Stability analysis of a borehole wall during horizontal directional drilling

In this paper, numerical simulation strategies are proposed and numerical analyses are performed to investigate the stability of a borehole wall during horizontal directional drilling in loose sand with an emphasis on the role of the filter cake in borehole stability. Two computational scenarios, one in the absence of a filter cake and one with the presence of a filter cake in a borehole wall, are investigated by considering both deep and shallow borehole situations. In the case where no filter cake is formed, the soil–drilling fluid interaction analysis shows that the effective pressure on soil particles will quickly decrease to zero even at a low drilling fluid pressure because of the rapid drainage of the drilling fluids into the loose sands. This conforms to the classical liquefaction criterion, indicating that static (flow) liquefaction-based soil crumbling and sloughing will occur even at a very low drilling fluid pressure if an effective filter cake is not formed. Soil’s permeability effect on pore pressure and the transition to a steady flow are also studied. In the second scenario in which a filter cake is formed, the hydraulic fracture failures around the bores are investigated, which are caused by the expansion of the yielding zones. The yield zone sizes and critical drilling fluid pressures at the moment of hydraulic fracturing failure are calculated from the finite element analyses and the closed-form solution, which is based on classical plasticity theories. The critical fluid pressures from the finite element analyses and the closed-form solutions are very close, but there is a large discrepancy between the yield zone sizes.