Mud infiltration and travel paths during mud loss experiments on horizontal borehole stability

Abstract

Excessive mud pressure may endanger stability of horizontal boreholes and the success of drilling and reaming processes during Horizontal Directional Drilling (HDD). However, no laboratory studies have been undertaken to investigate the relationship between horizontal borehole instability (resulting in mud loss rather than borehole collapse) and mud infiltration and travel paths from the borehole to the ground surface. In this paper, exhumations after a series of mud loss experiments in a medium-scale test pit are used to explore mud infiltration into the sand surrounding the borehole, and the paths used to travel from the borehole to the ground. Different factors are investigated including burial depths, pump rates and the effect of multi-layered soil systems (dense sand covered by different thicknesses of loose sand or dense sandy gravel). Details of the mud travel paths from the experiments are presented. Water contents and average radial extent of mud infiltration around the boreholes are summarized. It is concluded that shear failure of the sand has a dominant role in defining the mud transport through coarse-grained soil. A primary crack appearing on the ground surface approximately above and parallel to the borehole, and secondary cracks transverse to the primary cracks then developed, linking the volumetric expansion of the sand due to mud intrusion and uplift at the ground surface. Ground movements are more dramatic in experiments involving high pump rate but the movements were reduced in experiments involving multi-layered soil systems.