Contamination of deep formation waters by drilling fluids: correction of the chemical and isotopic composition and evaluation of errors

Contamination of deep formation waters by drilling fluids is a problem that concerns most types of drilling operations (petroleum wells, geothermal boreholes) and is crucial in the course of feasibility and safety studies of potential radioactive-waste repositories. Residual contamination of formation-water samples has an important impact on the accuracy of the characterisation of the natural hydrogeochemical background of the study sites. Based on a literature review and on experience acquired on the sites of the French Agency for Nuclear Waste Disposal (ANDRA), this article proposes a general method for the correction of residual contamination, including estimates of the associated errors. The quantification of the contamination is based on tracing techniques and on a geochemical survey during the pumping test preceding sampling. The correction and estimation of errors require repeated measurements of the tracer(s) and the chemical and isotopic species during pumping. The method is applied to a pumping test in a research well in deep granite of the Vienne district (France) where multi-tracing of drilling fluids has been used.