Step by Step Evaluation of Hydrofracture Risks for Horizontal Directional Drilling Projects

Hydrofracture associated with HDD projects remains a serious concern for regulatory and permitting agencies, and the engineers, owners, and contractors that must obtain permits and regulatory approval for their projects. The cavity expansion model is the appropriate model for evaluating hydrofracture risks, specifically maximum allowable drilling fluid pressures, for the majority of HDD projects in soils. The tensile strength model, not covered in this paper, is more appropriate for HDD projects in rock, especially those with relatively shallow cover. Estimation of minimum required drilling fluid pressure to ensure cuttings are transported from the bore is also required. The solution for minimum required pressure derives from well-established fluid mechanics principles. This paper describes the cavity expansion model, presents step by step instructions for its application, and illustrates the process using example calculations from actual HDD case histories. This paper is intended to help engineers become comfortable with the approach, the results predicted, and their significance. Continued research is also encouraged to measure actual pressures on projects and compare against predicted minimum required and maximum allowable pressures. More accurate evaluation of hydrofracture risks, though, is only one step in reducing actual hydrofracture risks. Coupled with improved evaluation, improvements in practice regarding management of drilling fluid properties and drilling methods are needed. Contingency measures such as conductor casing, relief wells, and piezometers should also be considered for reducing risks on difficult, challenging bores.