Study on Resistivity Response Characteristics Based on Digital Core Characterization of Occurrence Variation of Oil and Water in Heavy Oil Reservoirs

Abstract

Water production occurs in the heavy oil-oil layers of the third member of the Shahejie Formation in the Zhanhua Sag of the Jiyang Depression, with virtually no water-free production period. A contradiction exists between the “movable water” produced during extraction and the absence of movable water indicated by well logging responses. The reason lies in the unique “water-in-oil (w/o)” occurrence state within the heavy oil reservoirs, which not only leads to simultaneous oil and water production but also causes inaccuracies in saturation calculations using Archie’s equation, thereby affecting reserve assessment. Taking the third member of the Shahejie Formation in the Sanhecun Block of the Zhanhua Sag as an example, the study first extracts pores and matrix from computed tomography (CT) experimental data using threshold segmentation. Using morphological image analysis techniques, an erosion operation is applied to the pores to form a one-voxel-thick water film on the surface, representing water-wet characteristics. Next, the opening operation, which is an image analysis technique, is applied to the remaining pores after erosion to obtain the oil and water components, and the water saturation is calculated. Then, an erosion operation is applied to the oil component, and the remaining oil component after erosion is converted to water. Subsequently, a portion of the water component is extracted and subjected to the opening operation, yielding oil and water components. The extraction range as well as the structuring element (SE) radius of the opening operation was adjusted while keeping the water saturation of the entire digital core constant. Finally, the matrix, water film, the oil removed by erosion, converted water, remaining water after extraction, and the oil and water obtained from the opening operation of the extracted water are combined to form a “w/o”-type digital core. Use the finite element method (FEM) to simulate its resistivity (RT) response characteristics and then calculate the correction range for oil saturation. The results show that under the same RT, the oil saturation of the normal digital core is higher than that of the “w/o”-type digital core. The correction magnitude increases as porosity decreases. At a given porosity, the correction magnitude generally decreases with increasing RT. The correction range is 2.1–4.7%. This finding highlights the complexity of oil and water occurrence in heavy oil reservoirs and provides a basis for proposing subsequent saturation calculation methods and reserve assessment.