Page 104 - Applied Petroleum Geomechanics

96 Applied Petroleum Geomechanics

Horsrud (2001) proposed the following correlation for estimating rock
strength, mainly for high-porosity Tertiary shales:
UCS ¼ 0:77V p 2:93 (3.7)

where UCS is in MPa; V p is in km/s. Using the P-wave interval transit time
from sonic log, Dt, (in ms/ft), the following correlations were obtained
(Horsrud, 2001):
2:93
UCS ¼ 0:77ð304:8=DtÞ (3.8)
3:23
E ¼ 0:076ð304:8=DtÞ (3.9)
where Young’s modulus (E) is in GPa.
Lal’s and Horsrud’s correlations predict rock strengths fairly well
for shales in the Gulf of Mexico and the North Sea where high-porosity,
unconsolidated Tertiary-aged, or younger shales are dominant.
Therefore, these correlations after certain calibrations can be used to
estimate UCSs in weak shales. In the above strength correlations,
Horsrud’s correlation predicts a higher shale strength than Lal’sequation
for the shale with higher sonic velocity (or lower transit time). Fig. 3.8
presents the shale UCS calculated from sonic well log data by using
Lal’s and Horsrud’scorrelationsinanoil explorationwell inthe Gulf
of Mexico.

3.2.1.2 From porosity
Rock strength and porosity have good correlations for various rocks, and
porosity can be obtained from lab tests or from density and sonic logs. In
the following equations, the rock strength UCS is in MPa and porosity is
in percent (%). For shales in the North Sea (geologically young and weak
shales, mostly high-porosity Tertiary shales), Horsrud (2001) proposed the
following correlations for estimating rock UCS:

UCS ¼ 243:6f 0:96 (3.10)
Lashkaripour and Dusseault (1993) obtained a correlation based on 13
data points from publicly available sources and their own testing of shales.
The mean of the UCS is approximately 79 MPa, and 9 of these 13 points
have 10% porosity or less. The following is their correlation:
UCS ¼ 193:4f 1:143 (3.11)