Hydrologic testing during drilling: application of the flowing fluid electrical conductivity (FFEC) logging method to drilling of a deep borehole

Drilling of a deep borehole does not normally allow for hydrologic testing during the drilling period. It is only done when drilling experiences a large loss (or high return) of drilling fluid due to penetration of a large-transmissivity zone. The paper proposes the possibility of conducting flowing fluid electrical conductivity (FFEC) logging during the drilling period, with negligible impact on the drilling schedule, yet providing important information on depth locations of both high- and low-transmissivity zones and their hydraulic properties. The information can be used to guide downhole fluid sampling and post-drilling detailed testing of the borehole. The method has been applied to the drilling of a 2,500-m borehole at Åre, central Sweden, firstly when the drilling reached 1,600 m, and then when the drilling reached the target depth of 2,500 m. Results unveil eight hydraulically active zones from 300 m down to borehole bottom, with depths determined to within the order of a meter. Further, the first set of data allows the estimation of hydraulic transmissivity values of the six hydraulically conductive zones found from 300 to 1,600 m, which are very low and range over one order of magnitude.