Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 528

Figure 16.15 Principle of the hydraulic fracturing method (Ljunggren et al. 2003)
Water is normally used as the test fluid. In rock salt, brine or gas replaces the water. In
the latter case the method is referred to as “pneumatic fracturing” (Rummel et al. 1995).
The procedure of an HF test will be demonstrated by means of the example of a test
conducted in claystone (WBI 2010). The records of packer pressure, hydraulic pressure
and flow rate versus time, obtained in this test, are plotted in Fig. 16.16.
Prior to fracturing, a permeability test is often carried out, depending on the rock mass
permeability, such as a slug test or a pulse test (Section 15.8.2). In the test, the results of
which are represented in Fig. 16.16, a pulse test was carried out, yielding an equivalent
rock mass permeability coefficient of k RM = 4 · 10 -11 m/s.
Subsequently, water is pumped at constant flow rate into the test section. The applica-
tion of water pressure to the borehole wall generates tangential tension. The pressure
is increased until a tensile fracture is initiated. After reaching the peak pressure, the
so-called “breakdown pressure”, which in this case amounts to p b = 6.51 MPa, the
hydraulic pressure drops slightly due to loss of water into the fracture. Then pumping is
stopped and the hydraulic line is shut. This is followed by a pressure decay, first quickly
and then more slowly. The pressure at which the fracture closes is considered as the
transition level between fast and slow pressure decay and denoted as “shut-in pressure”
p s . A few minutes after shut-in, the hydraulic line is vented and the first pressurization
cycle termed “frac cycle” is completed.