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284 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

well logging, and pressure test analyses. A less common source is
geochemistry. The application of geochemical techniques to oil and gas
exploration has only recently achieved widespread acceptance among
exploration geologists; however, it is beyond the scope of this text.
It is not the purpose of this section to make complete discussion of
core analysis, well logging and well testing but rather to highlight the
significance of the measuring techniques.

ANALYSIS

All phases of the petroleum industry rely directly or indirectly on the
knowledge of reservoir rock properties. Analysis of rock samples yields
valuable data basic to exploration, well completion, and evaluation of
oil and gas reserves. Drill bit cuttings, because of their size and mode
of recovery, essentially provide qualitative information. The necessity
of recovering and examining large reservoir rock samples led to the
development of coring techniques. The first coring tool appeared in
1908 in Holland. In 1921, H. E. Elliot of the United States introduced
the first effective coring tool by successfully combining an inner core
barrel with a toothed bit. Four years later, considerable improvements
were made to Elliot’s device to include a removable core head, a core
catcher, and a stationary inner barrel, to which various refinements
have been added [41]. Currently, several types of coring devices are
available: diamond cores, rubber and plastic sleeve cores, percussion
and continuous sidewall cores, and cores recovered in a pressure core
barrel. Each one of these devices offers certain advantages. The selection
is generally dictated by the type of reservoir rock and objectives of the
core analysis.
Three coring methods are practiced: conventional, wireline, and
sidewall. Conventional coring, which refers to core taken without regard
to precise orientation, encompasses arrange of coring devices and core
barrels. The main disadvantages of conventional coring is that coring
equipment requires that the entire drill string be pulled to retrieve the
core; however, the corresponding advantage is that large cores, 3 to
5 in. in diameter and 30 to 90 ft long, may be recovered. In the wireline
coring method, the core may be retrieved without pulling the drill string
by using an overshot run down the drill pipe on a wireline. The cores
obtained by this method are small, approximately 1 to 2 in. in diameter
and 10 to 20 ft in length. Other advantages include downhole durability
and higher core recovery.
Sidewall coring is necessary when it is desirable to obtain core samples
from a particular zone already drilled, especially in soft rock zones
where hole conditions are not conducive to openhole drillstem testing.