Formation Damage From Acid Treatments

Abstract

Summary Well treatments with only HCl are potentially damaging in formations that contain aluminosilicates minerals (clays, feldspars, and micas) because HCl reacts significantly with aluminosilicates, leaching aluminum and forming hydrated silica. Fluid flow past leached mineral particles liberates mineral fragments and amorphous silica within the pore network. These fines can block pore throats, reducing the rock permeability. Both field data and reservoir-condition coreflood test results are consistent with this chemical reaction and permeability impairment mechanism. The average permeability reduction in our corefloods was 35%. Introduction Acids have been used widely for increasing or restoring the permeability of subterranean formations. Acid reactions can produce several side effects, however, that decrease formation permeability. In particular, mud acid can spawn a host of precipitates as it reacts. As acid dissolves matrix and matrix cement, mineral grains are liberated and can migrate to plug pores. Recognition of these and other acid-induced damage mechanisms has led to a variety of different formulations of acids and additives. An unacknowledged potential source of damage from acid treatments arises from the reaction of HCl with aluminosilicate minerals. This reaction is largely overlooked in the oil industry. In fact, it has been stated that HCl does not react with clay minerals, although the extraction of iron from chlorites by HCl has been mentioned as a potential source of damage. Gdanski and Peavy appear to have been first to point to the consumption of HCl upon aluminosilicate minerals in well treatments. Their analysis, however, was based on samples of returns from a mud-acid treatment, and HCl/aluminosilicate chemistry in the presence of HF is greatly complicated by the numerous precipitation/dissolution reactions involving fluorine. Whatever the role of HCl in the presence of HF, workers in fields such as catalyst activation and aluminum production have established that HCl alone reacts to a significant degree with aluminosilicates. For a generic aluminosilicate, this reaction can be written as where M represents collectively all the metal elements commonly found in aluminosilicates (Na, K, Mg, Ca, Fe, etc.). Balancing mass and charge in this reaction imposes two relations between the stoichiometric coefficients: y=3w+mv, and z=4x-3w-mv. At low pH, the precipitation of amorphous silica limits the aqueous concentration of H4SiO4, and the reaction becomes +x(amorphous silica)+ . Aluminum and metal ions are extracted from the mineral, leaving behind a siliceous residue. Because aluminum is an essential constituent of aluminosilicate mineral lattices, its removal by HCl must weaken the structure of the mineral particles. The fraying and fragmentation of aluminosilicates exposed to strong acid are well documented. This physical degradation into mineral fragments and amorphous silica particles has important implications for well treatments. The injection of HCl into argillaceous or feldspathic sandstones will create fines, independently of the formation of precipitates, the dissolution of cement, and the presence of HF. The potential for formation damage from fines migration is well established. Consequently, the assumption that HCl cannot hurt and might help is not always valid in sandstone-matrix stimulations, preflushes, and cleanup treatments. In this paper, we develop analytical expressions for aluminum extraction resulting from HCl flow in porous media. Comparing these expressions to reservoir-condition experiments suggests a consistent mechanism for the HCl/aluminosilicate reaction. This reaction is shown to be responsible for the permeability losses observed in the corefloods. We then consider the potential for damage from the HCl/aluminosilicate reaction in the field.