Page 125 - Well Logging and Formation Evaluation

CHAPTER 7

ROCK MECHANICS ISSUES

While rock mechanics can be a very complicated subject, there are a
few basics that all petrophysicists will need in their day-to-day work,
which will be covered here. In a normal reservoir, the formation rock is
subject to greatest stress from the overburden. This stress arises from the
weight of rock above and can be measured by integrating the density log
to surface. Since density logs are not usually run to surface, a common
working assumption is that the overburden stress is approximately 1psi/ft.
The vertical strain (i.e., compaction) caused by this stress is offset by
the formation pressure, which helps “support” the rock. Because the struc-
ture is usually partially open-ended, the fluid will take only a proportion
of the overburden stress. However, in overpressured reservoirs where the
fluid is not free to escape, the formation pressure may become close to
the overburden pressure. The net effective vertical stress seen by the
formation is given by:

s z = P overburden - P formation (7.1)

This is actually not the true effective vertical stress, which for given
conditions of P overburden and P formation would result in the same strain in the
sample if applied with zero pore pressure. This will now be demonstrated.
Let K m equal the bulk modulus of the matrix, when the pore pressure
equals the vertical stress, defined by:

K m = stress strain = ( P overburden – P formation ) (d V V). (7.2)
m
Let K b equal the bulk modulus of the dry rock, as measured in a normal
core measurement, defined by:

115