Aspects of mud properties in geophysical logging of shallow water bores

Abstract

In order that the whole suite of geophysical logging methods may be employed, it is necessary that the borehole contain drilling mud. Usually in shallow water bores this mud consists simply of a clay component and make-up water, although during drilling it also contains particles and fluids derived from the formations penetrated. The major purposes of the mud, from the driller's point of view, are to lubricate and cool the bit, to prevent caving especially in unconsolidated sediments and to carry bit cuttings to the surface. While the mud composition may be relatively simple, the physical, electrical and chemical system comprising the mud column and formation is rather complex. Mud filtrate, a fluid extract of the mud, is expressed and enters the formations under the influence of differential hydrostatic pressure; consequently, a mud cake is formed on the borehole walls; and the filtrate itself provides electrochemical contrasts with the formation waters. The system is further complicated by the frequently unsystematic approach to the specification and control of drilling mud, and the general lack of understanding of the system as a dynamic entity. The volume of influence of all well logging methods therefore includes drilling mud, mud cake, invaded and uninvaded formation, these latter being wholly or partially saturated by filtrate and formation water respectively. For purposes of quantitative log interpretation, especially in regard to electrical logs, it is a necessary prerequisite that the electrical properties of the mud and its derivatives be either measured directly, or derived through established relationships with readily measured properties. A limited laboratory experimental study has indicated the relationships between the electrical and physical properties of the mud system and the influence of solids content. Field testing of these relationships showed that they are extensively modified in both form and magnitude by formation particles and waters acquired during drilling. It is apparent that relationships are strongly dependent on several factors, and should be evaluated separately for individual borehole/mud/formation systems.