An analytical model of the initiation pressure for multilayer tree-type hydraulic fracturing in gas-bearing coal seams

Hydraulic fracturing is able to effectively enhance the permeability of coal seams and thereby achieve the appreciable methane drainage effect. Tree-type fracturing technology is an advance method that is expected to solve the problems of high initiation pressures and fracturing blank areas of conventional fracturing technology. The existing fracture initiation pressure models cannot accurately predict the initiation pressure when multiple tree-type branch boreholes (TTBBs) interact. An analytical model of calculating the initiation pressure during hydraulic fracturing under the condition of multiple TTBB layers was established in this study. The model focuses on analyzing the mutual influence of TTBBs in different layers and considers the influence of coal seam bedding planes, joints and coalbed methane pressure on the initiation pressure. The circumferential stress distribution around TTBB T-2 under different TTBB arrangement parameters was analyzed. It was found that under the influence of TTBBs in adjacent layers, the circumferential stress of T-2 first increased and then decreased with the distance to the fracturing borehole, and finally approached the stress value of a single TTBB. Theoretical initiation pressures under different TTBB arrangement parameters and coal seam geological parameters were calculated. The theoretical initiation pressure decreased with increasing number of TTBBs and TTBB layers because of the induced stress caused by the water pressure in the TTBBs. Additionally, the tree-type fracturing initiation patterns under most parameters entailed tensile failure along coal bedding surfaces. These results could help advance the development of hydraulic fracturing and provide dependable assistance for tree-type fracturing site applications.

Automated summary

An analytical model of calculating the initiation pressure during hydraulic fracturing with multiple tree-type branch boreholes (TTBBs) was established in this study, considering the mutual influence of TTBBs in different layers. The circumferential stress of TTBB T-2 was found to first increase and then decrease with the distance to the fracturing borehole under the influence of adjacent TTBBs, eventually approaching the stress value of a single TTBB. The theoretical initiation pressure decreased with increasing number of TTBBs and TTBB layers.