Are the LWD Resistivity Logs Telling the Whole Story?

Resistivity logs are crucial data in formation evaluation, as they are used to compute water saturation, an input parameter for hydrocarbon resource assessment. Therefore, it is important that these logs are reliably and accurately measured. Resistivity logs are acquired by logging tools usually conveyed either by electric wireline or Logging While Drilling LWD equipment. These measured resistivity logs need to be corrected for various borehole, environmental and other effects to derive the true formation resistivity. Wireline conveyed resistivity tools are usually run in wells under static condition, where the mud invasion process has virtually ended. However, the LWD resistivity logs, acquired under dynamic well conditions while the well is being drilled, are subjected to more perturbing effects than the wireline resistivity logs. The industry standard LWD resistivity tool uses the Electromagnetic Wave Propagation principle, which measures the attenuation and phase shift of the EM wave as it travels in the formation. These measurements are then converted into formation resistivity, using appropriate transforms. It is therefore important to understand the measurement principle, accuracy and range, depth of investigation and vertical resolution of the LWD resistivity logs. The various environmental effects influencing the LWD resistivity logs, such as borehole, eccentricity, polarization horns, invasion, formation anisotropy, thin and adjacent bed effects should also be taken into account when using LWD resistivity logs. Sometimes, the LWD resistivity logs may exhibit strange or anomalous behaviour due to some of these environmental influences. In certain cases, if the causes of these spurious logs are not properly understood, they may be misinterpreted, which can lead to wrong and costly decisions. In this paper, some case histories of strange looking LWD resistivity logs and the explanations behind their behaviour are presented. These potential issues were resolved by working closely together with the LWD service provider.