fluid,was analyzed to establish the calculation models of initiation pressure for three initiation patterns and matching judging criteria.Meanwhile,the influence on the hydraulic fracture caused by the orientation of horizontal wellbore and stress state is analyzed.Based on the simulation experiment,considering the maximum normal stress criterion,the theory judging criterion for the three situation,including the hydraulic fracture is controlled in the layer,hydraulic fracture propagate along the rock layer interface and hydraulic fracture penetrate into the barrier layer.