Experimental investigation examining influence of pump rates on horizontal borehole stability in sand

Abstract

Pump rate is one of the most important parameters during the drilling and reaming processes associated with Horizontal Directional Drilling. Pump rate influences the mud velocities at various locations within the borehole, and must be sufficient to transport soil debris to the ground surface. However, if mud pressure is too high or low, stability of the horizontal borehole will be in danger. Furthermore, after mud pressure reaches the maximum value the borehole can support, mud may flow to the surface. None of the tests reported in the literature recorded the influence of pump rate on maximum mud pressure (i.e. stability of the horizontal borehole) or the ground movements. Meanwhile, the stress state close to the borehole influenced by mud pressures has never been measured. Therefore, this paper investigates the effect of three different pump rates applied in five mud loss experiments. Mud pressure histories in these experiments are presented to explore any relationship between the maximum mud pressures or ground movements measured during the experiments and the pump rates used during the test. It was found that the maximum mud pressure is independent of the pump rates, but at the highest pump rates, the ground surface movements were much more dramatic, producing larger surface uplift than that seen at lower pump rates. Earth pressure sensors (null gauges) were buried near the borehole to measure axial, radial and hoop stresses during these experiments and these measurements are compared with closed form theoretical solutions and results from numerical analysis. Shear failure still dominated the failure mechanisms for the sand in all these experiments and mud travelled up to the ground surface through a shear plane that developed from the borehole.