Rock joint and rock mass behaviour during pressurised hydraulic injections
The hydro-mechanical effects of high pressure fluid injections into
jointed rock are considered mostly in the context of Hot Dry Rock (HDR)
geothermal energy systems.
In Part I, the mai n aspects ari sing from the HDR research at the "Camborne School of Mines (CSM) and Los Alamos Nat iana 1 Laboratory (LANL) projects are reviewed. Previous approaches to fluid-rock
interacti ons at these projects and important observed phenomena are
highlighted. Fundamental aspects of rock joint geometry, mechanical
behaviour and flow regimes within jointed rock are also reviewed. These
aspects are then related to possible conditions in HDR systems.
The role of in situ stress conditions is of great significance in this
study and is reviewed theoretically and in detail for both the CSM and
LANL project sites. The revi ew incl udes a comprehensi ve seri es of
measurements, by different techniques, organised and interpreted by the
author at the CSM project.
In Part II, model development, the emphasis is on intermediate fluid
pressures which are too high for simple diffusion alone and too low for
tensile hydraulic fracturing. The dominant mechanical activity is one
of joint shear.
Strike-slip shearing due to fluid injection is examined in two
dimensions with the numerical model FRIP, which has been extended by
the author. Similar behaviour is examined in three dimensions with an
analytical model which is linked to microseismic observations. This
model explains the observed phenomenon of downward shear growth. Joint
distribution and mechanical properties, and their effect on fluid
diffusivity, are examined and used in analytical models of fluid
pressure pulse propagation, tracer transport, and rock stress increment
All models are used to help interpret field data, mainly from the CSM
project. The models are also of potential application to hydrocarbon
reservoir stimulation, liquid waste disposal and leakage from high
pressure water tunnels.