Mud acid, which is composed of HCl and HF, is commonly used to remove the formation damage in sandstone reservoirs. However, many problems are associated with HCl acid, especially at high temperatures. To overcome many of these drawbacks, organic-HF acids have been used as an alternative to mud acid. However, very limited research has been performed to reveal the reactions between organic-HF acids and minerals in sandstone reservoirs.

In this study, formic-HF and acetic-HF acids were examined to react with various clay minerals (kaolinite, chlorite, and illite), in comparison with mud acid. A series of acid mixtures with different ratios and concentrations were tested. Inductively coupled plasma (ICP), scanning electron microscopy (SEM) and 19F nuclear magnetic resonance (NMR) were employed to follow the reaction kinetics and products. Core flood experiments on sandstone cores featured with different mineralogy, with dimensions of 1.5 in. × 6 in. were also conducted at a flow rate of 5 cm3/min. The core effluent samples were analyzed to determine concentrations of Ca, Mg, Fe, Si, and Al by ICP.

Both formic-HF and acetic-HF acids are much milder than mud acid. The species and amounts of reaction products of different clay minerals in organic-HF acids depend on mineral type, acid composition, and ratio. This conclusion is further confirmed by core flood experiments, in which sandstone cores with different mineral compositions give quite different responses to the same acid mixture. This paper will discuss the detailed chemical reactions that occurred within cores and were followed by chemical analysis of core effluent samples.

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