Evaluation of Organic-Hydrofluoric Acid Mixtures for Sandstone Acidizing

Mud acid, which is a mixture of hydrochloric (HCl) and hydrofluoric (HF) acids, is typically used to stimulate sandstone formations. The success rate of mud acid is limited mainly because of its high spending rate, high corrosion rate, and incompatibility with HCl sensitive minerals; i.e., illite. Organic (acetic, formic and citric) HF acid mixtures, in comparison, have a retarded nature and a low corrosion rate, and are compatible with sensitive sandstone formations; therefore, they can be used as an effective stimulation fluid alternative to regular mud acid. In this study, dissolution tests of kaolinite, illite or chlorite in acetic, formic and citric with 3 wt% HF acid was conducted. A coreflood was conducted to investigate the ability of organic acids, when combined with HF acid, to remove carbonaceous and siliceous minerals from Berea sandstone plugs at 250 °F. The core effluent samples were analyzed using inductively coupled plasma (ICP). The core plugs were analyzed using the X-ray diffraction (XRD). All of the tested organic acids were effective in dissolving calcium during the coreflood preflush stage without any indication of precipitation; however, only citric acid extracted aluminum during the preflush, while formic and acetic acids were not effective in leaching aluminum. Additionally, low amounts of aluminum were detected in the effluent of acetic-HF during the main flush and severe damage to the core plug was noticed. Although formic-HF acid extracted a significant amount of calcium and aluminum, there was no change in permeability before or after the treatment. Citric-HF acid dissolved a significant amount of calcium and aluminum, and an apparent increase in permeability was subsequently observed. In addition, a higher permeability ratio was obtained with low HF acid concentrations. This work provides new insights into the applications and potential limitations of organic-HF acids.