Phosphonic-Based Hydrofluoric Acid: Interactions With Clay Minerals and Flow in Sandstone Cores

Abstract

Summary Regular mud acid, which is composed of hydrochloric acid (HCl) and hydrofluoric (HF) acid, has been extensively used to remove formation damage in sandstone reservoirs; however, many problems may occur during stimulation treatments with this acid. To overcome many of these challenges, a phosphonic-based HF acid system has been used as an alternative to mud acid. However, very-limited research has been performed to investigate the interactions of phosphonic-based HF acid with clay minerals in sandstone reservoirs. In this study, a phosphonic-based HF acid system with two HF concentrations (1.5 and 3 wt%) was used to evaluate the solubility of various clay minerals (kaolinite, bentonite, chlorite, and illite) as a function of time and temperature. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) were used to identify of the reaction products. The pH of the solutions was measured by use of an HF-resistant electrode. The concentrations of key cations in the supernatant were analyzed by use of inductively coupled plasma optical emission spectrometry. The 19F, 31P, and 27Al liquid nuclear-magnetic-resonance (NMR) spectroscopy experiments were used for the first time to evaluate the reaction of this specific phosphonic-based acid with clay minerals. Coreflood experiments on Berea sandstone cores were conducted at 300°F and a flow rate of 2 cm3/min by use of full-strength phosphonic-based HF acid (3 wt% of HF) and mud acid (12 wt% of HCl and 3 wt% of HF). No aluminum fluoride (AlF3) precipitate was identified by EDS and X-ray-diffraction analyses of the solid samples after kaolinite, bentonite, and illite reacted with full-strength phosphonic-based HF acid. Large amounts of AlF3 were noticed in the chlorite samples after being treated with a full-strength phosphonic-based HF acid. The concentration of soluble silicon decreases in the spent acid after full-strength phosphonic-based HF acid reacted with clay minerals at 302°F. This indicated a secondary reaction that occurred at high temperatures, decreasing the ratio of silicon/aluminum. This result was further confirmed by the 19F NMR results at high temperature. The 19F NMR results obtained after the high-temperature reaction showed that HF acid in the full-strength phosphonic-based-HF acid solution was completely consumed in 30 minutes when it reacted with clay minerals at a weight ratio of 10:1 at 302°F. Coreflood tests showed significant permeability improvement to Berea sandstone when using full-strength phosphonic-based HF acid compared with regular mud acid.