Abstract
Wettability is a significant factor in the exploration and development of shale oil. Currently, shale wettability has yet to reach a unified understanding. The contact angle is widely used in the study of shale wettability. However, the pre-treatment of the shale profoundly affects the contact angle. In this paper, the contact angle errors introduced by the pre-treatment of samples are discussed. Shale wettability is influenced by many factors, and there is not yet a systematic study of its influencing factors. Based on the above issues, the shale of the northern Songliao Basin was taken as the subject. The wettability of the different lithofacies is characterized by an improved contact angle method. The compositional characteristics of the shales and oil in the study area were analyzed. Fresh minerals, a single component of oil, and different temperature/pressure conditions were set up to investigate the influencing factors of shale wettability. The studies show that Organic matter abundance and thermal maturity have a positive correlation with oil-wet. Siliceous minerals are positively correlated with water-wet. Carbonate and clay minerals are negatively correlated with water-wet. The mineralogical composition of the shale, the composition of the oil, the characteristics of the aqueous media, the asphaltene deposits on the surface, temperature, and pressure all impact wettability. The affinity of minerals for hydrocarbons is iron minerals > carbonate minerals > clay minerals > siliceous minerals. Minerals are more hydrophilic at low salinity conditions. The deposition of non-hydrocarbons and asphaltenes renders the surface oleophilic. Increasing temperatures will reduce the hydrophilicity of the “oil-water-rock”.
Highlights
Shale wettability is the tendency for oil to expand or adhere to the pore surface (Roshan et al, 2016)
Single components of oil, and different temperature/pressure conditions were set up to investigate the factors influencing the wettability of the shale oil reservoirs
Oil Components The physicochemical properties of shale oil are related to its composition, which varies with the maturity of the organic matter
Summary
Shale wettability is the tendency for oil to expand or adhere to the pore surface (Roshan et al, 2016). The surface absorption by oil will change from hydrophilic to lipophilic (Drummond et al, 2004) This reduces the capillary resistance of oil entering the inorganic minerals pores, making it easier to enrich. The surface of shale pore throats is composed of very complex minerals and organic matter, and this composition makes the surface both oil-wet and water-wet (Yang et al, 2019). Asphaltenes in shale oil can break the water film on the surface of inorganic minerals in shale and adsorb to its surface, causing changes in wettability 5) Temperature and pressure conditions (Zhang et al, 2018). Referring to the delineation scheme of Liu (Liu et al, 2019) for the shales of the Qingshankou Formation in the southern Songliao Basin, the lithofacies were delineated in terms of the macrostructure (Figure 3), TOC, and mineral composition of the samples. The samples are divided into high organic matter (TOC ≥ 2 wt%), medium organic matter (1 wt% ≤ TOC < 2 wt%), and low organic matter (TOC < 1 wt%)
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