Mechanical Model for Determining the Crossover Point of Slurry Flow inPull-back of Horizontal Directional Drilling Installations

摘要

At present, various predicting models are developed to predict pull-back loads forrnHDD projects and results at the end-point of pull-back derived from them fit quiternwell with the results obtained in field tests. However, the maximum loads thatrnusually occur in the process of pull-back can not be predicted by any of them becausernof slurry drag effects not considered adequately. There is a crossover point in thernprocess of pull-back where slurry reverses its flow direction while the maximumrnloads usually occur, since drag effects take a large proportion in total pulling forcernfor small and medium scale HDD installations. The paper presents a model torncalculate pressure drop of slurry in concentric annulus assuming laminar flow ofrnpower-law fluid. Crossover point is located by comparing pressure drops of twornkinds of annulus (pipe in borehole and drill string in borehole). Pressure drops arerncalculated based on equation prescribed by the Society of Petroleum Engineers (SPE)rnas well. The result calculated from the model agrees with HDD field test data morernaccurately compared with the SPE equation. It is observed that the rheologyrnparameters under low shear rates have a significant impact on pressure droprncalculation.