Correlation of Borehole Images in Conductive and Non-Conductive Muds to Understand Carbonate Heterogeneity: Case Study from Sichuan Basin, China

The heterogeneous carbonate reservoir is the most important exploration target in Sichuan basin. It is challenging to identify the potential flow units in the tight carbonate reservoir. High-resolution borehole resistivity image can reveal the dissolution features such as vugs or fractures in the pay zone. Operators in Sichuan Basin have acquired lots of borehole resistivity image using water-based mud imager in key exploration wells to identify potential reservoir. The oil-based mud has become increasingly popular in exploration and development because it reduces complex drilling practices reducing drilling costs. The new generation oil-based imager is used in these new wells. The borehole resistivity image can be obtained from both OBM (oil base mud) imager and WBM (water base mud) imagers. It is important to understand the response differences through the comparisons between different borehole imaging technologies in the same wellbore. The objective of this study is to understand the responses and refine the rule of oil-based mud borehole image interpretation in tight carbonate reservoir. The operator has logged both WBM and OBM imager in the same wellbore at different times. The WBM resistivity image is acquired in water-base mud conditions in the beginning. Then drilling fluid system was changed to OBM, and 200 meters were drilled further in the borehole. Finally, the OBM imager was logged in the hole. A comparison between the WBM resistivity images and OBM resistivity images are carried out. Increased understandings of the image log responses in this vuggy or fractured carbonated reservoir were obtained. The borehole resistivity image from WBM and OBM imager is highly correlatable over most of the logging interval as they both show similar resistivity response (bright / dark) on both images for the same geologic feature. The new generation OBM resistivity imager is proven to have similar applications as the WBM resistivity could offer. In fact, the OBM borehole images has higher resolution than the WBM borehole image in most of the intervals, especially in high resistivity formations. It was found that the responses of vugs or fractures can be interpreted slightly different in some intervals. Only the unfilled vug will have resistive response in OBM image while it will be very conductive in the WBM image because it was invaded by the resistive or conductive mud. The responses of induced fractures or natural fractures will be slightly different, which is influenced by formation conditions such as Rt/Rm. The induced fracture could be conductive or resistive under different logging conditions. This case study improved the understandings of the OBM imager response in tight carbonate reservoirs. These observations can be used as a reference for borehole image interpretation in similar reservoirs.