Page 195 - Applied Petroleum Geomechanics

190 Applied Petroleum Geomechanics

3
water density (1.03 g/cm ) is also needed for overburden calculation.
Integrating this composite density (Eq. 6.2), overburden stress and over-
burden gradient can be obtained, as shown in Fig. 6.1.
It should be noted that the density log may be affected by the borehole
quality. When the borehole has washouts or breakouts, the density tool
cannot give a reliable density measurement. In this case the density log data
need to be corrected before using it for overburden stress calculation.

6.1.2 Overburden stress from empirical equations
6.1.2.1 Overburden stress for offshore drilling
Formation overburden stress gradient in onshore drilling can be estimated
using 1.0e1.1 psi/ft (0.0227e0.025 MPa/m). However, it is not recom-
mended for deepwater overburden gradient interpretations. Traugott (1997)
presented the following empirical equation for overburden stress gradient
based on drilling and well logging data in the Gulf of Mexico:

r Z w þ r ðZ Z w Z ag Þ
w
a
OBG ¼ (6.4)
Z
where OBG is the overburden stress gradient in offshore wells, in ppg; Z is
the true vertical depth (TVD) in the Kelly Bushing (KB) level; Z w and Z ag
are the water depth and air gap, respectively; the density of sea water
r w ¼ 8.5e8.7 ppg; r a is the average density of the sediments in ppg, and
0:6
Z Z w Z ag
r ¼ r þ (6.5)
ml
a
3125
where r ml is the density at the mudline (sea floor) and r ml ¼ 16.3 ppg
3
(1.956 g/cm ); the depth unit is ft.
It should be noted that the depth datum used for overburden gradient
calculation in Eq. (6.4) is the KB level for drilling purpose. From Eqs. (6.4)
to (6.5), overburden stress can be obtained from the following equation:

0:6

Z Z w Z ag
s V ¼ r Z w þ 16:3 þ ðZ Z w Z ag Þ =19:25
w
3125
(6.6)
where s V is in psi; water density is in ppg and the depth is in ft.
Barker and Wood (1997), assuming plastic deepwater formations,
derived an expression for the cumulative average density from the mudline
to a depth of interest using leak-off test (LOT) data from 70 deepwater
wells. The essence of the plastic formation assumption is that the pressure