Radially converging tracer test in a low-angle fracture zone at the Finnsjoen site, central Sweden. The fracture zone project - phase 3

摘要

Performance and results of a radially converging tracer test in a low-angle major fracture zone in crystalline rock are described. The extensive, about 100 m thick, zone 2 was encountered by means of borehole investigations at depths ranging from 100 to 250 metres at the Finnsjon site, central eastern Sweden. The zone studied (zone 2) consists of highly conductive, metre thick interconnected minor shear and fracture zones (sub-zones) with low conductive rock in between. The objective of the tracer test was primarily to determine flow and transport characteristics in a major fracture zone. Secondly new equipment, experimental design and methods of interpretation were developed, tested and improved. The converging flow field was created by pumping in a central borehole from a packed-off interval enclosing the whole thickness of zone 2. Tracer breakthrough was registered from all nine injection points, with first arrivals ranging from 24 to 3200 hours. Evaluated flow and transport parameters included; flow porosity, dispersivity, flow wetted surface, fracture aperture and hydraulic conductivity in fracture flow paths. Directional variations were found in the flow and transport parameters determined, which is concluded to be due to heterogeneity and/or anisotropy. This conditions is more pronounced at depth in zone 2. Results from the tracer test also clearly show that the upper boundary of zone 2 is highly conductive and consistent over hundreds of metres. Within zone 2, and between upper and lower margins, interconnected discrete minor shear and fracture zones (sub-zones) constitute flow paths of considerable variable residence times. The dispersion within the sub-zones of zone 2, expressed as Peclet numbers ranged from 16 to 40. Flow porosity was determined to be 0.001-0.05 in the upper sub-zone and 0.01-0.1 in the intermediate and lower ones and flow wetted surface area per volume of rock was calculated to be within 1-92 m(sup 2)/m(sup 3). 68 refs, 61 figs, 40 tabs. (Atomindex citation 25:037438)