A revolutionary family of treating fluids designed for the stimulation of critical, hot or exotic oil and gas wells has been developed through application of detailed chemical and engineering studies.1–3 Formulations based on the hydroxethylaminocarboxylic acid family of chelating agents have now been used to successfully increase production of oil and gas from wells in a variety of different formations. Included in the field test matrixes were new and producing wells drilled into carbonates and sandstone formations. The temperatures of the wells treated ranged from 230°F to 370°F (110–187°C) bottom hole static temperature (BHST).
Because these formulations do not contain high concentrations of corrosive mineral or organic acids (the formulations are less acidic than carbonated beverages), very low corrosion rates of the tubulars can be achieved by application of small amounts of special, inexpensive corrosion inhibitors. The mild fluids also are highly retarded so high-temperature carbonates can be stimulated and sensitive sandstone formations are not damaged. The fluids have reduced health, safety, and environmental (HSE) footprints because:
they are much less toxic to mammals as well as to aquatic organisms than mineral acids or organic acids such as HCl, HF or formic acid;
the fluids are returned to the surface at pH values between 5 and 7 and frequently can be added to normal well production fluids without neutralization;
because of much lower corrosion rates for corrosion resistant alloys (CRAs), lowered concentrations of Ni and Cr will be in the well returns compared with conventional acids that also may contain antimony (as a corrosion inhibitor).
While mineral acids can be very effective stimulation fluids at low temperatures, the use of HCl-based fluids at high temperatures (generally defined as above 200°F (93°C)) can cause many problems. The major concerns are damage to corrosion resistant tubular materials, toxicity of the fluids and inhibitors, too rapid attack on the formation (carbonates) and massive damage to clays in sandstone formations. Alternative fluids based on the hydroxethylaminocarboxylic acid (HACA) family of chelating agents can be formulated to alleviate these problems.
This paper will describe the scientific basis for using these fluids in hot formations. We describe a completely new family of matrix stimulation fluids based on HACA chemicals, which has a unique ability to be tailored to specific formation conditions. Because of the high acid solubility of HACA chemicals, formulations of low- as well as high-pH fluids have been produced. A major application will be stimulating high-temperature carbonate formations where mineral acids cannot be pumped fast enough to produce wormholes unless these are retarded by the formation of emulsions. In addition, this paper describes results from laboratory tests and field treatments using chelating agent fluids for matrix stimulation of high-temperature sandstone formations. Laboratory experiments have been conducted up to 400°F (204°C), and have included rotating disk tests using carbonate specimens to determine the kinetics and core flood tests using carbonate and sandstone cores to validate dissolution mechanisms and to qualify formulations for use in field applications. Results from field applications up to 370°F (187°C) are presented.