Page 27 - Petrophysics

2 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

of the petrophysical and fluid transport properties of the more complex
subsurface samples of rocks associated with petroleum reservoirs.
This body of experimental data and production analyses of artificial
systems, surface rocks, and subsurface rocks make up the accumulated
knowledge of petrophysics. Although the emphasis of this text is
placed on the analyses of small samples, the data are correlated to
the macroscopic performance of the petroleum reservoirs whenever
applicable. In considering a reservoir as a whole, one is confronted with
the problem of the distribution of these properties within the reservoir
and its stratigraphy. The directional distribution of thickness, porosity,
permeability, and geologic features that contribute to heterogeneity
governs the natural pattern of fluid flow. Knowledge of this natural
pattern is sought to design the most efficient injection-production system
for economy of energy and maximization of hydrocarbon production [ 13.
Petrophysics is intrinsically bound to mineralogy and geology because
the majority of the world’s petroleum occurs in porous sedimentary
rocks. The sedimentary rocks are composed of fragments of other
rocks derived from mechanical and chemical deterioration of igneous,
metamorphic, and other sedimentary rocks, which is constantly
occurring. The particles of erosion are frequently transported to other
locations by winds and surface streams and deposited to form new
sedimentary rock structures. Petrophysical properties of the rocks
depend largely on the depositional environmental conditions that
controlled the mineral composition, grain size, orientation or packing,
amount of cementation, and compaction.

MINERAL CONSTITUENTS OF ROCKS-A REVIEW

The physical properties of rocks are the consequence of their mineral
composition. Minerals are defined here as naturally occurring chemical
elements or compounds formed as a result of inorganic processes. The
chemical analysis of six sandstones by emission spectrography and X-ray
dispersive scanning electron microscopy [2] showed that the rocks
are composed of just a few chemical elements. Analysis of the rocks
by emission spectroscopy yielded the matrix chemical composition
since the rocks were fused with lithium to make all of the elements
soluble in water, and then the total emission spectrograph was analyzed.
The scanning electron microscope X-ray, however, could only analyze
microscopic spots on the broken surface of the rocks. The difference
between the chemical analysis of the total sample and the spot surface
analysis is significant for consideration of the rock-fluid interactions. The
presence of the transition metals on the surface of the rocks induces