| dc.description.abstract | Regular mud acid, which is composed of HCl and HF, has been extensively used to remove the formation damage in sandstone reservoirs; however, many problems may occur during stimulation treatments with this acid. To overcome many of these drawbacks, phosphonic-based HF acid systems (HF and a phosphonic acid) have been used as an alternative to mud acid. However, very limited research has been performed to investigate the interactions of phosphonic-based acid systems with clay minerals in sandstone reservoirs.
Phosphonic-based acid has been used in the field. Reactions of this acid with clays and its propagation in sandstone cores have not been examined previously. Therefore, the main objectives of this study are to: 1) investigate the reaction of phosphonic-based acid systems with clay minerals; 2) evaluate the effect of acid concentration, temperature, and reaction time on the acid performance; 3) determine the nature of precipitate that might form during the reaction of this acid system with various clay minerals, and 4) conduct coreflood tests to determine the impact of this acid system on core permeability.
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 (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to identify the reaction products. The pH of the solutions was measured using a HF resistant electrode. The concentrations of key cations in the supernatant were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). 19F, 31P, and 27Al liquid nuclear magnetic resonance (NMR) spectroscopy were used for the first time to evaluate the reaction of this phosphonic-based HF acid system with clay minerals. Coreflood experiments on Berea sandstone cores were conducted at 300°F at a flow rate of 2 cm3 /min using full strength phosphonic-based HF acid (3 wt% HF) and mud acid (12 wt% HCl/ 3 wt% HF).
No AlF3 precipitate was identified by EDS and XRD analyses of the solid samples after kaolinite, bentonite, and illite reacted with full strength phosphonic-based HF acid system. Large amounts of AlF3 were noticed in the chlorite samples after being treated with a full strength phosphonic-based HF acid system. The concentration of soluble Si 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 Si/Al. This result was further confirmed by the 19F NMR results at high temperature. 19F NMR results also 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 both 77 and 302°F. Coreflood tests showed significant permeability improvement to Berea sandstone when using full strength phosphonic-based HF acid systems compared to regular mud acid. | en |
