The nature of stress state: numerical and laboratory experiments and some field observations

Abstract

Simulations of the stress state in a region of discontinuously fractured rock revealed a strongly heterogeneous, self-organized stress distribution displaying internally balanced strains and corresponding energy storage. The explicit simulations follow a sequence which may be inferred to be irreversible and strongly dependent on interaction between fractures, implying a time-dependency. The state of the simulated reservoir displays a sensitivity characteristic of marginal stability. These numerical observations are compared with new and previous laboratory and field experiments spanning a wide range of size, stress magnitude and stress difference. The characteristics of the state of the simulated reservoir, laboratory samples and field behaviour are found to be consistent with the numerical predictions. A stress memory is evident. This supports previous conclusions that the concept of stress state as uniform, constant and resulting from averaged values of stresses or displacements, presently applied at remote and artificial boundaries, is grossly oversimplified and flawed.

Towards an improved understanding of the mechanical properties and rheology of the lithosphere: an introductory article to ‘Rock Deformation from Field, Experiments and Theory: A Volume in Honour of Ernie Rutter’

“…They develop the theoretical background to the work, and compare results with stress magnitudes obtained using independent methods in a North Sea reservoir. With a view on the general state of stress in the crust, Harper & Hagan (2015) present a reappraisal of how we should think about stresses that are locked into rocks at depth. They argue that terminology borrowed from engineering such as 'residual stress' is not very useful in geomechanics and propose the use of the term 'inherent stress' to describe the stresses that a rock can record in it from a previous stress state the rock was under.…”