there is an increase in susceptibility opposite zones of cavitation. If the drilling 

 mud had a very low susceptibility, the susceptibility log would have decreased 

 opposite zones of cavitation. To aid in this interpretation, a drilling-mud sus- 

 ceptibility measurement is made on a circulated-mud sample. Although differ- 

 ences between sands and shales can be measured, the most significant anomalies 

 are generally associated with permeable zones or zones containing oxides of iron. 

 Magnetite is the dominant oxide of iron that produces high magnetic suscepti- 

 bility. 



The good correspondence between the self-potential log and the suscepti- 

 bility log suggests that enrichment of iron oxides in these zones has marked 

 them. This concept has prompted the use of magnetic trace material in drilling 

 muds to mark permeable zones or zones of lost circulation. The procedure 

 involves adding a finely divided black iron oxide to the mud system. Such a 

 tracer material is known commercially as "Magnafloat Grade B" and is marketed 

 by the Foote Brothers Mineral Company. It is a black iron oxide which is 

 primarily magnetite, with 90 percent of the particles less than 10 microns in 

 diameter. The weight of the drilling mud must be kept high enough that the 

 hydrostatic head of the fluid column in the bore hole is greater than the natural 

 pressure on the fluids in the formation. Under this condition some of the drilling 

 fluids will tend to enter permeable formations. The depth of entry of whole 

 mud into a reservoir rock is usually small and varies with pore size and the 

 bridging properties of the mud. Once the pores are bridged, the filter cake 

 builds up on this bridge as a result of filtration. This process continues, with 

 the total volume of filtrate being proportional to the square root of time if the 

 filter cake is undisturbed. Naturally, the abrasive action of the drill bit and pipe 

 will limit the maximum thickness. The rate of filter invasion and increase in 

 thickness of the filter cake are a factor of filter-cake permeability and thickness. 

 Therefore, the build-up is a function of the mud properties and not a quantita- 

 tive measure of either permeability or porosity of the formation. No filtration 

 occurs, and therefore no filter cake is formed on the wall of the borehole, 

 where impermeable rocks are penetrated. Mud solids are found in a concentration 

 of two or three times to as much as thirty to forty times their concentration in 

 the mud. Consequently, the filter cake formed by a mud containing iron-oxide 

 powder will be proportionally high in magnetic susceptibility. This phenomenon 

 is very similar to the theory of operation of the microlog. 



Because tracer methods rely on variations in the magnetic susceptibility of 

 the filter cake, the contact magnetic susceptibility tool (Minimag) is most use- 

 ful in such measurements. An example of a tracer-log study is shown in Figure 

 22-5. Other standard electric and radioactive logs are included, as well as 

 permeability and porosity determinations from core analysis. During the drilling- 

 operations, circulation was lost when the bit was at 2030 feet. This circulation 



432 



