It is very difficult to evaluate shale wettability due to the large number and complex relationship of mineral components, and it is particularly important to identify the key factors affecting shale wettability for the efficient development of shale gas. Taking Longmaxi Formation shale in the southern Sichuan Basin as the research object, using a variety of experimental methods, this paper conducts qualitative analysis and quantitative characterization of the mixed wettability of shale, and establishes a model of mixed wettability to calculate the contact angle of organic matter and quantitatively evaluate the hydrophilic property of organic matter. Moreover, through the combination of analytic hierarchy process and grey correlation analysis, this paper establishes a multi-level evaluation system, clearly identifies the key factors impacting shale wettability, and forms a new evaluation method for shale wettability. The results show that in Longmaxi Formation the surface shale is characterized by mixed complex heterogeneous wettability being lipophilic, hydrophilic and oil-wet; the hydrophilic property of shale is positively correlated with the clay content. However, when the clay content is higher than a certain value, the hydrophilic property is inhibited; under formation temperature, the hydrophilic property of shale is stronger. With the increasing soak time of shale in liquid, the hydrophilic property of shale is gradually enhanced; there is an optimal liquid salt concentration making the surface contact angle of shale the smallest. The test range of KCl is from 4% to 6% under the current experimental condition; the average contact angle of organic matter is 73.8°, and its wettability is weakly hydrophilic, indicating that the water can enter into organic pores, but showing the weaker hydrophilic property as compared with inorganic pores; clay content has the greatest effect on shale wettability, followed by total organic carbon content, liquid salt concentration and temperature.
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2020, Vol. 41 
