Characteristics and Model of Heterogeneous Wettability in a Marine-Continental Transitional Shale Reservoir: Insights from Organic Matter, Minerals, and Microstructure

Abstract

Heterogeneous wettability is characterized by a spotted-wet or mixed-wet state. It has a great influence on reservoir evaluation and the efficient development of shale gas reservoirs. In this study, reservoir properties of several core plugs from the Permian Shanxi–Taiyuan formations were studied. These formations are typical marine-continental transitional shale located in the Southern North China Basin. To investigate the effects of reservoir properties on heterogeneous wettability, we measured the water–air contact angle and compared it with other properties such as organic petrology, organic geochemistry, mineralogy, and microstructure features, including pore-fractures and surface roughness. The results reveal a negative correlation of vitrinite content with contact angle, in addition to the high clay content and residual polar groups within type III kerogen, indicating that the Shanxi–Taiyuan shales with a high maturity level still have a higher affinity to water. The contact angle of the core samples decreases with increasing surface roughness, partially due to the influence of pore-fracture development. The crossplots indicate that the majority of pore-fractures that exist in the shale preferentially tend to be water-wet. Therefore, the heterogeneous wettability of shale is dominated by the random mixture and arrangement of hydrophilic and hydrophobic components as well as the complexity of the microstructure, such as the rough pore wall. Furthermore, on the basis of improving and examining the Cassie model, a triangle method and a detailed workflow for evaluating the heterogeneous wettability of shale are proposed by comprehensively analyzing “three factors”, including pore structure, mineral components, and organic matter. The test results demonstrate that the Shanxi–Taiyuan shales are mainly water-wet (controlled by organic factor) and neutral-wet, which evidently differ from the marine Longmaxi shale with a wide distribution covering the zones of strongly water-wet, weakly water-wet (controlled by pore factor), and weakly water-wet (neutral-wet). The proposed approach can be applied to promptly and comprehensively evaluate and predict the heterogeneous wettability of shales during shale oil and gas exploration.