Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 531

Figure 16.17 Example of a hydraulic fracturing test, impression packer record, orientations and
magnitudes of principal normal stresses

Sometimes it is difficult to identify fracture orientation. Then techniques employed by
Lee & Haimson (1989) can be used to remove subjectivity in the determination of frac-
ture orientation.

The test evaluation is based on records of hydraulic pressure versus time, and flow rate
versus time, as illustrated in Fig. 16.16 and the following simplifying assumptions:
-

the rock mass is homogeneous, isotropic and brittle,

- one principal normal stress is oriented parallel to the borehole axis,

- plane strain is maintained in the test section during pressure build-up and fracturing.

If the in-situ stress state is not influenced by topography and tectonics and generally at
great depths, the vertical stress can be assumed to be a principal normal stress. Thus, the
HF method is most suitable for measurements in vertical boreholes and at great depths.
In the following, therefore, test evaluation is described only for the case of vertical bore-
holes. In addition, elastic rock mass stress-strain behavior, and no infiltration of the
fracturing fluid into the rock mass and the absence of pore fluid in the rock mass are
assumed, that is, the intact rock is impermeable and non-porous. Other interpretation
methods such as the poroelastic model (Haimson & Fairhurst 1967, Haimson 1968,
Detournay et al. 1989, Schmitt & Zoback 1989) and the fracture mechanics model de-
scribed in Abou-Sayed et al. (1978), Rummel (1987) and Rummel & Hansen (1989) are
not considered here.