AAPG Methods in Exploration No. 13, Chapter 17:
Stress-dependent Flow in Fractured Rocks at Sellafield, United Kingdom, by S. F. Rogers
and C. J. Evans, Pages 241 - 250
from:
AAPG Methods in Exploration No. 13: Geological Applications of Well Logs, Edited
by M. Lovell and N. Parkinson
Copyright © 2002 by The American Association of Petroleum Geologists. All rights
reserved.
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.