Page 188 - Introduction to Petroleum Engineering

LOG CALIBRATION WITH FORMATION SAMPLES 175
The dipmeter logging tool uses three or more microresistivity sensors mounted
on caliper arms to make measurements on different sides of the borehole wall.
Information recorded by the dipmeter logging tool is recombined by computer to
provide planar orientation including bedding dip. Acoustic and optical borehole
imaging tools can provide similar information about bedding dip by providing
images of the borehole wall.

9.6 LOG CaLIbRaTION WITH FORMaTION SaMPLES

Conclusions derived from logging data are subject to varying levels of error. To
reduce that error and build confidence in interpretations of logs, it is useful to com-
pare the logs to samples from the formation. The cuttings collected during the mud
logging process provide one set of samples. Other samples can be collected with
coring operations.

9.6.1 Mud Logs
The mud log is obtained during the drilling operation. It includes several tracks: rate
of penetration, depth, composition of gas liberated at the surface from the drilling
mud, plus word and strip chart descriptions of cutting samples.
The wellsite geologist (mud logger) usually collects samples of cuttings every
10 ft of drill progress. The geologist examines the cuttings to determine lithology and
the presence of any oils for the formations in contact with the bit. To do this, the geol-
ogist must account for the travel or lag time from the bit to the shale shaker as the
mud carries the cuttings to the surface. These shale shaker samples can also contain
bits of rock from shallower formations. Consequently, the sample may not be entirely
representative of the environment encountered by the drill bit at a particular location,
but they are the first direct evidence of lithology obtained from the borehole. Part of
the duty of the mud logger is to identify the portion of the sample that was most
recently cut by the bit. These descriptions of samples, as well as the samples them-
selves, are routinely compared to open‐hole logs.
The rate of penetration track shows the amount of time it takes to drill through a
foot of rock and indicates the hardness of the formation. Rock hardness inferred from
the rate of penetration gives information about the rock type, which can be compared
to the lithology logs.
Oil and gas shows in the mud log can be compared to indications of hydrocarbon
in the porosity and resistivity logs.

9.6.2 Whole Core
Core samples are obtained by replacing the drill bit with a ring‐shaped coring bit.
The coring bit drills into the rock and captures a cylindrical volume of rock through
the hole in the bit. The rock sample is collected in a core barrel in the lower portion
of the bottom‐hole assembly (BHA). The need to replace the drill bit with a coring
bit, obtain a core, and then replace the coring bit with a drill bit to resume drilling