Analysis of Field Downhole Annular Pressure Data in HDD Operations

Current hydraulic fracture models to estimate the maximum allowable annular drilling fluid pressure (formation limit pressure) before hydrofracture or blowout will occur during horizontal directional drilling (HDD) operations are largely based on cavity expansion theory. The most common of the models employed in the HDD industry is the Delft equation, which was developed by Luger and Hergarden. However, since then, numerous methods have been proposed which produce a wide range of potential maximum annular pressures that should be applied in design. This paper explores some of the more common formation limit pressure models and identifies which model appears to better predict the actual maximum formation limit pressure based on downhole annular pressure data collected in HDD pilot hole operations in cohesive soils where we observed inadvertent returns (IR) of drilling fluid. Based on the analysis and data comparison presented in this paper, it was concluded that the cavity expansion theory for undrained soils and the Queen’s model provide realistic predictions of formation limit pressures in clays.