Analysis Of Environmental Corrections For Propagation Resistivity Tools

Measurement While Drilling (MWD) propagation resistivity tools require several environmental corrections to achieve accurate results. The environmental effects that require correction include borehole effects, invasion, dielectric effects, anisotropy, and effects of nearby formations. An effective strategy has been developed to address each correction even when more than one affects a given data set. This strategy starts with a borehole correction followed by deconvolution. Invasion, anisotropy, and dielectric effects are then evaluated simultaneously. These three effects are evaluated together because errors result if they are evaluated separately. Analyzing one effect alone is an attempt to explain all of the differences in the data curves with that one effect. This attempt will fail when other effects contribute to separation of the data curves. In addition, the improperly corrected data will not be easy to interpret. The best-fit solution to the data set must be found with a model that includes all three effects. This strategy has been tested using both computed data and field data. The results show that anisotropic effects often cause the largest residual error in highly dipping boreholes. In highly resistive pay zones, the dielectric effects cause a significant error, but these effects can usually be removed from the data during the processing. Residual errors due to side bed effects are usually small (that is, the deconvolution works well). When the relative dip angle is more than 70 degrees, however, the deconvolution does not work well and residual errors are large. Invasion effects can usually be eliminated from the data resulting in an accurate measurement of Rt, but the values of Rx0 and are often difficult to determine with the shallow or nonexistent invasion typical of MWD logging. After analysis, corrections are made for all effects except invasion. The data are then displayed at four different fixed depths of investigation for quantitative identification of any invaded zones.