Abstract
The interaction between shale and water is the key factor of wellbore instability in shale gas reservoirs. In this work, the influence of water on shale is studied from the perspective of water activity. Two kinds of shale samples from Sichuan Basin, Southwest China, were characterized using X-ray diffraction (XRD), total organic carbon (TOC) content tests, swelling ratio tests, field emission scanning electron microscope (FE-SEM), uniaxial compressive strength (UCS) and Nitrogen adsorption experiments. The relationships between mineralogical composition, TOC content, swelling ratio and water activity were studied respectively, and then the effect of water activity on microfractures was discussed. Based on the Frenkel–Halsey–Hill equation, fractal dimensions of shales were obtained by the Nitrogen adsorption experiments. And the relationship between water activity and various factors was analyzed. The results show that the water activity of shale is positively correlated with the total content of clay minerals (TCCM), especially the montmorillonite content. When the water activity of external solution was higher than that of shale, shale samples expanded due to chemical potential difference. The swelling ratio increased with increasing water activity of external solutions and shales. Shale swelling led to the development of microfractures, which in turn caused the decrease of strength. Therefore, under higher water activity conditions, shales were more prone to develop microfractures, and thus had lower strength. The adsorption data indicated that the fractal dimensions of shale samples ranged from 2.75 to 2.82, what was heterogeneous and irregular. And fractal dimension showed a good positive correlation with the water activity of shale. What is more, the study also shows that the use of low water activity working fluid in shale gas formation (especially in the formation with high water activity) can inhibit hydration and maintain formation stability.
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