SMALL-SCALE HYDRAULIC FRACTURE EXPERIMENTS IN A FRICTIONAL-COHESIVE MATERIAL

摘要

Directional drilling uses pressurized mud when drilling through fluvial and glaciofluvial materials to maintain borehole stability and remove cuttings. Failure of the soil surrounding the borehole is controlled by the shear strength of this frictional ground and this influences the maximum allowable mud pressure that may be applied to the borehole. While there have been a number of theoretical studies of mud loss mechanisms, there have been few if any experimental investigations to examine the efficacy of the proposed design equations.A horizontal directionally drilled borehole is examined in the laboratory using experiments in a uniformly graded sand and a layered sand and gravel system. Measurements of mud pressure and surface displacements associated with blowout are compared to analytical calculations. During the experiments, the downhole mud pressures to determine the borehole pressure that leads to mud loss and surface displacements were measured. Physical information obtained during the experiments was used to assess the effectiveness of various analytical models to estimate the mud loss pressures and displacements. Use of the existing analytical solutions was found to be questionable (unconservative) during pilot borehole drilling (the configuration studied in these experiments).