Estimation of the maximum allowable drilling mud pressure for a horizontal directional drilling borehole in fractured rock mass

The drilling mud loss risk is a major problem for horizontal directional drilling (HDD) projects when it was drilling in fractured rock stratum. The existing theoretical calculation methods for the maximum allowable drilling mud pressure in HDD borehole are particularly designed for soils, so they are not suitable when rocks are encountered. The maximum allowable drilling mud pressure was analyzed in this study for a fracture zone of a maxi-HDD project crossing Yangtze River. The geotechnical investigation disclosed multiple layers of soils and rocks: the upper part two sand layers and the lower part one sandstone layer. The fracture zone in the sandstone layer was investigated by digital panoramic borehole camera technique, so that the detailed fracture orientations and locations can be identified. The maximum drilling mud pressure for the upper part two sand layers was calculated using Delft equation. At the same time, the evolution of drilling mud pressure in the fracture zone was studied using numerical modeling method. Combing with the calculation results of both Delft equation and numerical modeling, the maximum allowable mud pressure at the fracture zone of the HDD borehole was obtained. The numerical modeling found that the persistent fracture system in the fracture rock mass has very low resistance to the drilling mud pressure propagation, so a low drilling mud pressure needs to be used when it was drilling/reaming in the fracture zone. The effects of fracture aperture, the drilling mud viscosity, and fracture roughness on the mud pressure evolution were also analyzed.