A new electrical imaging logging method in oil-based mud for low-resistivity formation based on concave electrode couples

Abstract

Electrical imaging logging can provide high-resolution information and occupies a pivotal position in reservoir exploration and evaluation. To realize clear electrical imaging in high-resistivity oil-based mud (OBM), researchers have tried various methods. In the existing methods, there are some worrisome issues such as the challenge in clear imaging in low-resistivity formation, the complication and robustness in the inversion of quantitative interpretation and few obtained parameters etc. In this paper, a gratifying way with the help of concave electrode couple is presented and analyzed particularly to focus on the quantitative evaluation in low-resistivity formation. The imaging pad structure and its work principle are first introduced. Then, a systematical and mathematical statement is carried out to obtain desired formation resistivity and standoff between imaging pad and borehole wall. Furthermore, sensitivity analysis is implemented to make clear the performances of measured values including equivalent formation resistance, tool factor and calculated standoff under the influence of multiple parameters. Finally, several imaging tests are shown to demonstrate the effectiveness of this new method presented in this paper. The results shown that profiting from the change of traditional electrode array to concave electrode couples, the formation resistivity and standoff between the pad and borehole wall in low-resistivity formation can be measured simultaneously. Another major advantage of this new method is that the unknown OBM parameters are unnecessarily mastered, and the entire process is free from complicated iterative computation. This study provides another new choice for the electrical imaging logging in OBM.