Effects of L-glutamic acid, N, N-diacetic acid as chelating agent on acidification of carbonate reservoirs in acidic environments

Abstract

For matrix acidification in carbonate reservoirs, the use of chelating agents can avoid the problems associated with conventional acids, such as excessive acid-rock reaction and extreme corrosion on a well tubular. L-glutamic acid, N, N-diacetic acid (GLDA) has been used as a chelating agent for carbonate reservoir stimulation owing to its high water solubility, effective dissolution effect on carbonate, lower corrosion to metals than conventional acid, and high biodegradability. In this study, the dissolution and acidification effect of GLDA on Baota limestone, as well as its corrosion effect on a well tubular and iron control capabilities, were evaluated. Meanwhile, the mechanism of GLDA on Baota limestone was studied by inductively coupled plasma optical emission spectrometer (ICP-OES), zeta potential, and core flooding experiments at low pH, which was adjusted using a conventional acid. The results indicated that 5% GLDA with the pH adjusted by acetic acid (pH = 4, HAC) exhibited the most significant acidification effect on the Baota limestone. Its dissolution rates were 75.2 and 94.4% at 25 and 80 °C, respectively, after 16 h. The surface of the Baota limestone was mainly positively charged in the aqueous solution; thus, the negatively charged GLDA was easily absorbed and chelated. In terms of iron control and corrosion inhibition, 5% GLDA (pH = 4, HAC) performed better than 5% GLDA, 5% HAC, and 5% HCl. The corrosion rate of 5% GLDA (pH = 4, HAC) on P110 metal test pieces were only 0.74 and 14.43 g/(m2·h) at 25 and 80 °C, respectively, for 24 h, which showed more noticeable corrosion inhibition effects than 5% HCl (2.08 and 56.10 g/(m2·h) at 25 and 80 °C, respectively). Based on the core flooding experiment and computed tomography (CT) scanning, it was determined that 3% GLDA (pH = 4, HAC) had the most significant acidification effect on the Baota limestone at an injection rate of 1 mL/min. The permeability and porosity of the Baota limestone increased from 0.12 to 5.8 mD (approximately 48x) and from 0.44 to 2.22%, respectively. The findings of this study may provide an effective reference guide for matrix acidification design of carbonate reservoirs.