Page 240 - Reservoir Geomechanics

221 Determination of S 3 from mini-fracs

of this technique has been related to determination of S Hmax for scientific purposes or
for application to problems in civil engineering, but as mentioned above, at relatively
shallow depth.
As alluded to above, it is important to recognize the reasons why hydraulic fracturing
is not a viable method for determination of S Hmax in relatively deep and/or hot wells.
First, most wells are cased at the time of hydraulic fracturing and hydrofrac initiation
in the perforations made through the casing and cement and into the formation. In this
case, hydraulic fracturing tests still yield accurate estimates of the least principal stress
(becauseonceafracturepropagatesawayfromthewellbore,itpropagatesperpendicular
to the least principal stress), but the equations above are no longer relevant because the
stress concentration around the well does not govern fracture initiation. Second, during
a leak-off test, the open hole section below the casing may not be well described by
the Kirsch equations. In other words, in classical hydraulic fracturing, we assume the
well is circular and there are no pre-existing fractures or faults present. When doing
classical hydraulic fracturing in open hole, it is common to use wellbore imaging
devices to assure this. In the case of leak-off tests, this is never done and there can
be serious washouts at the bottom of the casing (or other irregularities) that might
affect fracture initiation. The most important reason that hydraulic fracturing cannot
be used to determine S Hmax in oil and gas (or geothermal) wells is that it is essentially
impossible to detect fracture initiation at the wellbore wall during pressurization. In
other words, equation (7.2) assumes the P b is the pressure at which fracture initiation
occurs. In point of fact, depending on the stress state, the breakdown pressure may not
be the fracture initiation pressure (as originally discussed by Zoback and Pollard 1978;
Hickman and Zoback 1983). Regardless of the stress state, however, in oil and gas
wells it is straightforward to show that it is essentially impossible to detect the pressure
at which the fracture initiates at the wellbore wall. To see this, consider the volume
of fluid associated with conducting a hydrofrac, V s , consisting of the well, pump and
surface tubing, etc. which has a system compressibility, β s ,given by

1
V s
β s = (7.5)
V s
P
which can be rewritten as
1

P =
V s
β s V s
If we divide both sides by
t,we obtain

P 1
V s
= (7.6)

t β s V s
t
which expresses the rate at which the pressurization rate
P/
t,asa function of
1
V s
the rate at which the system volume, changes with time, . Thus, for a constant
V s
t
pumping rate, the change in pressurization rate (the parameter being observed during