Experimental Investigations of the Mineral Wettability in Shale and its Influence Factors

exploration in North American, multiple continental oil fields in China have vigorously put forward its evaluation and exploration of shale oil. However, the outcomes of shale oil drilling in China are not well as expected. One of the important reasons is that the fluidity of shale oil is poor, resulting in the low effectiveness of production. The shale oil fluidity is, to some extent, determined by its occurrence status, which is influenced by not only the pore structure characteristics, but also the reservoir medium or surrounding rock. It means that occurrence status is closely related to the oil-water-rock interaction. Wettability is a comprehensive parameter to characterize the rock-fluid interaction, and it has important significance in understanding the characteristics and mechanism of oilwater occurrence in shale. The previous studies have mostly focus on the wetting characterization of quartz and calcite to represent the sandstone and carbonate reservoir, respectively. It will cause error when directly applying those results to the shale that has complex mineral composition. In addition, the effect of the chemical properties of formation water on the wettability is still controversial (Berg et al., 2010). The purpose of the work is to investigate the wetting characterization of six common minerals in shale by measuring the contact angle of droplet in the mineral surface in air (sessile drop) and in water (pendant drop). The effect of calcium chlorideand sodium bicarbonatetypes of formation water and salinity on the contact angle in ‘oil component-formation water-mineral’ three-phase system have also been stressed. 1. The contact angle between distilled water in the air and the common minerals (i.e., quartz, calcite, dolomite, feldspar, pyrite) ranges from 22.05 to 55.37, indicating that the clean surface of non-clay minerals are hydrophilic, but with different degree of hydrophilicity. As a result of high hydrophilicity or permeability, the distilled water in the albite surface spreads quickly and permeates into the orebody. Kerosene, N,N-Dimethyldodecylamine (NNDMDA), 3-Dodecylthiophene represent the oil component of low carbon alkane mixtures, nitrogen compound and sulfur compound, respectively. These oil components infiltrate the minerals in the air, and spread on the mineral surface quickly. For a given mineral, the contact angles between the liquid and minerals are decreased with decreasing liquid surface tension. In our experiments, the polarity and surface tension of kerosene, NN-DMDA, 3Dodecylthiophene increases gradually, resulting in the reducing extent of spreading. 2. The pendant drop method was used to measure the contact angle between kerosene, NN-DMDA, 3Dodecylthiophene and mineral surface in aqueous phase. The contact angle is defined between the water-oil interface and the solid surface, and is measured though the high polar phase, in this case water. The contact angles of all the experiments are less than 90o indicating that the minerals are hydrophilic, and the interfacial tension between water and mineral are smaller than that between oil components and mineral. In oil-water systems, the wettability of the rock minerals can vary with the oil composition properties. The contact angles between NN-DMDA with minerals are the highest in our experiments, indicating that the NNDMDA has the best affinity with mineral. 3. The adsorption of polar components of crude oils onto the mineral surface in aqueous phase is sensitive to the brine type and salinity (Anderson, 1986; Shabib et al., 2014). This work discusses the influence of brine type and salinity on the wettability of minerals with NN-DMDA. In the presence of aqueous solutions of calcium chlorides, the contact angles are less than 90 for all the minerals. The contact angles of quartz, potassium feldspar, dolomite and XIE Liujuan, LU Shuangfang, LI Junqian, HU Yongjie, ZHANG Pengfei, CHEN Jianjun and ZHANG Ping, 2016. Experimental Investigations of the Mineral Wettability in Shale and its Influence Factors. Acta Geologica Sinica (English Edition), 87(supp.): 170-171.