Influence of Overlying Layers of Loose and Dense Sand or Gravel on the Drilling Mud Pressure Causing Mud Loss from Horizontal Boreholes

Stability of horizontal boreholes under the pressure of drilling mud is a key consideration during horizontal directional drilling. Based on the successful application of experimental and numerical studies in uniform sand undertaken by the authors, an experimental study was carried out to investigate the relationship between the maximum measured mud pressure Pmax and influence of sand layers (i.e., a dense sand layer covered by different thicknesses of loose sand or dense sandy gravel). Mud pressure histories from experiments involving boreholes in stratified deposits were explored for the first time. A numerical study was also conducted, and the numerical results were close to the Pmax values observed in the tests. Earth pressure sensors were buried near the boreholes and the measured data were also compared to the results of numerical solutions. For the system involving an overlying layer of dense sandy gravel, it was conservative to use the Pmax calculation for uniform granular layers. However, for the system involving an overlying layer of loose sand, a reduction factor needed to be added; this was developed from a parametric study. Ground surface failure was less dramatic for these layered systems than seen previously for uniform sand.