Evaluation of carbonate rocks integrity after sequential flooding of chelating agent solutions

Recently chelating agents flooding have been emerged as an effective enhanced oil recovery (EOR) technique for carbonate and sandstone reservoirs. The exact recovery mechanisms leading to the significant incremental oil recovery from the use of those chemicals are not fully understood. The main objective of this paper is to investigate the impact resulting from the application of chelating agent having different concenrtations as an enhance oil recovery fluid on the limestone rock integrity. Core flooding experiments were conducted using Indiana limestone core samples under reservoir conditions using different concentrations of chelating agents at the tertiary mode after seawater flooding. In those tests, significant improvement in the permeability of all core samples used indicting significant rock dissolutions. The effect of rock dissolution created by chelating agent on the rock properties was investigated. Computed tomography (CT) scan and nuclear magnetic resonance (NMR) measurements were conducted on the core samples before and after flooding experiments. Experimental results show that, channels/wormholes have been created inside the core samples which may lead to bypass more oil; then increase the residual oil saturation and reduce the ultimate oil recovery. Therefore, the optimum concentration of chelating agent must be determined to maximize the oil recovery without severe rock dissolution. Small improvement of rock porosity was observed, while the absolute permeability was increased significantly, with more permeability improvement observed in the core sample closer to the core inlet. Ion concentrations in the effluents were measured to evaluate rock dissolution casued by chelating agent flooding. Significant increase in the Ca⁺² concentrations was observed which indicates considerable rock dissolutions was taking place. This result revealed that leaching of calcium ions from the rock lattice should be controlled and optimized in such a way to alter the rock wettability to produce more oil, and prevent severe rock dissolutions which will result in creating wormholes inside the core samples. This can be achieved by controlling the chelating agent concentration at small level, and maintaining the solution pH at moderate to high levels to avoid strong acidic conditions. In addition, multicomponent ion exchange (MIE) phenomenon was investigated by measuring the Mg⁺² concentration, slight decrease in Mg⁺² concentrations below the reference concentration during chelating agent injection indicate replacement of calcium ions by the adsorbed magnesium on the rock surface.