Page 203 - Geothermal Energy Systems Exploration, Development, and Utilization

4.4 Investigations Ahead of Stimulation 179

Wellbore
Leak off
Fracture
Injection Friction propagation rock
strength
Stress intensity
Elastic opening factor
pressure support
of fracture walls

Figure 4.2 Top view of a fracture propagation due to hydraulic stimulation (Fokker, 2007).
of core or cutting material should be performed. This certainly is of particular
importance in case the well is to be stimulated chemically.
A routine core analysis will provide key rock matrix properties like porosity
and permeability to which a wireline analysis can be calibrated. A complementary
special core analysis may be performed focussing on thermal properties (thermal
conductivity and possibly radiogenic heat production) and fracture mechanical
parameters (e.g., fracture toughness).
Within the process of planning a hydraulic stimulation treatment, frac modeling
is a key step, for example, to understand the ongoing processes as shown in
Figure 4.2. The geomechanical parameters most important for these models are
the Young’s modulus E, the Poissson ratio ν, the angle of internal friction φ,
and the poroelastic constant η; From these the single, most important parameter
certainly is Young’s modulus E since it has a nearly direct relationship with the
net pressure, fracture geometry, and fracture width. Young’s modulus can easily
be measured in the lab, or derived from sonic logs, and it is recommended in
case hydraulic stimulation measures are being planned, to take core samples, and
conduct triaxial tests in order to determine the elastic parameters Since these
parameters may largely vary between the various lithological units over which the
hydraulic fracture will grow, they should be determined for each of the main unit
ahead of the stimulation, in order to enable the frac treatment to be planned with
sufficient accuracy.
Every case-specific upfront investigation program will be compiled according to
both the stimulation method of choice and the particular local geological conditions.
However, in case a chemical stimulation is being envisaged, the program will
certainly focus on mineralogical content and potential hydrothermal alterations,
where in the case of hydraulic stimulation treatments, the focus will shift toward
the lithological characterization and the rock mechanical properties of the matrix
material, and a detailed description of the local stress field.
A joint interpretation of the structural geological setting, results of an image log
analysis (Figure 4.3), wireline, and if applicable, lab-based stress field investigations