Analysis of a pilot crosshole fluid logging test at the BK-site, Grimsel Test Site

A pilot crosshole fluid logging test was performed in the BK-area of the Grimsel Test Site (GTS). The fluid logging test ran from January 29, 1990 until February 8, 1990. The pilot crosshole fluid logging test consisted of logging various boreholes in the BK-area with temperature-conductivity logs while a constant injection of tracer with constant concentration occurred somewhere in the well array. As each borehole was opened, flushed, and logged, connected fractures to the injection zone would produce traced fluid. From this experiment additional knowledge was hoped to be gained concerning the inter-borehole connections within the BK-area. In addition, the test technique is experimental and for this reason interest also centered around implementation and performance of the crosshole fluid logging experiment. The pilot crosshole fluid logging study was a success based upon the objectives of the experiment. The study (1) identified fractures with some form of connection between the injection borehole and the logged observation boreholes, (2) identified fractures at the observation wells which were apparently not connected to the injection well, and (3) because of the success in 1 and 2 validated the crosshole fluid logging technique for fracture system characterization. With the testing method described in this report it was possible to determine a great amount of information on fracture locations in a very short time span (in this case 9 days of testing). In total, 53 individual inflowing fractures were clearly defined and isolated to a specific borehole depth. Of these, 34 were connected to the injection borehole as indicated by tracer inflow and 19 were apparently not connected and did not receive tracer breakthrough. (author) figs. tabs