Significant advances in acidizing chemistry have led to the introduction of sequestering agents, such as hydroxypolycarboxylic acids, followed by chelating agents, to mitigate precipitation reactions. The initiative to obtain fluids with an improved environmental footprint has led to the redesign of treatment fluids to possess distinct advantages, such as stability at higher temperature, broader pH activity, and stronger complex formation. In the area of hydrofluoric (HF) acidizing chemistry, the conceptualization of the unique HF acid reactions on clays and silica surfaces—namely, kinetic controls over the so-called primary, secondary, and tertiary reactions—has facilitated fluid designs that can handle such varied reactions. The work presented here describes the development of a new acidizing fluid containing an environmentally relevant chelating agent and an aminpolyocarboxylic acid.
The chelating agent is fully biodegradable, according to the Organization for Economic Co-operation and Development (OECD) protocols, is stable in fluid media from pH 1 to 7 and at high temperatures, and stabilizes the dissolved ions during an acidizing treatment. In HF acidizing, the chelant performance has been tested at 0.6 mol/L and HF acid concentrations from 0.5 to 2%, pH of 2.5 to 4, including a stabilizing agent to mitigate the precipitation of fluorosilicates or fluoroaluminates, and is effective in temperature ranges from 200 to 300°F. Laboratory tests show it to be effective in maintaining in solution dissolved aluminum (3000 to 10 000 mg/L), calcium (5000 mg/L), and iron (6000 mg/L). The use of nuclear magnetic resonance (NMR) spectroscopic analysis revealed additional dissolved-fluoride-containing species that has not been previously reported. Moreover, the chelating agent can also be used when stimulating carbonate rocks in concentrations from 0.2 to 0.6 M with a pH of 1 to 4 and is effective from 125 to 350°F. The representative pore volume breakthrough (PVbt) curves provide an indication of the distinct reactivity of this chelant.