Friction loss calculation and wellhead pressure prediction in CBM radial drilling

The paper analyzed the pressure loss of drillpipe system for CBM radial drilling and high pressure water jet breaking mechanism, and listed the formula of various parts of resistance coefficient and nozzle jet power in drill pipe system, presented the relationship between hydraulic impact broken pressure and physical and mechanical parameters, and the relationship between hydraulic erosion velocity and dimensionless impact pressure by theoretical analysis and laboratory tests. The results show that the critical breaking pressure calculated by the weighted average method agrees with the measured values. At the 3~8 times critical breaking pressure, erosion rate is about 0.4 times as much as dimensionless impact pressure. According the requirements of construction, by pressure prediction step, the wellhead pressure of field operation was predicted, the predicted pressure was 3~5 MPa lower than construction pressure.