Acidizing Sandstone Reservoirs With Aluminum-Based Retarded Mud Acid

Abstract

Summary Aluminum chloride (AlCl3) has been used as a retarding agent for mud acid for a long time; its applications are studied in the laboratory and tested in the field. The theory and mechanism of AlCl3 retardation were investigated in many works involving mud acidizing and reservoir-permeability enhancement. This paper furthers this investigation with solubility tests, coreflood tests, and 19F nuclear magnetic resonance (NMR) to better understand the mechanism of AlCl3 working as a retarding agent in mud acid. The reactivity of Al-based retarded mud acid (15 wt% HCl, 1.5 wt% HF, and 5 wt% AlCl3·6H2O) with clay minerals and sandstones at different conditions has not been examined fully. To enhance the acid performance and to minimize formation damage, a systematic investigation of the interactions between the Al-based retarded mud acid and clay minerals in sandstone reservoirs is provided in this study. Furthermore, for the first time, 19F NMR spectroscopy was used to follow the reactions of Al-based retarded mud acid with clay minerals. Solubility tests were performed to evaluate the retardation of the Al-based retarded mud acid when reacted with kaolinite, bentonite, and illite. Inductively coupled plasma (ICP) and 19F NMR were used to analyze the concentrations of key cations and components in the supernatant, whereas the scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to identify the reaction products and to explore the possibility of the presence of any precipitation. Coreflood tests of sandstone cores were also conducted. This study shows that AlCl3 can retard the reaction of hydrofluoric acid (HF) with kaolinite, bentonite, or illite at 75 and 200 °F in Al-based retarded mud acid. Even with 5 wt% AlCl3·6H2O added in the acid system, no AlF3 precipitate was observed in any of the solubility tests. 19F NMR results showed that AlF4− and AlF3 were the only Al-F species existing in the spent Al-based retarded mud acid. H2SiF6 and HSiF5 were also identified. Coreflood tests showed significant permeability improvement to Berea sandstone when Al-based retarded mud acid was used, and the enhancement diminished when the temperature increased to 300 °F. Computed-tomography (CT) scan showed deeper penetration of Al-based retarded mud acid than mud acid at 75 °F, and the penetration reduced when temperature increased to 200 °F. On the basis of these results, new mechanisms were developed to better understand the reaction of Al-based retarded mud acid and clay minerals.