高钙镁气田水中碳酸钙结垢行为研究与趋势预测

Scaling Behavior and Scaling Potential Prediction of Calcium Carbonate in Gas Field Water with High Calcium and Magnesium Contents

高钙镁气田水具有高钙镁离子、高氯根离子含量、高浊度和低pH等显著特点,在后续处理中易发生碳酸钙结垢现象。通过分析结垢前后钙离子和碳酸氢根离子的浓度变化,发现高钙镁气田水中碳酸钙的结垢类型为碳酸氢根型结垢,经X射线衍射(XRD)和扫描电子显微镜(SEM)表征分析后确定结垢物为球霰石型碳酸钙。以朗格利尔理论为背景,以球霰石溶度积和碳酸氢根离子活度系数为相关参数建立了基于球霰石饱和指数的碳酸钙结垢趋势预测模型,并在实际水结垢中实现精准预测,准确率达90%。

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.