Chelating agents are used to remove various inorganic scales, including sulfates and carbonates. They are also used as stand-alone stimulation fluids and as iron control agents during acidizing treatments. The main chelating agents used in the field, include: ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), N-(hydroxyethyl)-ethylenediamineteriaacetic acid (HEDTA), and recently, L- glutamic acid-N, N diacetic acid (GLDA). One of the concerns with these chelates is their thermal stability at elevated temperatures. A few studies examined the thermal stability of these chelating agents, and the impact of thermal degradation products on permeability. The objectives of the present study are to: 1) Examine the thermal stability of several chelating agents and their salts up to 450°F, and 2) assess the effect of thermal decomposition products on the permeability of carbonate and sandstone cores with different initial permeabilities.

We prepared solutions (0.4 to 0.6 M) of HEDTA, GLDA, NTA, EDTA and their salts. The solutions of these chelates were heated at various temperatures and times (2 to 12 hrs.). The concentration of chelate was determined using a new analytical technique that was based on titration with FeCl3. The products of thermal decomposition of chelates were determined using MS technique.

Core flood tests were conducted on Berea sandstone and Indiana limestone to determine the effect of thermal degradation products on the permeability of these cores. Coreflood tests were conducted at 325°F and 3 cm3/min.

Most chelates decomposed at temperatures greater than 350°F. Among monovalent salts, potassium salt was found to be the most stable one. Chelates with two nitrogen atoms were more stable than those with one nitrogen atom. For example, diammonium salt of EDTA is more stable than diammonium salt of GLDA. Analyses of chelate solutions after heating using MS technique highlighted that the decomposition products included: iminodiacetic acid, formic acid, and α-hydroxy acids. Results of the coreflood indicated that some of the thermal decomposition products can cause formation damage. This paper will summarize the results obtained, and explain how chelates can be used to improve field treatments, especially at high temperatures.

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