Chelating agents have been used in the oil industry as iron control agents and scale removers, and recently as effective stand-alone stimulating fluids in matrix acidizing, especially for deep wells where using hydrochloric acid is restricted due its corrosion problems. The ultimate goal from using chelating agents is to create highly conductive wormholes that connect the formation to the wellbore. Glutamic acid diacetic acid (GLDA) is a new chelating agent that can be used for this purpose. The objective of this work is to study the reaction of GLDA with calcite and investigate the effectiveness of the created wormholes by both kinetics and transport studies that have been performed experimentally in the laboratory.
The reaction of GLDA with calcite was investigated by measuring the rate of dissolution using the rotating disk apparatus. The effect of initial pH (1.7, 3.8, and 13) and disk rotational speed (100-1800 rpm) on the rate of reaction was studied at 150, 220 and 300°F. Pink Desert limestone cores 1.5 in. diameter and 0.65 in. length were utilized. GLDA transport and its effect on wormhole creation were investigated in core flood experiments using samples of 1.5 in. diameter and 6 in. length. The cores were scanned using CT-scan before and after the injection of GLDA solutions into the cores. Core flood experiments were conducted at temperatures of 200 and 300°F.
The calcite dissolution rate was found to be a strong function of temperature and increased significantly by increasing the temperature from 80 to 300°F. Increasing the pH from 1.7 to 13 resulted in a reduction in the rate of dissolution. GLDA reacted with calcite by one of two mechanisms; hydrogen ion attack and calcium complexation reaction. The GLDA chelation ability (expressed as a percentage of the total rate of dissolution) decreased by increasing temperature, but was not affected much by changing the disk rotational speed.
Acid diffusivity was determined at pH 1.7, 3.8, and 13 and the data was used with core flood results to determine the Damköhler number. An optimal Damköhler number was found in all experiments that corresponds to a minimum pore volume required to breakthrough the cores. Increasing temperature or reducing the pH increased the optimum Damköhler number with a reduction in the minimum pore volume required to breakthrough.