Page 118 - Petrophysics 2E

POROSITY 91

ENGINEERING CLASSIFICATION OF POROSITY

During sedimentation and lithification, some of the pore spaces
initially developed became isolated from the other pore spaces by
various diagenetic and catagenetic processes such as cementation and
compaction. Thus, many of the pores will be interconnected, whereas
others will be completely isolated. This leads to two distinct categories
of porosity, namely, total (absolute) and effective, depending upon
which pore spaces are measured in determining the volume of these
pore spaces. The difference between the total and effective porosities is
the isolated or non-effective porosity. Absolute porosities is the ratio of
the total void space in the sample to the bulk volume of that sample,
regardless of whether or not those void spaces are interconnected.
A rock may have considerable absolute porosity and yet have no fluid
conductivity for lack of pore interconnections. Examples of this are lava,
pumice stone, and other rocks with vesicular porosity.
Effective porosity is affected by a number of lithological factors
including the type, content, and hydration of the clays present in the
rock, the heterogeneity of grain sizes, the packing and cementation of
the grains, and any weathering and leaching that may have affected the
rock. Many of the pores may be dead-ends with only one entry to the
main pore channel system. Depending on wettability, these dead-end
pores may be filled with water or oil, which are irreducible fluids.
Experimental techniques for measuring porosity must take these facts
into consideration.
In order to recover oil and gas from reservoirs, the hydrocarbons
must flow several hundred feet through the pore channels in the rock
before they reach the producing wellbore. If the petroleum occupies
non-connected void spaces, it cannot be produced and is of little interest
to the petroleum engineer. Therefore, effective porosity is the value used
in all reservoir engineering calculations.

GEOLOGICAL CLASSIFICATION OF POROSITY

As sediments were deposited in geologically ancient seas, the first fluid
that filled pore spaces in sand beds was seawater, generally referred to as
connate water. A common method of classlfying porosity of petroleum
reservoirs is based on whether pore spaces in which oil and gas are
found originated when the sand beds were laid down (primary or
matrix porosity), or if they were formed through subsequent diagenesis
(e.g., dolomitization in carbonate rocks), catagenesis, earth stresses,
and solution by water flowing through the rock (secondary or induced
porosity). The following general classification of porosity, adapted from