Hydraulic fracture experiments in a frictional material and approximations for maximum allowable mud pressure
Elwood, David E. Y.
Directional drilling , Hydrofracture
Directional drilling has become a popular construction method used by municipalities, contractors and engineers alike for the construction of new subsurface pipelines while minimizing impact on the surrounding community. Pressurized drilling fluid is used during the advancement of the borehole through granular materials to maintain borehole stability throughout the drill path. It is believed that 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, there have been few if any experimental investigations to examine the efficacy of the proposed design equations. The basis of this research considers a horizontal directionally drilled borehole and compares the analytical findings with those obtained from a series of smaller and larger-scale laboratory experiments for a uniformly graded sand and layered sand - sand and gravel case. The analytical solutions are considered reasonable for hydrofracture during pullback, but may not be directly applicable to blowout during pilot borehole drilling such as the model researched in the experiments. During the experiments the downhole mud pressures were continuously monitored and observations of composite drilling fluid and sand material were made. Smaller-scale experiments were carried out to determine the response of the horizontal stresses resulting from internal pressurization acting on the sidewalls of the test cell. During the larger-scale experiments, the surface displacements were measured to better understand the influence of an increase in the soil volume with surface displacement. iii Through the course of the research, physical information has been collected regarding the changes that a clean sand undergoes when in contact with a drilling fluid, the ability of a borehole to resist internal loading, and the effectiveness of the various analytical models currently used to estimate the peak allowable internal fluid pressures. In addition, physical data has been collected regarding the displacement of a surrounding material during the introduction of drilling fluid into a horizontally drilled borehole