Drilling Fluid Observations and Recommendations for U.S. Polar Program, WAISCORES Drilling Project

Boring ice to depths in excess of about 300 meters requires a fluid with a density closely matched to that of ice to prevent lithostatic pressure from causing plastic collapse of the borehole; the latter frequently results in loss of the drilling equipment. The fluid, or mixture of fluids, must simultaneously satisfy criteria for density, low viscosity, frost resistance, as well as workplace safety and environmental compliance over both the short term (e.g., fire hazard and acute toxicity) and long term (chronic toxicity, local and global environmental degradation). The fluid must also satisfy other criteria, for example those stemming from the analytical methods employed on the ice core.

A number of different fluids and fluid combinations have been tried in the past. Since GISP2 (1990-1993) the US Polar Program has utilized a single-component fluid system, n-butyl acetate, but the toxicology, flammability, aggressive solvent nature, and longterm liabilities of n-butyl acetate raises serious questions about its continued application.

The European community, including the Russian program, has concentrated on the use of two-component drilling fluid consisting of low-density hydrocarbon base boosted to the density of ice by addition of halogenated-hydrocarbon (s.l.) densifier. Many of the proven densifier products are now considered too toxic, or are no longer available due to efforts to enforce the Montreal Protocol on ozone-depleting substances.

A number of compounds suggested as replacements for ozone-depleting substances such as HCFC's were investigated. Most of these are unsuitable to ice drilling applications and can be dismissed out-of-hand due to toxicity, flammability, unsuitable density, and so forth.

Alternatives categorized as hydrofluorocarbons (HFC's) and hydrofluoroethers (HFE's), may prove to exceed the engineering performance of the now-obsolete densifiers: simultaneously providing high density, low viscosity, materials compatibility, very low toxicity, high safety, convenience in handling, and low environmental liability. Detailed engineering-related testing tailored to our application would begin by procuring large samples of each compound, as soon as practical. Following a preliminary set of engineering tests to assure ice and drill-materials compatibility, samples of fluid would be supplied to the science community for compliance testing in their analytical streams. Because both HFC's and HFE's evaporate cleanly, and drawing upon experience with the similar HCFC compounds they replace, interference with scientific analyses is not anticipated with either HFC or HFE densifiers.