WELLOG Well Evaluation – an Overview
WELL
EVALUATION:
The
reason for running a Well Log is to help locate an Oil, Gas or Water bearing
formation. Other reasons may be related
to defining bed thickness of Coal or other minerals.
In
order to analyze a formation, three types of logs are available. The types are
Lithology, Resistivity, and Porosity. Each will be discussed in detail in
subsequent parts of this webinar.
When a
well logging unit arrives on location, a geologist or engineer approaches the
evaluation task in the following manner:
The
first step is to run a “base log” , normally a resistivity log, which is used
to correlate formations with previous logs run in the local area. This base log
helps establish the structural position of the well.
Information
from the base log that indicates lithology or formation type is examined to
determine which zones have sufficient porosity and permeability to be of most interest
in production. The resistivity anomalies
are then further evaluated.
No
definite conclusions can be made at this point regarding the commercial value
of the well. Further information on porosity must be obtained in order to make
a quantitative evaluation.
Note:
The porosity logs only “respond to variation in porosity”. Therefore, logging
engineers are advised that the porosities calculated from tool response may be
subject to correction after further evaluation.
When
Lithology, resistivity, and porosity logs are available then the analyst has
sufficient information to proceed with a numerical analysis of porosity and
saturation. This data combined with
other geological information provides the basis for determination of the
commercial value of a well.
WHAT
ASTM says about Log Interpretation:
The American Society for Testing and
Measurement is actively involved in testing measurement processes and setting
acceptable standards.
ASTM, founded in 1898, is a developer
and publisher of technical information designed to promote the understanding
and development of technology and to ensure the quality of commodities and
services and the safety of products.
WELLOG strongly recommends the
following ASTM publication:
“ASTM standards on Ground water and
Vadose Zone investigations – Drilling, Sampling, Geophysical logging, Well
Installation and Decommissioning. “Stock #: Drill99
The website is http://www.astm.org
Quoting ASTM:
“The full potential of a logging
program cannot be realized until the logging measurements are interpreted. Log
interpretation should start at the time of data acquisition and should continue
as an iterative process through-out the project.”