A simple approach to mud invasion correction for dual laterolog resistivities is proposed in this paper. This approach was derived from an idealized formation model for laterolog resistivity measurements. In this model, the measure current is assumed to flow horizontally into the formation and the invasion radius is assumed to be less than the radial depth of investigation of the shallow-laterolog. Instead of a "pseudo- geometrical factor", the tool calibration coefficient was introduced into the laterolog response equations. As a result, the correction for the mud invasion effect on the laterologs can be made without knowing the diameter of mud invasion into the formation. Once the true resistivity of the un-invaded formation is determined from the dual laterologs, the invasion diameter can be further estimated for medium- to high-porosity formations.
The invasion of mud fluid into porous, permeable, rocks during the drilling process is generally unavoidable. This invasion, which may range from a few centimeters in high-porosity zones, to a few meters in low porosity zones, has a significant influence on the resistivity of the porosity zones, has a significant influence on the resistivity of the formation. As a result, an invasion profile is usually observed as illustrated in Figure 1. For matters of simplicity, a step-profile is assumed in Figure 1 where the formation is laterally divided into five segments: the borehole, the mud cake, the flushed zone, the transition zone, and the un-invaded zone. The measured resistivity is usually considered to be the sum of the weighted values of the resistivities of each segment. As one of the major parameters in formation evaluation, the true formation resistivity is critical to determine the hydrocarbon saturation in the uninvaded formation. However, since most wireline tools are exposed to the distorted formation by the borehole fluid and its invasion into the formation, the apparent resistivity readings recorded by wireline logging tools must be corrected for these effects.
Wireline logging companies have attempted to minimize the effects of borehole fluid and mud fluid invasion on their resistivity tool designs. The laterolog tool is so designed that the borehole fluid effect is eliminated by forcing the measuring electric current horizontally into the formation.