Page 114 - Petrophysics

CHAPTER 3

POROSITY AND

P E RM EAB I LITY

The nature of reservoir rocks containing oil and gas dictates the
quantities of fluids trapped within the void space of these rocks, the
ability of these fluids to flow through the rocks, and other related physical
properties. The measure of the void space is defined as the porosity
of the rock, and the measure of the ability of the rock to transmit
fluids is called the permeability. A knowledge of these two properties
is essential before questions concerning types of fluids, amount of
fluids, rates of fluid flow, and fluid recovery estimates can be answered.
Methods for measuring porosity and permeability have comprised much
of the technical literature of the oil industry. Other reservoir properties
of importance include the texture, the resistivity of the rock and its
contained fluids to electrical current, the water content as a function
of capillary pressure, and the tortuous nature of the interstices or pore
channels.
The texture of sedimentary rocks is determined largely by grain
shape and roundness, grain size and sorting, grain orientation and
packing, and chemical composition. A specific combination of these
variables may reveal information about diagenetic and catagenetic
processes and mechanisms operating during transportation, deposition,
and compaction and deformation of sedimentary materials. In some
cases, texture may yield some information about formation permeability
and porosity. For example, fine-grained sandstones with poorly sorted
angular grains will generally have lower porosity than sandstones
composed of coarse, well-sorted grains. Variation in permeability may be

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