Drilling fluid technology in ice sheets: Hydrostatic pressure and borehole closure considerations

The drilling of deep boreholes in ice sheets requires that the hole is filled by a liquid to compensate the ice-overburden pressure. Density is the most important drilling fluid property, so the hydrostatic pressure of the fluid column should be sufficient to prevent closure of the borehole. However, it is impossible to prevent borehole closure at all, because the upper part of glaciers contains a lot of air inclusions and the density of snow–firn zone is much less than the rest stratum, and the expansion coefficient and the compressibility of fluids and ice are rather different. That is why at all accounts there are parts in the borehole, where the ice pressure is higher than the pressure of fluid. One of the main ideas of ice deep drilling technology is the limitation of the closure rate by controlling the density of the drilling fluid. First, it is proposed to choose the depth of the pressure equilibrium and the average fluid density at average temperature in the borehole. Then, the hydrostatic pressure and pressure difference at the borehole walls can be estimated. Finally, the diameter changes in the interval with negative pressure difference are predicted and compared with maximal safety values. This method allows to choose the correct fluid density and the fluid level below surface for the concrete drilling site and to ensure accident-free drilling in ice sheets.