Chelating agents are used in the oil and gas industry mainly to: remove inorganic scales, control iron during acid treatments, and stimulate carbonate/sandstone reservoirs. The main chelating agents used in the field, include: ethylenediaminetetraacetic acid (EDTA), N-(hydroxyethyl)-ethylenediaminetriaacetic acid (HEDTA), and recently glutamic acid-N, N diacetic acid (GLDA). As with all organic acids, one of the main concerns with using these chelates is their stability at elevated temperatures. Therefore, the objectives of the present study are to: 1) assess thermal stability of various chelates, and 2) examine the effect of thermal degradation products on the permeability of carbonate and sandstone cores.
We prepared solutions (0.4 to 0.6 M) of HEDTA, GLDA, EDTA, and their salts. The solutions of these chelates were heated at various temperatures up to 400°F and times (2 to 12 hrs.). The thermal stability of these chelating agents was determined by measuring the concentration of the chelates before and after heating using a titration method utilizing FeCl3 as a titrant. Mass spectrometry was used to determine the degradation products. We injected GLDA to high permeability Indiana limestone, and investigated the effect of soaking of GLDA at high temperature on the final permeability of the core. Coreflood tests were conducted at 340°F and 5 cm3/min.
Chelating agents degraded at temperatures greater than 350°F. The decomposition products included: iminodiacetic acid, acetic acid, and ⍰-hydroxy acids. The addition of salt and raising pH improved the thermal stability of chelates. Thus, careful design of the chelate-based treatment fluids can prevent loss of functionality at elevated temperatures in the field, enhancing the versatility of these solutions, and minimizing formation damage due to decomposition products.