The Breakdown Pressure Calculating Model for Open Hole CompletionCBM Well Hydraulic Fracturing

摘要

The open hole completion CBM borehole wall intersects with the weak surface such as cleats and fractures. Inthe process of hydraulic fracturing, the fractures may origin from coal body or cleats, which makes the rupture mechanismand rupture models of the borehole wall being different from the conventional reservoirs. The previous model forcalculating breakdown pressure of open hole completion borehole wall considering tension failure has poor applicabilityfor calculating breakdown pressure. Considering the spatial relationship of the intersection of borehole wall and cleats,analyzing the stress state of borehole wall rock and cleats wall, and basing on elastic mechanics and fracture mechanics,the breakdown pressure calculation model for CBM open hole completion hydraulic fracturing was established. In themodel, fractures initiate from coal rock body, tensile failure along with face cleats, shear failure along with face cleats,tensile failure along with butt cleats and shear failure along with butt cleats five kinds of damage modes were considered.According to example calculation, the breakdown pressure of HX-L1 well calculated using the model is 14.81 MPa. Theactual pressure obtained by bottom hole pressure gage is 15.42 MPa, and the relative error is 3.96%. The calculated resultagrees with the actual conditions. It can be concluded that the model can be used to calculate the breakdown pressure foropen hole completion CBM well hydraulic fracture.