Introduction to Induction Logging and Application to Logging of Wells Drilled With Oil Base Mud

Abstract

Abstract A new logging method, called induction logging, is described; it measuresthe conductivity, or resistivity, of the strata traversed by a bore hole. Theapparatus, which is briefly described, comprises a coil system coupled with theground by induction, so that there is no need for direct contact with the mud, or with the formations. For that reason, the method is particularly useful inoil base mud, and it was first applied to that case. A discussion concerning the respective contribution of each region of groundin the vicinity of the coil system is given. Through the concept of ageometrical factor, it is demonstrated that the total signal, which measuresthe apparent conductivity, is the summation of the partia1 products -geometrical factor by conductivity - for the different regions of the groundunder consideration. It is thus possible to establish, for each type of coilarangement, investigation characteristic as well as departure curves andcorrection charts for beds of various thicknesses. This should greatlyfacilitate the quantative interpretation of the field logs, even in cases wheresuch interpretation would have been difficult on electric logs taken withelectrodes. It is also shown that focussing combinations of coils can be used, whichgive, directly on the recorded log, a value close to the true conductivity, even for fairly thin beds, thus eliminating in most cases the need forcorrections. These focussing systems also give a sharper definition of theboundaries between successive strata. T.P. 2641