Page 231 - Reservoir Geomechanics

212 Reservoir geomechanics

supplies it; hence the pressure drops). The difference between the LOP and FBP is a
complex function of the conditions immediately surrounding the well (especially when
a frac is being initiated through perforations). If pumping continues at a constant rate,
the pumping pressure will drop after the FBP to a relatively constant value called the
fracture propagation pressure (FPP). This is the pressure associated with propagating
the fracture away from the well. In the absence of appreciable near-wellbore resistance
mentioned above (i.e. if the flow rate and fluid viscosity are low enough), the FPP is
very close to the least principal stress (e.g. Hickman and Zoback 1983). Hence, the FPP
and LOP values should be similar. It should be emphasized that a distinct FBP need
not be present in a reliable mini-frac or XLOT. This correspondence between the LOP
and FPP is the reason why, in typical oil-field practice, leak-off tests are taken only to
the LOP, rather than performing a complete, extended leak-off test.
An even better measure of the least principal stress is obtained from the instantaneous
shut-in pressure (ISIP) which is measured after abruptly stopping flow into the well,
because any pressure associated with friction due to viscous pressure losses disappears
(Haimson and Fairhurst 1967). In carefully conducted tests, constant (and low) flow
rates of ∼200 liter/min (1 BBL/min), are maintained and low viscosity fluid (such as
water or thin oil) is used and pressure is continuously measured. In such tests, the LOP,
FPP, and ISIP have approximately the same values and can provide redundant and
reliable information about the magnitude of S 3 .Ifa viscous frac fluid is used, or a frac
fluid with suspended propant, FPP will increase due to large friction losses. In such
cases the fracture closure pressure (FCP) is a better measure of the least principal stress
than the FPP or ISIP. In such, tests, the FCP can be determined by plotting pressure
√
as a function of time and detecting a change in linearity of the pressure decay (Nolte
and Economides 1989). However, if used inappropriately, fracture closure pressures
can underestimate the least principal stress and care must be taken to assure that this is
not the case.
Figure 7.3 illustrates two pressurization cycles of a mini-frac test conducted in an
oil well in Southeast Asia. Note that the flow rate is approximately constant at a rate
of ∼0.5 BBL/min during the first cycle (in which 10 BBLS was injected before shut-
in), and was held quite constant during the second (in which 15 BBLS was injected
before shut-in). It is not clear if a constant FPP was achieved before shut-in on the
first pressurization cycle, but it is quite clear that it was on the second. Pressures after
shut-in are shown for the two tests. The ISIPs were determined from the deviation in the
rate of rapid pressure decrease to a more gradual decay on the linear plots of pressure
as a function of time. The FCP’s were determined from the deviation from linearity in
√
the time plots that are shown. As shown, these two pressures vary by only a few tens
of psi. Once the hydrostatic head is added to the measured values, the variation between
these tests results in a variance of estimates of S hmin that is less than 1% of its value.
Figure 7.4 shows a compilation of pore pressure and LOT data from the Visund field
in the northern North Sea (Wiprut, Zoback et al. 2000). Pore pressure is hydrostatic