Regular mud acid, which is composed of HCl and HF, has been extensively used to remove the formation damage in sandstone reservoirs; however, many problems may occur during stimulation treatments with this acid. To overcome many of these drawbacks, sandstone acid systems (HF and a phosphonic acid) have been used as an alternative to mud acid. However, very limited research has been performed to investigate the interactions of sandstone acid systems with clay minerals in sandstone reservoirs.
In this study, sandstone acid with different HF concentrations was used to evaluate the solubility of various clay minerals (kaolinite, bentonite, chlorite, and illite) as a function of time, and temperature. The Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) techniques were used to identify the reaction products. The concentrations of key cations in the supernatant were analyzed using ICP. 19F Nuclear Magnetic Resonance (NMR) spectroscopy was utilized for the first time to determine the reaction of sandstone acid with clay minerals.
Sandstone acid did not leach aluminum from chlorite. No AlF3 precipitate was identified by SEM analysis of the precipitates. The secondary reaction between clays and HF occurred at high temperatures and, as a result, the ratio of Si/Al decreased. 19F NMR results showed that all HF acid in the sandstone acid solution reacted with clays at a weight ratio of 10:1, while HF was detected in the sandstone acid solution after it reacted with sand particles for 24 hours at the same weight ratio. This work examined interactions occurring with the reaction of sandstone acid with clay minerals that have not been discussed before, and highlighted the effect of acid strength, reaction time, and temperature on the performance of sandstone acid.