Chapter 17
Stress-dependent Flow in Fractured Rocks at Sellafield, United Kingdom

S. F. Rogers¹
C. J. Evans
British Geological Survey
Keyworth, Nottingham, U.K.

¹Currently with Golder Associates, Edwalton, Nottingham, U.K.

ABSTRACT

The relationship between flow of fluid through a fractured rock mass and the prevailing stress field is poorly understood. Recent work has suggested that the fractures most likely to act as fluid pathways are those that are oriented in the rock mass such that the ratio of shear stress to normal stress is high. These critically stressed fractures have the potential to dilate as a result of localized failure, resulting in the enhancement of permeability. Detailed data about fractures, stress fields, and flows were available from a borehole near Sellafield, west Cumbria, drilled as part of the United Kingdom's radioactive-waste disposal program. In Nirex borehole RCF3, 100 short-interval tests were carried out continuously over lengths of approximately 1.5 m. These tests provide a record of a 160-m section of borehole with almost unprecedented control over the relationship between permeability and fracture orientation. The data allow testing of the hypothesis that the occurrence of flow is dependent on the state of stress. Shear and normal stresses were calculated for all discontinuities identified from borehole-image logs in the hydraulic-test zone. When these stress data are compared with transmissivity of each test section, this deduction seems to be justified: In the set of most-transmissive zones, induced stresses on fractures show a very high shear/normal stress ratio. Orientations of discontinuities in these flow zones are consistent with those predicted by theory.