New Diversion Fluid for Chelate-Based Acid Systems

Abstract

Chelating agents have been used in the upstream oil and gas industry as a scale remover, iron control agent and acidizing fluid. Despite several studies on diversion techniques for conventional acid systems (i.e., HCl or organic acid), the application of diverting systems has rarely been studied during acidizing jobs using chelating agents. In this study, a new diverting system is introduced and experimentally evaluated during the stimulation of carbonate cores by a chelate-based acid. The diverting system introduced in this study is a unique blend of viscoelastic surfactants based on sulfobataine and amine oxide chemistries. The new diverting system was examined during carbonate acidizing by Glutamic acid N, N-diacetic acid (GLDA). The diverting system, which is specially designed for high temperature applications, was used during dual coreflooding experiments at 130°C. In these evaluations, Indiana limestone cores with various permeability contrasts were used. The pressure behavior, permeability improvement/reduction, and the production flow rates from each core plug were measured. Finally, to visualize the wormhole propagation inside each core, CT scanning was conducted after the treatments. From the results, it was found that the diverting system can block the formation for a certain period of time (<2 hours). During this period, the diverting system shows the highest viscosity and can temporarily block the high permeability cores (>100 mD) which diverts the injected acid to the lower permeability core. It was found that the acid diversion lasts for a longer time when the system has a higher permeability contrast. From the CT-scan images, it was found that the wormholes were propagated up to 50% across the low permeability core when 0.3 PV of the diversion fluid was injected. With enough volume of the diversion fluid, the acid system was fully diverted, and the treatment fluid achieved full-length wormholes in both high permeability (HP) and low permeability (LP) cores. It was also shown that required volume of the diverting fluid is a function of permeability contrast. It was demonstrated this pre-flush diversion technique can be successfully applied and it needs less VES volume compared to the continuous VES injection with acid. The novel formulation introduced in this study is a cost-effective solution specially designed for the chelate-based acid systems. The proposed formulation can also be optimized to resolve diversion challenges in other applications at high temperature.