Abstract

Formation evaluation, based on logs, conventional cores, sidewall cores, cuttings, wireline formation testing, or other methods, is in every case inevitably affected by the process of invasion of mud components into the formation. Examples of the effects of fluids invasion, fines invasion, and in the case of oil muds, possible emulsion breakdown on wireline logs can be used to diagnose problems with the mud system.

The properties of some commonly used mud systems are discussed from several different points of view. The selection of mud type and adjustment of mud properties for optimisation of drilling, formation evaluation, and eventual production are discussed. The compromises in mud properties which are frequently made, but which may not be clearly recognised are pinpointed.

The often overriding factor in mud type and properties selection is mud cost. The relative costs, which include indirect costs in both evaluation and eventual production efficiency, may not be fully accounted for. Insufficient or imprecise formation evaluation from logs in some mud systems exemplify such indirect costs.

The components of some commonly used mud systems may be adjusted to give a best compromise in terms of drilling, evaluation, and production, and still be compatible with overall cost optimisation.