A Novel Technique for Continuous Improvement of Injectivity and Improving Oil Recovery for Carbonate Reservoirs

Abstract

Injecting water into underground formation has always been used in petroleum industry for reservoir pressure maintenance, improving oil recovery and as a disposing produced water. The incompatibility between the reservoir fluids and the may injected water cause severe precipitation, and then reduce the well injectivtiy due to formation damage. Several treatments are required to remove the damage and recover the well injectivtiy. The goals of this research are to propose an environmentally friendly and cost effective technique to improve and maintain well injectivity during seawater flooding for carbonate reservoirs, and to improve oil recovery. The impacts of this technique on the injector performance and the formation rock integrity are assessed and presented. This approach involves adding few chemicals (such as chelating agent) in small concentrations to the injected seawater to reduce the injection pressure and create more favorable flowing conditions. The proposed chemicals are environmental friendly, stable and can be prepared using seawater without any dilution. Also, they do not induce any precipitation or corrosion problems, therefore, no chemical inhibitors (such as anti-corrosion or anti-scaling agents) are required. Several coreflooding tests were carried out at reservoir condition using limestone core samples and chelating agents with different concentrations. In these tests, considerable reduction in the injection pressure was observed indicating that, the injectivity is increased significantly and no formation damage was taking a place. The compatibility between injected fluids and the reservoir fluid was investigated, by designing and conducting several solubility tests. Solutions with different concentrations were prepared from GLDA and seawater over wide range of pH at reservoir temperature. The optimum conditions, in term of pH and concentration, were defined to avoid ion precipitations and formation damage. The effect of injected fluid on the rock properties was investigated by conducting computed tomography (CT) scan and nuclear magnetic resonance (NMR) measurements, before and after flooding experiments. Small improvement of rock porosity was observed, while the absolute permeability was increased significantly, with more permeability increment was obtained closer to the injection face. Moreover, the produced effluents from the coreflooding experiments were analyzed by measuring the ion concentrations to evaluate the reaction between the injected fluid and the formation rock. Significant increase in the calcium concentrations was observed which indicates considerable rock dissolutions was taking place. This result revealed that leaching of calcium ions from the rock lattice, which may alter the rock wettability to more water-wet behavior. Furthermore, multicomponent ion exchange (MIE) phenomenon was investigated by measuring the magnesium concentration, slight decrease in Mg+2 concentrations below the reference concentration during chelating agent injection indicates replacement of calcium ions by adsorbed magnesium on the rock surface.