Geochemical Logging of Drilling Fluids in the Fractured Basement

Abstract

Geochemical logging technique are not frequently used during geothermal drilling operations. Several experiments were conducted on two boreholes drilled in the fractured basement of the Cezallier are (Massif Central, France). In 1985, the Deep Geology Program (GPF) started drilling a 500 m deep borehole with percussion drilling and a 1400 m deep corehole. The main goal of this geochemical logging experiment was to forecast and detect the water producing zones penetrated by the drill, and to set up a methodology suitable for various types of drilling and environments. This technique consists of sampling the drilling fluid at regular depth intervals, filtering the samples and analyzing for selected chemical parameters in a field laboratory. Four chemical tracers of the deep geothermal fluid were analyzed, Li, C1, SiO2, and HCO3, and a geochemical log was established by plotting their variations with depth.