Relationship between reservoir mineralogy and optimum sandstone acid treatment

Mud acid treatments are normally designed by approximating the complex mineralogy of a sandstone using a 'lumping' procedure. Minerals are classified as either fast reacting or slow reacting and the rates of their reaction with hydrofluoric acid are determined by analyzing the acid effluent from acidized core plugs. For most treatments carried out at modest temperatures and at reasonable rates, this approach is satisfactory. In this paper, we show that at higher reaction temperatures, the simple two-mineral dissolution model does not apply because an intermediate product of the HF reaction with quartz, feldspars, and clays (H₂SiF₆) begins to react further with both clays and feldspars. This new reaction must be included to model the data. The additional reaction, not observed at lower temperatures, has important consequences when it does occur. For example, the acid injection rate is no longer a critical factor. In fact, slow rates in deep formations are preferred. The analysis presented here shows that retarded acids are unnecessary. Contrary to previous concepts, mud acid itself provides a deep penetrating capability. This surprising result may account for the high percentage of successful treatments even when carried out under a wide range of differing conditions.