Page 128 - Well Logging and Formation Evaluation
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118 Well Logging and Formation Evaluation
lateral stress. In such a test, the formation is pressured up using the mud
until close to the point at which it fractures. Hence, this pressure (after
correcting for the weight of the column) will be equivalent to the weakest
lateral stress. When V p and V s are measured in a well using a dipole sonic
tool, one can directly calculate Poisson’s ratio, and this is normally done
as standard in the log print presented to the client.
Exercise 7.1. Net Effective Stress
You need to decide the appropriate hydrostatic lab pressure to use for some
core plug measurements. Relevant data are as follows:
Depth: 12,000ft
Overburden gradient: 1psi/ft
Formation pressure gradient: 0.435psi/ft
Poisson’s ratio of the rock: 0.35
1. At what pressure should the measurements be performed?
2. Suppose you are now told that the poroelastic constant for the samples
is 0.85. What pressure should you use?
