Oil-based mud electrical imaging logging responses of carbonate strata

ObjectivesThis study aims to characterize the oil-based mud (OBM) electrical imaging logging responses of carbonate strata, highlighting the response characteristics of dissolution vugs, fractures, stratigraphical structures, and special minerals. MethodsTests were conducted on logging data acquisition of a drilled well within the Dengying Formation in the Sichuan Basin before and after OBM was replaced with water-based mud (WBM). Based on the fine-scale core-to-log (CTL) calibration, this study compared the WBM and OBM electrical imaging logging responses of different geological and pore structures. Furthermore, the characteristics of OBM electrical imaging logging responses were further discussed from the measurement principles of the instruments. Results and Conclusions The results indicate that compared to the fullbore formation microimager (FMI)—a WBM tool, OBM tool Quanta Geo (NGI) exhibits a wider resistivity measurement range and higher dynamic focusing capability, thus reflecting more geological phenomena in images. The NGI tool proves sensitive to high resistivity, thereby clearly reflecting the internal structural characteristics of high-resistivity, tight massive strata. For low-resistivity geological features such as micritic laminae, argillaceous bands, and stylolites, both the NGI tool and FMI microimager show effective reflections, which are roughly consistent with the response characteristics in logging images. Nevertheless, the NGI tool presents more distinct textures and lithological boundaries. For open fractures, dissolution vugs, or larger-scale dissolution pores, they are predominantly manifested as high-resistivity bright patches in logging images obtained using the NGI tool. However, small dissolution vugs show both high-resistivity bright and low-resistivity dark responses on the dynamic and static logging images from the NGI tool. The complicated response patterns represent a key technical bottleneck restricting the transition of OBM electrical imaging logging from qualitative identification to quantitative evaluation.The research results provide a reliable geological-logging calibration basis for the qualitative identification and quantitative evaluation of carbonate fracture-cavity reservoirs under OBM conditions. They strongly support the industrial application of OBM electrical imaging logging technology in deep to ultra-deep carbonate formations in the Sichuan Basin, and promote the transition of this logging technology from the exploratory testing stage to large-scale production applications.