Control Pressure and Stabilize the Borehole

Two types of pressure are exerted on the borehole during drilling, formation pressure and hydrostatic pressure. Formation pressure can collapse the borehole if it is not overcome by hydrostatic pressure pushing back against the formation. Hydrostatic pressure is the weight or density of the volume of drilling slurry pushing against the formation. In order to have hydrostatic pressure, the drilling slurry must push back against the formation with minimal penetration into the formation. In unconsolidated permeable formations, the hydrostatic pressure occurs when the weight of the fluid is in contact with the impermeable deposits (filter cake) placed on the sides of the borehole by the drilling slurry. The filter cake and the hydrostatic pressure thereby control the formation pressure, reduce slurry loss and prevent caving, resulting in hole stabilization.

Normal water weighs 8.34 lbs/gal, with the Shore Pac polymer fluid at 8.4 lbs/gal, it is necessary to maintain the slurry level 2 to 3 meters (7 to 10 feet) above the surrounding groundwater level. This 2 to 3 meters of hydrostatic pressure, or head, exerts the pressure necessary (approx. 450 to 640 lb/ft2, or 2000 to 3000 kg/m2), to support the walls of the excavation and is required to assure hole stability. Without this positive pressure exerted by the slurry column against the sidewall, soil overburden pressures will cause the excavation to collapse. The natural soil overburden pressures forced against the positive pressure exerted by the slurry column result in little or no leach ability to the surrounding formation.