Research on Fine Characterization Method of Fractures and Vugs in Carbonate Reservoirs Using Electric Imaging Based on Electric Field Calibration: A Case Study of Penglai Gas Field, Sichuan Basin

The identification of fractures and vugs using micro-resistivity imaging logging constitutes a critical approach in reservoir evaluation. Carbonate reservoirs exhibit complex storage space types (with developed pores, vugs, and fractures) and diverse rock structures. The dark low-resistivity features observed on imaging logs may correspond to various geological phenomena such as fractures, vugs, structural characteristics, or special mineral fillings. In the absence of core calibration, using traditional grayscale threshold segmentation for image recognition of dissolution fractures and vugs can easily lead to misjudgment, resulting in overly optimistic assessments of reservoir quality in logging interpretations. As micro-resistivity imaging reflects variations in the micro-resistivity of wellbore lithology, in order to clarify the factors that cause resistivity changes, it is necessary to calibrate the imaging (wellbore) electric field characteristics with the surface electric field of the rock core. This study first conducted macro-micro resistivity measurement experiments on rocks, revealing that the primary factors affecting rock resistivity relate to mineral composition variations and structural changes. Subsequently, a novel full-diameter core surface resistivity scanning and imaging device were developed to continuously and automatically measure the electric fields of core surface, using this device the surface electric field distribution imaging of the core is quickly generated, then a geological-imaging features matching relationship can be established. Finally, the electric field gradient skeleton line method is employed to quantitatively characterize electric field distributions, summarize the quantitative identification and classification modes of different geological objects, improve the accuracy of effective fracture identification and quantitative calculation, and provide key basis for reservoir effectiveness evaluation. The developed core scanning imager demonstrates significant application value in petrophysical experimental research.