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.