Abstract:
Shale gas is an unconventional natural gas resource, and the hydraulic fracturing technology is frequently used in the exploitation process of shale gas, resulting in a large amount of wastewater. The gas field water with high calcium and magnesium contents has some obvious characteristics, such as high calcium and magnesium contents, high turbidity, high chloride ion content and low pH value. Scaling problem is the most challenging task, especially the scaling problem of calcium carbonate in the post-treatment process of the water produced from shale gas. Due to the ignorance of the polycrystalline nature of calcium carbonate, it has been found that the prediction accuracy of the existing scaling prediction methods is low. Moreover, the influences of impurity ions on both the scaling crystal form and the prediction accuracy of index have not been explored yet. In this paper, the scaling potential prediction index was determined to provide effective guidance for the subsequent scale inhibition based on the investigation of the scaling behavior of calcium carbonate in brine with high calcium and magnesium contents. Through the analysis of the change of the key ion concentration before and after scaling and characterization of scaling by X ray polycrystalline diffraction and scanning electron microscopy, it was found that scaling occurred with bicarbonate dissociation in vaterite crystal form. According to Langelier's theory, the vaterite saturation index(VLSI) was determined by using the soluble product of vaterite and the activity coefficient of bicarbonate as the key parameters, which could predict the calcium carbonate scaling in the gas field water with more than 90% accuracy, and the prediction accuracy was higher than that of other calcium carbonate polycrystal saturation index, Chinese standard recommended saturation index and Chinese standard SDSI.