A Composite Lithology Log
While Drilling
Eli Tannenbaum, Brod Sutcliffe, Andrew Franks
A new method for producing a computerized composite lithology log
(CLL) while
drilling by integrating MWD (measurement while drilling) and surface data is
described.
At present, lithology logs are produced at the well
site by mud loggers. They
provide basic description and relative amounts of lithologies. Major
difficulties are encountered in relating the cuttings to their original
formations due to mixing in the drilling mud while transporting to the surface,
sloughing shales, flawed sampling, etc. This results in a poor control on the
stratigraphic
sequence
and on the depth of formation boundaries. A composite
log
can be produced after drilling with additional inputs such as wireline,
petrography, and paleontology. This process is labor intensive and expensive.
The CLL integrates three types of data (MWD mechanical, MWD geophysical, and
surface cuttings) acquired during drilling, in three time stages: (1) Real Time.
MWD drilling mechanical data including the rate of penetration and the downhole
torque. This stage would provide bed boundaries and some inferred lithology.
This would assist the driller with immediate drilling decisions and determine
formation tops for coring, casing point, and correlation. (2) MWD Time.
Recomputation of the above by adding MWD geophysical data (gamma-ray,
resistivity, neutron-density). This stage would upgrade the lithology inference,
and give higher resolution of bed boundaries. (3) Lag Time. Detailed analysis
of
surface cuttings to confirm the inferred lithologies. This last input will
result in a high-qualit CLL with accurate lithologies and bed boundaries.
The log
will serve the geologist as
well
as the driller, petrophysicist, and
reservoir engineer. It will form the basis for more comprehensive formation
evaluation while drilling by adding hydrocarbon and MWD
log
data.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.