Page 177 - Introduction to Petroleum Engineering

164 WELL LOGGING
reservoir fluid samples. Examples of open‐hole logs include resistivity logs, nuclear
logs, sonic logs, and borehole imaging.
Some tools are designed for cased‐hole logging. For example, cement‐bond logs
are used after setting casing to determine the quality of the bond between casing and
cement. If the bond is not sufficient, then remedial operations are needed before
proceeding with any completion operations. After the cement‐bond log, the next
cased‐hole log is a gamma radiation log for correlating depths of target formations
for completion operations in cased holes. Temperature logs and flow‐rate logs may
be used to identify sources of fluids in completed wells. A spinner flow meter can
measure flow rates at different locations in the wellbore. Carbon–oxygen logs use
gamma‐ray spectroscopy to measure carbon content in hydrocarbons and oxygen
content in water. A low carbon to oxygen ratio implies the presence of water, while a
high carbon to oxygen ratio implies the presence of hydrocarbons.

9.1.3 Depth of Investigation
Some logging tools measure properties in the first few inches of the formation, while
other tools measure properties deeper into the formation. This depth of investigation
is usually characterized as shallow, medium, or deep and can range from a few inches
to several feet. The lithology, or mineral composition, of the formation can be
determined by shallow measurements. One purpose of resistivity logs is to estimate
brine saturation in the formation, as an indication of the presence of oil and gas in the
formation. For these estimates to be useful, deep measurements that penetrate beyond
the transition zone are needed.

9.2 LITHOLOGY LOGS

Lithology logs indicate rock type. Most hydrocarbon accumulations are found in
sedimentary rocks. The most important conventional reservoir rocks are sedimentary
rocks classified as clastics and carbonates (Figure 9.3). A clastic rock is composed
of clasts, or fragments, of preexisting rocks or minerals. Sandstones are compacted
sediment, while shales or mudrock is laminated sediment. Carbonates are produced
by chemical and biochemical sources. A single well can encounter several different
types of rocks, as illustrated by the stratigraphic column in the figure. The strati-
graphic column depicts the layering of rock strata in a column of rock. We consider
three types of lithology logs in this section: the gamma‐ray log, the spontaneous
potential (SP) log, and the photoelectric effect (PE) log.

9.2.1 Gamma‐Ray Logs
A gamma‐ray tool detects gamma‐ray emissions from radioactive isotopes. Gamma‐
ray logs imply the presence of shale when there is a high gamma‐ray response. Clean
(shale‐free) sands or carbonates tend to have a low gamma‐ray response. Depth of
investigation is about 1.5 ft.