Page 46 - Petrophysics

20 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Krumbein [19] established a set of images for visually estimating
roundness, ranging from a roundness of 0.1 to 0.9. Later, Pettijohn
[20] defined five grades of roundness as: (1) angular, (2) subangular,
(3) subrounded, (4) rounded, and (5) well rounded. The degree of
roundness is a function of the maturity of the particle. The particles
are more angular near their source just after genesis and acquire greater
roundness from abrasion during transportation to a depositional basin.
The texture of clastic rocks is determined by the sphericity, roundness,
and sorting of the detrital sediments from which they are composed. The
sphericity and roundness are functions of the transport energy, distance
of transport from the source, and age of the particles. Young grains, or
grains near the source, are angular in shape while those that have been
transported long distances, or reworked from preexisting sedimentary
rocks, have higher sphericity and roundness.

DEVELOPMENT USE OF PETROPHYSICS
AND

The study of fluid flow in rocks and rock properties had its beginnings
in 1927 when Kozeny [21] solved the Navier-Stokes equations for fluid
flow by considering a porous medium as an assembly of pores of the
same length. He obtained a relationship between permeability, porosity,
and surface area.
At about the same time the Schlumberger brothers introduced the first
well logs [22]. These early developments led to rapid improvements of
equipment, production operations, formation evaluation, and recovery
efficiency. In the decades following, the study of rock properties and fluid
flow was intensified and became a part of the research endeavors of all
major oil companies. In 1950 Archie [23] suggested that this specialized
research effort should be recognized as a separate discipline under the
name of petrophysics. Archie reviewed an earlier paper and discussed
the relationships between the types of rocks, sedimentary environment,
and petrophysical properties. Earlier, in 1942, Archie [24] discussed
the relationships between electrical resistance of fluids in porous media
and porosity. Archie proposed the equations that changed well log
interpretation from a qualitative analysis of subsurface formations to
the quantitative determination of in situ fluid saturations. These and
subsequent developments led to improvements in formation evaluation,
subsurface mapping, and optimization of petroleum recovery.
The Hagen-Poiseuille equation [25], which applies to a single, straight
capillary tube, is the simplest flow equation. By adding a tortuosity
factor, however, Ewall [25] used pore size distributions to calculate
the permeability of sandstone rocks. The calculated values matched the