Relationship of Drilling Mud Resistivity to Mud Filtrate Resistivity

Abstract

Abstract The effect of suspended solids on the resistivity of slurries is discussedand the relationship between drilling mud resistivity and mud filtrateinvestigated. It is concluded that it is erroneous to substitute mudresistivity for mud filtrate resistivity in electric log calculations. Arecommendation is made that both the bud resistivity and the mud filtrateresistivity be determined when electric logs are run. Introduction The electric log is influenced not only by the resistivity of the drillingmud in the borehole at the time of logging but also by the resistivity of thedrilling mud filtrate. Sherborne and Newton investigated the relationship of mud resistivity to mudfiltrate resistivity and concluded that, "The resistivity of the mud inmost cases closely approximates that of its filtrate," and "In fact, with the exception of Aquagel and its filtrate, the figures for any particularmud and filtrate are almost identical." Present practice is to determineonly the drilling mud resistivity and apply this same value to calculationsinvolving the mud filtrate. The purpose of this study is to reexamine thefactors governing the relationship between mud resistivity and mud filtrateresistivity. Effect of Borehole Fluid on the Electric Log Resistivity Log The resistivity log may be modified by the resistivity of the borehole fluidin two different ways:The apparent resistivity of a formation may be different from the trueresistivity of the formation because of the flow of some current through thedrilling mud in the borehole. Therefore the resistivity of the mud is animportant factor.The apparent resistivity may differ from the true resistivity, if aformation is invaded by mud filtrate, because of displacement by the mudfiltrate of some of the interstitial fluid in the formation. In this case theresistivity of the mud filtrate rather than the resistivity of the mud is theimportant factor. Self Potential Log The self potential arises, in part, from electrochemical effects resultingfrom the interaction of connate waters in porous formations and the fluid inthe borehole. A theory of the electrochemical component of the self potentialin boreholes has been recently set forth by Wyllie. In the above equationresistivities have been substituted for activities of the ions in the fluids.It is therefore apparent that the resistivity of the mud filtrate is morenearly representative of the activities of the ions than is the resistivity ofthe mud. However, it is possible that in some instances the ionic activities ofcations from certain clays may contribute to the total cationic activity of thedrilling fluid to such an extent that the mud resistivity is more nearlyrepresentative of the activities than the filtrate resistivity. This isparticularly the case when the resistivity of the mud is less than theresistivity of the mud filtrate. In addition the apparent self potential may be influenced by the resistivity ofthe drilling mud because of current flow through the borehole. T.P. 2512