Assignment Instruction: A sedimentary formation is the product of rock particle
deposition, an ongoing process in which an underlying
layer of particles is buried and forced downward by
succeeding layers. As deposition and burial proceed,
the underlying layer is subjected to a progressively
increasing overburden pressure. This increased
overburden results in compaction--a reduction in
the thickness and bulk volume of the underlying layer.
Compaction also takes place when fluids are withdrawn
from a subsurface formation. Fluid withdrawal causes a
decrease in pore pressure, which in turn increases the
grain-to-grain, or matrix pressure of the rock
particles and causes them to compact.
The amount of compaction that occurs in a given
formation depends only upon the difference between the
vertically applied overburden pressure and the fluid,
or pore pressure. Thus, the uniaxial compaction of a
rock sample of thickness h can be expressed as
DVb/ Vb = Dh/h
Depending on geological conditions, subsurface
compaction may be associated with varying degrees of
subsidence, or sinking of the ground at the
surface.
Compaction and its associated effect of surface
subsidence can, in certain situations, have significant
effects on reservoir management strategies and field
operations. In this Assignment, you will build your
general understanding of these phenomena and see how
they can influence reservoir engineering decisions. You
will also look at a system that has been developed by
PDVSA to describe and predict compaction and subsidence
in areas where they are of particular concern.
By the time you complete this assignment, you should
be able to identify situations where compaction and
subsidence may be important considerations in a
reservoir management strategy, and you should be able
to begin using computer-based tools for predicting
compaction and subsidence under various conditions.