Well designed and properly controlled mud. Method requirements and subsurface characteristics can dictate the need to use a drilling fluid, and some drilling methods require specific types of fluid.

Method requirements and subsurface characteristics can dictate the need to use a drilling fluid, and some drilling methods require specific types of fluid. A drilling fluid is defined as a “water- or air-based fluid used in the water well drilling operation to remove cuttings from the hole, to clean and cool the bit, to reduce friction between the drill string and the sides of the hole, and to seal the borehole.” The hydrostatic head of the drilling fluid maintains pressure on the borehole wall and prevents its collapse. Water-based drilling muds build a filter cake or rind on the borehole wall. This exerts a positive hydrostatic pressure against the borehole wall, preventing inflow of ground water into the borehole. It also helps maintain borehole stability, which helps to prevent invasion of the fluid into the borehole wall and reduce cross contamination between aquifers.

During drilling, a mud engineer should be present to monitor drilling-fluid viscosity and circulation pressure and to implement the procedures needed to minimize invasion of drilling fluid into the borehole wall and avoid causing fractures in the geologic material. With proper control, drilling mud should only penetrate the borehole to 0.25 inches, making subsequent well development relatively easy.

Common drilling fluids include potable water; water with additives either of clay (usually bentonites or “mud”), clay mixtures (amended bentonite), bentonite or polymers (chemical foams); and compressed air. Some bentonite drilling additives contain petroleum or other organic compounds, and some drillers add diesel fuel to bentonite. Additives modify fluid characteristics (such as density or viscosity) in order to address a change in borehole or geologic conditions during drilling. Drilling additives in contact with potable water aquifers must meet the requirements of NSF International Standard 60-1988. Compressed air introduced into the drill stem also can be used to enhance circulation of the drilling fluid and can help move cuttings to the surface, but must be filtered adequately to avoid introducing significant quantities of oil.

Residues from drilling fluids can alter sample chemistry. Potential effects on water chemistry from “pure” (unamended) bentonite result primarily in changes in the major ion chemistry of ground water from the well. If well development has insufficiently removed residues of bentonite-based drilling fluids, the exchange of cations in the clay matrix with possible organic and inorganic constituents occurring in ground water could result in data showing lower or higher constituent concentrations. Other effects of drilling-fluid contamination on ground water can be indicated by elevated concentrations of sulfate, chloride, phosphate and organic carbon; metals; absorbed organic compounds; an altered cation exchange capacity, pH and chemical oxidation demand. Polymeric drilling fluids contain organic compounds that enhance biologic degradation of the drilling fluid, but this biologic activity can cause long-term variations in the chemistry of ground water samples that could be difficult to reverse.

The drilling fluid must be removed after drilling. A poorly designed and improperly controlled drilling fluid process results in invasion of mud to geologic materials that can cause damage to the borehole and to cores, as well as affecting the chemical composition of ground water samples collected from the well. If a proper mud-control program is not implemented, fluid removal can involve an intensive and repetitive effort during well development.

This article is provided through the courtesy of the U.S. Geological Survey. It is excerpted from Report 96-4233, “Selection and Installation of Wells, and Supporting Documentation,” written by Wayne Lapham, Franceska Wilde and Michael Koterba.