Hydro-Mechanical Behaviour of a Pressurised Single Fracture: An In-situ Experiment

Doctoral thesis, 1999

Exploration of ground water resources, grouting, disposal of nuclear waste and extraction of geothermal energy are examples of activities which require consideration of the relationship between effective stress and transmissivity. However, this relationship is poorly understood and very few investigations have been conducted in field scale. The objective of the study presented in this thesis was to achieve a better understanding of the hydro-mechanical in-situ behaviour of a single fracture during conditions of increased fluid pressure. The study includes a literature survey, predictions of deformation behaviour based on roughness measurements on rock cores and data from the literature, a field scale experiment and analysis of the flow pattern in the investigated fracture using the simulated annealing technique. The test site is situated at the Röda Sten Rock Laboratory (RSRL), in Göteborg Sweden, 70 metres below ground surface. At the laboratory seven sub-vertical boreholes were drilled within an area of 16 m2. A subhorizontal fracture that within its extension incorporates the boreholes was selected for the tests. The relationship between effective stress and transmissivity was investigated at five pressure steps where the entire fracture was pressurised. To avoid jacking the pressure never exceeded the normal stress. In order to clarify the transmissivity distribution within the fracture; the hydraulic communication between the boreholes was studied and hydraulic tests were performed in all seven boreholes. The transmissivity variability within the fracture under natural conditions was significant and the difference between the highest and the lowest transmissivity was a factor of 10. The overall transmissivity of the fracture was increased by a factor of 20 when the effective stress decreased by 0.8 MPa. This was in accordance with the predictions. Fracture stiffness back calculated from hydraulic field tests is in general lower than fracture stiffness evaluated from laboratory tests. The reason for this is discussed based on a conceptual model of fracture deformation. The stiffness of the studied fracture is in the lower range of the values found in the literature. The importance of differentiation between mechanically and hydraulically evaluated stiffness when used as input in prediction work is emphasised.