The usefulness of the electrical resistivity log in determining reservoircharacteristics is governed largely by:
the accuracy with which the trueresistivity of the formation can be determined;
the scope of detailed dataconcerning the relation of resistivity measurements to formationcharacteristics;
the available information concerning the conductivity ofconnate or formation waters;
the extent of geologic knowledge regardingprobable changes in facies within given horizons, both vertically andlaterally, particularly in relation to the resultant effect on the electricalproperties of the reservoir.
Simple examples are given in the following pagesto illustrate the use of resistivity logs in the solution of some problemsdealing with oil and gas reservoirs. From the available information, it isapparent that much care must be exercised in applying to more complicated casesthe methods suggested. It should be remembered that the equations given are notprecise and represent only approximate relationships. It is believed, however, that under favorable conditions their application falls within useful limits ofaccuracy.
The electrical log has been used extensively in a qualitative way tocorrelate formations penetrated by the drill in the exploitation of oil and gasreservoirs and to provide some indication of reservoir content. However, itsuse in a quantitative way has been limited because of various factors that tendto obscure the significance of the electrical readings obtained. Some of thesefactors are the borehole size, the resistivity of the mud in the borehole, theeffect of invasion of the mud filtrate into the formation, the relation of therecorded thickness of beds to electrode spacing, the heterogeneity of geologicformations, the salinity or conductivity of connate water, and, perhaps ofgreatest importance, the lack of data indicating the relationship of theresistivity of a formation in situ to its character and fluid content.
On the Gulf Coast it is found that the effects of the size of the boreholeand the mud resistivity are generally of little importance, except when dealingwith high formational resistivities or extremely low mud resistivities.Fortunately, little practical significance need be attached to the exact valuesof the higher resistivities recorded. Low mud resistivities are not common, butwhen this condition is encountered it may be corrected by replacing the mudcolumn. With the present advanced knowledge of mud control, invasion of mudfiltrate into sands can be minimized, thereby increasing the dependability ofthe electrical log. The effect of electrode spacing on the recorded thicknessof a bed is often subject to compensation or can be sufficiently accounted forto provide an acceptable approximation of the true resistivity of theformation.