Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 530

After the frac cycle at least three repressurization cycles called “refrac cycles” should
be conducted using the same fl ow rate as in the frac cycle. These cycles are carried out
to identify the so-called “reopenening pressure” p r and additional p s values. At the re-
opening pressure p r the slope of the pressure versus time plot decreases and then drops
slightly due to fl ow of water into the fracture. Lee & Haimson (1989) introduced a sta-
tistical method for identifying p r . In the hydraulic pressure versus time plot represented
in Fig. 16.16, reopening pressures were determined for the fi rst and the second refrac
cycles resulting in p r1 = 2.5 MPa and p r2 = 2.66 MPa, respectively. The shut-in pressure
was evaluated for the third refrac cycle yielding p s = 2.11 MPa.
After completion of the frac and refrac cycles a so-called “step-rate test” is often carried
out, in which the fl ow rate is raised stepwise to obtain an array of pressure levels at dif-
ferent fl ow rates. Step-rate tests allow the shut-in pressures derived from the preceding
frac and refrac cycles to be checked by means of a fl ow rate versus hydraulic pressure
plot using the following relationship derived from equations formulated by Perkins &
Kern (1961):
(16.16)
where subscripts 1 and 2 stand for two fl ow rate and hydraulic pressure levels, respec-
tively (Haimson 1978). Solving (16.16) for p s yields
(16.17)
The step-rate test plotted in Fig. 16.16 yielded shut-in pressures ranging from 1.95 to
2.3 MPa. Thus, good agreement with p s = 2.11 MPa derived from the third refrac cycle
could be achieved.
The orientation of the hydraulically induced fracture can be detected by means of an
impression packer with attached compass (Fig. 16.15, right). For this purpose, the im-
pression packer is wrapped by a transparent plastic sheet aligned with a marker line.
Then it is positioned in the test interval and infl ated to a pressure higher than the re-
corded shut-in levels for about 30 min. During this time, a photograph is taken from
the compass yielding the direction of the marker line with respect to magnetic north.
Fracture orientation is then obtained from the trace of the fracture on the plastic sheet.
Figure 16.17 (left) illustrates the impression packer record of the HF test whose results
are represented in Fig. 16.16, indicating two vertical fractures oriented at 135° with
respect to north.
Alternatively, the fracture orientation can be determined by means of borehole obser-
vation methods using an optical televiewer, for example (Section 13.5.2).