Logging and Testing
Throughout drilling, various logs are plotted to record
the well's progress. The record includes data about the type and thickness of
rock layers, based on the well site geologist's examination of cuttings brought
to the surface during drilling. The speed of penetration is also recorded
because the first sign of possible success is usually an increase in the rate of
penetration followed by the appearance of oil or gas traces in cuttings. If more
information is needed about the rocks, a special cylindrical bit may be used to
cut a core sample for analysis by geologists, reservoir engineers, geochemists and palynologists.
Further information is obtained by lowering a package of
instruments, called wireline logging tools, into the wellbore. The instruments
record and transmit information about the rock layers' thickness, porosity and
permeability and the composition of fluids, such as oil, gas or water, contained
A logging instrument can also be mounted on the string
above the bit to send information continuously during drilling. It sends signals
to the surface by means of pulses, like sonar signals, in the mud. Another
instrument, the measurement-while-drilling (MWD) tool, can similarly measure the
direction and precise location of the bit while drilling horizontal wells.
A common way to determine potential oil or gas production
is the drillstem test, which uses a special tool in place of the bit on the end
of the string. The tool has valves and rubber sleeves, called packers, that can
be controlled from the surface. First, the packers are expanded to isolate the
section of the hole to be tested. Next, valves on the tool are opened, allowing
liquids or gas from the formation to flow into the empty drill pipe. This gives
a good indication of the type and volume of the fluids in the formation, their
pressure and rate of flow.
Petroleum Communication Foundation. Our Petroleum Challenge: Exploring Canada's Oil and Gas Industry, Sixth Edition. Calgary: Petroleum Communication Foundation, 1999. With permission from the Centre for Energy.