Application of conventional propagation resistivity logging for formation boundary identification in geosteering

Abstract

Previous studies indicate that the polarization horn effect of conventional propagation resistivity (CPR) logs will occur once the logging tool is close to or passes through the boundary between high conductivity contrast layers with a small relative angle. Polarization horns cannot be used to distinguish between the top or bottom boundary. This situation disturbs the application of CPR in geosteering. Considering the cycle sequence features in sedimentary formation, e.g. fining upward sequence, coarsening upward sequence and massive cycle, they can be identified by analyzing the shapes of spontaneous potential (SP) or gamma-ray (GR) logs in logging geologic analysis. Previous studies mostly focused on the ideal medium formation models, which can only represent the massive cycle. Thus in this study, the fining upward sequence and coarsening upward sequence formation models are established, and the logging responses of CPR in the two formation models are simulated under the condition of horizontal well. The simulation analysis shows that polarization horns will either occur at the top boundary in the coarsening upward sequence or at the bottom boundary in the fining upward sequence, if the influence of formation thickness and mud invasion is ignored. The occurrence probability density chart is plotted to determine that polarization horns will occur in what formation resistivity contrast and deviation conditions. In order to identify the top or bottom boundary in sedimentary formation using CPR, a convenient method is established to facilitate geosteering. Firstly, the cycle type of the subject reservoir should be determined by analyzing the SP or GR logs of the offset wells. Secondly, the top boundary in coarsening upward sequence and the bottom boundary in fining upward sequence can be identified by using the occurred polarization horns of CPR logs. Finally, when the tool is close to the boundary, the rough distance from CPR tool to the top boundary (in coarsening upward sequence) or to the bottom boundary (in fining upward sequence) can be obtained. When the tool crosses the boundary, the logging measured depth of the corresponding boundary can be obtained. Two field geosteering examples can verify the two functions of the method, respectively. Based on cost-effective CPR logs, the method contributes to geosteer in sedimentary formation.