Development of Tensile Hoop Stress during Horizontal Directional Drilling through Sand

Although horizontal directional drilling has become commonplace, there are problems associated with high drilling mud pressures causing hydraulic fracture leakage of mud out into the environment. Initiation of tensile fracture is examined here, using finite- element analysis to represent the sand material and the annulus of filtercake that forms around the borehole. The analyses examine the soil response as mud pressures are increased, including shear failure in the sand material and the cohesive filtercake layer. The study identifies the initial geostatic stress conditions and the drilling fluid pressures that initiate tensile stresses in the filtercake. The effects of filtercake thickness, borehole depth, and the location of the maximum tensile stresses are studied. Significant discrepancies are found relative to limits currently used in the industry. Tensile fracture may be responsible for some mud loss, but simple use of drilling mud pressures that prevent tensile circumferential stress may be overly conservative.