A New Simultaneous Anisotropy And Dielectric Correction Algorithm For Lwd Resistivity Measurements

A correct resistivity measurement is crucial to an accurate formation evaluation. Wireline resistivity measurements are often affected by invasion, shoulder bed properties, and environmental effects, such as borehole size and tool position. Though Logging While Drilling (LWD) resistivity measurements may be affected less by these factors, the use of a high frequency (.4 to 2 MHz) induces other unwanted effects in the LWD resistivity measurements. The most significant effects are due to the dielectric constant and the resistivity anisotropy of the formation. They may affect log response even in homogeneous formations when other effects are small. These two effects can be present simultaneously. This paper demonstrates a new algorithm that simultaneously corrects for both dielectric and anisotropy effects when using multiple-spacing LWD resistivity measurements. This algorithm determines the true formation (horizontal) resistivity, the resistivity anisotropy, and the formation dielectric constant simultaneously. Several field examples are presented that demonstrate when the effect of the dielectric constant and the resistivity anisotropy, or both, had to be corrected to obtain the true formation resistivity from the LWD measurements. The results are compared with those without simultaneous corrections and where only enough resistivity data are available to correct for variations in a single formation property, either the dielectric constant or the formation resistivity anisotropy.