Effects of fluid rotary drilling on hydrochemical sampling results from deep boreholes in fractured Columbia River Basalt

Abstract

A field test was conducted at the Hanford site in south-central Washington State to assess the effects of drilling and complex drilling fluids on hydrochemical sampling results. Changes in groundwater chemistry from a pre-test baseline were related to the various fluid components. Chemical species having a source in one or more of the fluid additives increased in concentration, while those not having a source in the fluid decreased in concentration as a result of dilution within the formation. Isotopic species, primarily δ 13 C and 14 C , were dramatically affected by the high carbon content of the drilling fluids. The use of complex drilling fluids similar to those used in this test severely limits the usefulness of these environmental isotopes. Attenuation of drilling fluid components in the formation appeared to occur by physical as well as chemical mechanisms. Mass balance calculations indicated that extensive borehole development was required before most fluid components were removed from the formation. Tracers added to, or which were components of, the drilling fluids, were used to evaluate the efficacy of well development. It was concluded that: (1) a 1000-fold decrease in tracer concentration (C/C 0 = 10 −3) or a development ratio (development volume divided by volume lost during drilling) of 100 is required for many trace constituents; (2) significantly lower development ratios may be adequate for major chemical constituents or in cases where the solute concentration is greater in the formation fluid than in the drilling fluid; (3) extensive development (a development ratio of greater than 100) may be required for certain isotopic species ( 14C, 36Cl, 129I, etc.). Unless drilling fluid losses are controlled to less than a few thousand liters in the latter case, or a non-contaminated source of drilling fluid is used, the required development may be impractical and/or cost prohibitive.