Using resistivity data to study the waterflooding process: A case study in tight sandstone reservoirs of the Ordos Basin, China

Abstract

Water injection is one of the most common methods used to enhance the oil recovery of reservoirs with low permeability and strong heterogeneity. However, with the injection of water in the production process, the formation water salinity, oil saturation, and the distribution of the residual oil in the reservoir are altered, causing difficulties in the evaluation of the flooded layers. Consequently, it is crucial to understand the changes in reservoir resistivity when evaluating waterflooded layers with different wettabilities. Resistivity experiments for high-resistivity (HR) and normal-resistivity (NR) tight sandstone samples are carried out to simulate different types of waterflooding processes. Specifically, fresh water and the original formation water with higher salinity are separately used to displace oil in rock samples, including the HR and NR samples. Our results indicate that the salinity of injected water and the reservoir wettability can affect the reservoir resistivity after waterflooding. When fresh water is injected, the resistivity of NR and HR samples initially decreases, and then increases due to reduced oil saturation and increased salinity of the injected fresh water. In contrast, the resistivity of NR and HR samples decreases monotonically when the original formation water is injected. Our study also indicates that the formation water injection facilitates resistivity logging evaluation of high-resistance waterflooding layers better than fresh-water injection. To our knowledge, this is the first report of such a rich set of resistivity experiments in tight sandstone waterflooded reservoirs with complex wettability. Finally, combining waterflooding resistivity experiments with relative permeability, we have developed a fast, new, convenient method for evaluating the extent of waterflooding of a tight sandstone reservoir, which has been successfully applied to field-data examples for water-cut evaluation.