Estimation of maximum mud pressure in purely cohesive material during directional drilling

Abstract

Directional drilling requires the use of drilling mud to stabilize the borehole and return cuttings to the ground surface. High mud pressure can result in mud loss, ground heave, damage to buried infrastructure, and other serious problems as a result of either hydrofracture or blowout of the soil surrounding the borehole. First, past research on two ground failure mechanisms is reviewed. A new approach is introduced to estimate maximum mud pressure using a cavity expansion solution, where the non-unit lateral earth pressure coefficient at rest (K ₀) is explicitly considered. Finite-element analyses demonstrate that the new approach provides effective estimates of the extent of the plastic zone around the borehole under different geostatic stress conditions. Control of mud pressures to prevent the zone of shear failure extending more than halfway to the ground surface produces a reserve capacity of 20–30% against blowout. An examination of hydrofracture versus blowout indicates that blowout is the most likely mode of mud loss in normally and lightly overconsolidated clays. Hydrofracture is the expected cause of mud loss in heavily overconsolidated clays when the coefficient of lateral earth pressure exceeds 1.8.

Keywords:

• Horizontal directional drilling
• Hydraulic fracturing
• Blowout
• Maximum allowable mud pressure

Acknowledgements

The work reported here has been supported by the Natural Sciences and Engineering Research Council through the award of a Strategic Research Grant on pulled in place pipe installation.