reservoir geomecanics

190 Reservoir geomechanics
and then calculate
n∑
li
n∑
mi
l =
R 2 =
i=1
R
n∑
li
i=1
and m =
+
∑n
mi
i=1
i=1
R
The mean breakout direction is given by
(
θ m = tan −1 m
)
l
We define
k = N − 1
N − R
such that the standard deviation is given by
(6.11)
(6.12)
(6.13)
(6.14)
sd = 81◦ √
k
(6.15)
The rationale for the A, B, C and D quality assignments for drilling-induced tensile
fractures is similar to that for breakouts as discussed above. When 10 or more consistently
oriented tensile fractures are seen over a 300 m depth interval, such observations
are given an A quality. B and C quality involve fewer tensile fracture observations,
greater variation of orientation and a smaller depth interval. Again a standard deviation
greater than 25 ◦ is interpreted as an indication of unreliable data (quality D) such that
it should not be presented on maps.
As discussed in Chapter 7,inanopen-hole hydraulic fracturing stress measurement,
an isolated section of a well is pressurized until a tensile fracture is induced at the point
of least compressive stress around the well. Under ideal circumstances, impression
packers (oriented with respect to magnetic north) can be used to determine the azimuth
of the induced hydrofrac. However, this is both a time-consuming and difficult process
and rarely yields reliable results in oil and gas wells.
More on drilling-induced tensile fractures
In this section we return to the subject of drilling-induced tensile fractures to make a
few additional points. First, drilling-induced tensile fractures occur in vertical wells
only when there is a significant difference between the two horizontal stresses. In fact,
it is straightforward to show that the conditions for the occurrence of drilling-induced
tensile fractures around a vertical wellbore in the absence of excess mud weight or
wellbore cooling are essentially identical to the values of S hmin and S Hmax associated
with a strike-slip faulting regime in frictional equilibrium.