Page 202 - Geothermal Energy Systems Exploration, Development, and Utilization

178 4 Enhancing Geothermal Reservoirs
4.4
Investigations Ahead of Stimulation

The choice of the most suitable stimulation measure for any particular EGS
development project will depend on both, the specific objective of the intended
stimulation measure, and the geological and thermal conditions at the particular
location. The method chosen to be best suited on the other hand, will, besides the
local geological setting, determine the scope as well as the necessary depth of any
investigation ahead of stimulation measures. The prime purpose of any upfront
investigation program is to provide a framework for the decision on geometrical
and procedural frac layout, and key input parameters for a potential frac growth
modeling to be performed ahead of stimulation.
Apart from all potential differences in the necessary investigation program, the
decision on every stimulation measure will be based on a subsurface model: a
detailed, structural, and stratigraphical interpretation of 2D or ideally 3D seismic
data will deliver the main structural setting and features, qualitative information
on the prevalent stress regime, (normal-, thrust-, or strike-slip faulting), the fault
inventory, and information on the main stratigraphical and lithological units. If
necessary and relevant, these investigations may be complemented by local field
mapping or an outcrop analysis aiming at analyzing fault and fracture geometries
and hierarchies, rock petrography, mineralogy, or any potential hydrothermal
alteration.
Investigations may be extended on the well scale, by, for example, analyzing
well-based seismic data (vertical seismic profiling, VSP), allowing a more
detailed reservoir and fault delineation and identification of fractured and high
permeable zones. Dipmeter, full waveform sonic, and electrical or acoustic image
logs can provide information on fracture distribution patterns, orientations,
magnitude, and aperture with an even higher resolution. They can also serve
in discriminating between open and mineralized fractures. If applicable for the
geological setting under investigation, a detailed wire line log analysis will yield
further insight into lithological variations, porosity and permeability distribution,
the mineralogical composition, and fluid saturation of the matrix rock. Key
geomechanical parameters like Youngs modulus and poisson ration may be
derived from monopole and dipole sonic logs.
In case a hydraulic stimulation is planned, a detailed evaluation of the local stress
field will be performed aiming at ideally determining the full stress tensor for
the candidate EGS reservoir. A comprehensive review on techniques to determine
the orientation and magnitude of the in situ stress in deep wells was provided by
Zoback et al. (2003) Within this context it is sensible also to predict the proximity
to shear failure by performing a slip tendency analysis for the fault inventory (see
e.g., Morris, Ferrill, and Henderson, 1996).
In order to understand the fabric of the EGS reservoir under investigation, and
potentially to develop an empirical model relating rock type to failure resulting from
hydraulic stimulation, a detailed mineralogical and petrographical characterization