Hydraulic stimulation involves the injection of large volumes of fluid into the
target rock mass. Experience has demonstrated that the operation is often highly
effective in producing a permanent increase in rock mass transmissivity. The
mechanism is thought involve the shearing of natural fractures which tends to produce
dilation of the fracture walls and the opening of tubes at jogs in the fracture
trace. The generation of microseismicity and the permanent nature of the
transmissivity increases support this view. The shearing represents the relaxation
of the natural shear stresses within the rock mass triggered by the weakening of the
fracture by the elevated fluid pressure. The pore pressure increase required to
initiate shear failure on favourably oriented fractures is often very small, perhaps
as low as a few MPa. This reflect the tendency for the Earth's crust to be close to
failure, a state which is referred to as being critically stressed. As such, the
vast majority of the mechanical work of permeability/porosity creation is done by the
natural stresses. The stress state thus determines which fractures shear and thus
has a major influence of the geometry of the stimulated volume and flow paths that
develop therein. It follows that knowledge of the stress state is essential if the
process of reservoir creation is to be understood and modelled, which is a step
towards developing the means to control it. The confident determination of the
complete stress tensor is at best difficult and often not practical. Fortunately,
limited knowledge of the stress tensor suffices for reservoir engineering purposes,
but complete knowledge is needed for modelling. In this talk I will discuss the
importance of the individual attributes of the stress tensor (e.g. the orientation of
the maximum horizontal stress), and suggest strategies for their measurement.
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