Experimental Study Of Tree-type Hydraulic Fracturing Crack Initiation And Extension With Different Tree-type Branched Borehole

At present,the current hydraulic fracturing technology have many shortcomings.Such as the high initiation pressure is required,they can cause severe damage to roof and floor,fracturing imbalance may lead to a “blank area”,and so on.In order to solve the above problems,our team proposed a method of tree-type hydraulic fracturing.The method forms a tree-branched borehole in the coal seam through the cross-measure fracturing borehole by the self-propelled water jet device first.And if the cross-measure fracturing borehole is regarded as the tree trunk,the branched boreholes can be regarded as tree branches,and the cracks as twigs – hence the final result of this method should resemble a tree in three dimensions.The initial application shows that the method can greatly increase the extraction range and enhance the extraction effect.Based on the similarity simulation theory of rock mechanics,this paper investigates the initiation pressure,initiation location and the direction of crack extension with different number of branched boreholes,different length of branched boreholes and different angles between the four-branch drilling model and the principal stress by using a true triaxial hydraulic fracturing simulation system and RFPA2D-Flow software.The main results are as follows:(1)The law of initiation pressure in tree-type hydraulic fracturing with different number and length of branched boreholes is studied.With an increase in the number and length of branches in the borehole,there is a measurable decrease in the initiation pressure.In this simulation,the model with four branches reduced the initiation pressure by 69%,and the 20 m long branches model reduces the initiation pressure by 40% compared with the 5m long branches model.(2)The law of initiation location and extension in tree-type hydraulic fracturing with different tree-type branched boreholes is studied.The initiation locations of the model with zero branches are the bottom of the fracturing hole,the initiation locations of the model with four branches are the end of branched boreholes.With increasing the number of the boreholes,the initiation locations increase.With increasing the length of the boreholes,the initiation locations disperse.Within a certain range,the adjacent branched borehole will lead cracks extend to it and form the crack network in the plane of branched boreholes.With increasing the number of the branched boreholes,the more balanced the distribution of the resulting fracture network,the better the fracturing effect becomes.(3)The law of complex degree of fracture influenced by the angle between the branched borehole and the principal stress in tree-type hydraulic fracturing is studied.In the study,when there are multiple branched boreholes,will produce different angle between each branched borehole and the principal stress angle are not equal.If the difference of the angle is smaller,higher the complexity of the crack seam,more balanced of the crack seam.