Thermal Stability of Various Chelates that are Used in the Oilfield

Acid treatment, especially at high temperatures, is very challenging since HCl is really corrosive to the metal equipment. The use of HCl is associated with face dissolution, corrosion, and iron precipitation. Organic acids are weak and less corrosive than HCl but they have a limitation, which means that they can't be used at high concentrations. The next option would be chelating agents. Chelating agents are used in well stimulation, iron control during acidizing, and removal of inorganic scales. Chelates such as ethylenediaminetetraacetic acid (EDTA), N-(hydroxyethyl)-ethylenediaminetetraacetic acid (HEDTA), L- glutamic acid-N, N diacetic acid (GLDA), and nitrilotriacetic acid (NTA) are used in high-pressure/high-temperature oil and gas wells. GLDA is environmentally friendly, which makes it favorable. One of the concerns with these chelates is their thermal stability at high temperatures because if they degrade at high temperatures, they may lose their functionality. This study describes the thermal stability of these chelates, thermal degradation products, and some methods to improve their stability. The thermal stability is determined by measuring the concentration before and after heating using a complexo-metric titration utilizing FeCl? as a titrant. The degradation products are identified using Mass Spectrometry (MS). A series of experiments were run in the lab at varying temperatures (300 to 400?F) up to 12 hours, and the results shows chelates are not stable at temperatures greater than 350?F. Furthermore, chelates with two nitrogen atoms are more stable than those with one nitrogen atom. Iminodiacetic acid (IDA), acetic acid, and [alpha]-hydroxy acids are the decomposition products. There is a layer of black deposition after the chelates are heated, which is analyzed using Scanning Electron Microscope (SEM). Some coreflood tests are conducted using these degraded chelates to investigate the effect of these solid precipitates on the permeability of carbonate and sandstone cores. Increasing ionic strength and raising pH results in a higher thermal stability. Some salts such as, NH?Cl, KCl, Csformate, and NaBr are added to chelate solutions to enhance stability.