Air rotary drilling requires that care be taken both to prevent cross contamination of
subsurface materials and to prevent contamination or chemical alteration of ground water or
subsurface materials.
Mud Rotary and Water Rotary
The mud rotary and water rotary drilling methods involve the introduction of drilling
fluids (various drilling muds or water) into the borehole through the drill pipe to maintain an
open hole, provide lubrication to the drill bit, and remove drill cuttings.
Water rotary drilling is a rapid and effective drilling method for most geologic
materials. However, the water used as a drilling fluid tends to react with the surrounding
formation and ground water. For this reason, the utility of water rotary drilling is limited. In
addition, there are other problems associated with water rotary drilling. The identification of
water bearing zones is hampered by the addition of water into the borehole. In clay rich
sediments, the water may form a slurry that can rapidly cause plugging of the formation,
resulting in a well that is difficult to develop. In poorly consolidated sediments, drillers may
have a problem with caving of the borehole prior to installation of the well screen and casing.
In highly fractured rock, it may be difficult to maintain effective water circulation because of
water losses to the subsurface. The drilling fluids used in rotary drilling can grossly
contaminate upper or lower uncontaminated zones if a contaminated zone is penetrated.
Driving casing as the borehole is advanced can help resolve this problem.
While there are hydrogeologic conditions where mud rotary drilling is the best option
(e.g., where it is extremely difficult to maintain a stable borehole), mud rotary creates a high
potential for affecting aquifer characteristics and ground water quality. If the mud rotary
method is used, the drilling mud(s) should not affect the chemistry of ground water samples
or samples from the borehole, or adversely impact the operation of the well. To minimize the
influence to the surrounding formation and ground water, drilling muds should be limited to
water based, locally occurring clays. The following describes the type of adverse affects that
can occur to the aquifer, ground water quality, and/or well performance as a result of using
certain drilling muds. A more comprehensive review of the properties, applications, and
impacts of drilling fluids is given in Aller et al. (1989):
Bentonite muds form a filter cake on the sides of the borehole, thus reducing
the effective porosity of formations in the borehole, and compromising the
design of the well. Bentonite may also affect local ground water pH.
Additives to modulate viscosity and density may also introduce contaminants to
the system or force large, unrecoverable quantities of mud into the formation.
Some organic polymers and compounds provide an environment for bacterial
growth, which reduces the reliability of sampling results.
November 1992
6 12






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