Page 236 - Reservoir Geomechanics

217 Determination of S 3 from mini-fracs

measure of the least principal stress. If not, then the test must be considered a FIT
and the maximum pressure achieved cannot be used to estimate the least principal
stress. Second, was a stable FPP achieved? If so, the fracture clearly propagated away
from the well and the shut-in pressure is likely a good measure of S 3 . While these
two questions are straightforwardly answered when there is a good record of the test,
it is sometimes necessary to rely on a single reported value, not knowing whether it
refers to a reasonable estimate of the least principal stress. In fact, in some cases the
pressure–time record is approximated by a few distinct data points only, obtained by
reading pressure on a fluctuating gauge and estimating flow rate by counting pump
strokes. In such cases, determination of accurate LOT values is essentially impos-
sible. Values of LOT’s that are markedly lower than the expected trend for a given
area should also be treated with extreme caution as these tests may simply indicate a
poor-quality cement job rather than an anomalously low value of the least principal
stress.
Pressure-while-drilling (PWD) is a measurement-while-drilling (MWD) or logging-
while-drilling (LWD) sensor that continually measures annular pressures during the
drilling process and has the potential for providing information about the magnitude
of the least principal stress, especially in difficult drilling situations (Ward and Clark
1998). This measurement is generally taken some 5–10 m behind the bit and allows for
accurate downhole determination of mud weight, equivalent circulation density (ECD),
swab and surge pressures. Pressure values are transmitted to the surface in real time
during drilling and recorded downhole in memory that can be read when the bottom
hole assembly is brought to the surface or transmitted to the surface once circulation
is resumed. Figure 7.6 shows a number of drilling-related parameters as a function of
time (including PWD in the right column) that are measured during drilling opera-
tions. Note the step-like nature of the pressure drop that occurred when drilling was
stopped to connect a section of drill pipe. When drilling resumed the pressure abruptly
increased. This step in pressure allows us to define the difference between the ECD (the
equivalent circulating density which corresponds to the bottom-hole pressure during
drilling) to the static mud weight. In this case the viscous resistance to mud circula-
tion during drilling results in a difference between the ECD and static mud weight of
0.33 ppg.
There are three ways that PWD data can be used to better constrain the minimum
horizontal stress: improving the accuracy of leak-off test (LOT) measurements, iden-
tifying lost circulation incidents and identifying ballooning incidents. With respect to
LOT’s, it is important to recognize that such tests are normally recorded at the surface

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Figure 7.5. Determination of the least principal stress from step-rate tests conducted in a well in
Alaska. This test is similar to a conventional mini-frac or extended leak-off test except that the
injection rate is varied in incremental steps. The least principal stress (i.e. the pressure at which the
hydrofrac opens) is indicated by the distinct change in slope.