Evaluation of pore-scale wettability in the tight sandstone reservoirs of the Upper Triassic Yanchang Formation, Ordos Basin, China

Wettability is a key factor influencing multiphase fluid distributions and flow behavior in the reservoirs. The microscopic characteristics of the mixed wettability which is a special wettability type are unclear in sandstone reservoirs. An integrated study of petrographic, petrophysical and pore-scales wettability by Environmental Scanning Electron Microscope (ESEM) using the water condensation method were carried out on typical sandstone samples in the Chang-6 interval of Yanchang Formation, southeastern Ordos Basin, and the characteristics of mixed wettability were investigated. It shows that the oil-free sandstones are water-wet in general, while the oil-bearing sandstones are apparently mixed-wet. In the mixed-wet sandstones, the walls of different pores or even the same one pore display different wetting states. The mixed wetting nature of the tight sandstones was probably caused by the early phase oil charge. After oil charged into the reservoir, chlorite surfaces in contacting with pores adsorbed the polar compounds in the crude oil, and their wettability was altered locally from water-wet to oil-wet states. However, pore walls surrounded by quartz and feldspars mainly remained water-wet after oil charge.