Mechanism analysis of hydrochloric and acetic acids dissolving clay minerals

During acidification of the reservoir, the acid concentration will continually decrease as the reaction progresses. If the reaction time is long enough, the acid will eventually be exhausted. According to this phenomenon, the acidification of the reservoir can be regarded as the reaction between different concentrations of acid and minerals. Mineral dissolution is the common method for carbonate reservoirs to reform reservoirs and acid fracturing. In addition to carbonate minerals, carbonate reservoirs are likely to contain clay minerals, but less study has been done on acids with different concentrations and clay minerals. In this work, hydrochloric acid (HCl), acetic acid (HAC), and mixture acid (HCl + HAC) were used to dissolve chlorite, montmorillonite, kaolinite, and illite, respectively. The mechanism of acid dissolving clay minerals was analyzed by XRD, ICP-OES, and SEM. Moreover, we found that a high concentration of HCl has an anti-swelling and shrinkage effect on montmorillonite. The result shows that during the processes of HCl dissolving chlorite, montmorillonite and illite, there is SiO2 precipitate generated even at solutions’ pH of 0.5. In addition, chlorite may generate bischofite precipitate. Moreover, the effect of HCl dissolving chlorite is larger than that of other clay minerals. Also, by comparing the effects of various acids dissolving clay minerals, the effect of HAC dissolving clay minerals is lower than that of HCl, and the precipitate is not formed. The mixture acid dissolving clay minerals is better than that of single acid except for montmorillonite. For reservoirs with high chlorite content, the concentration of HCl can be appropriately reduced and the concentration of HAC can be increased; for reservoirs with high montmorillonite content, the concentration of HCl can be increased. Illite and kaolinite react weakly in HCl and hardly react with HAC.