Evaluation of new viscoelastic surfactant systems in high-temperatures carbonates acidizing

Viscoelastic surfactants (VES) have been introduced as acid divergent agents for easier cleaning and mixing advantage over in-situ gelled acid systems. Thermal stability and compatibility with the other additives, such as corrosion inhibitors, are the main problems with conventional VES. This work introduces five VES-based acid systems for diversion in matrix acidizing. A series of VES samples were developed, hereinafter named samples A, A1, B, C and C1 according to their chemical composition, and evaluated for various applications. The initial screening was conducted by viscosity measurements of the spent acid system at 150°F and shear rates of 10 and 100 s⁻¹. The effects of different corrosion inhibitors on the viscosity of the selected VES were then examined. The viscosity of the VES system was then measured as a function of temperature (77 to 300°F) and shear rate (10 and 100 s⁻¹) to evaluate thermal stability. The diversion characteristic of the VES systems was evaluated with single core flood experiments on Indiana limestone cores with a permeability range of 10-200 md. CT-scan imaging of the cores after the acid treatment was used to evaluate the structure and the propagation of the wormhole. Dual coreflood experiments were conducted on Indiana limestone cores with permeability contrast of 1.5-50 to examine the ability of the VES to divert the acid system on heterogeneous limestone formations. Phase separation was observed during the preparation of the systems with a samples C and C1 VES samples. High viscosity was found in the case of A and B VES systems with the viscosity of 2,500, and 350 cP at 10 and 100 s⁻¹, respectively, at 150°F. The viscosity of the Sample A1 and B VES systems were not affected by the tested corrosion inhibitor. The single coreflood experiments revealed the ability of the VES to divert the acid system. The breakthrough injected acid volumes were 0.7 and 0.6 in the case of Sample A1 and Sample B, respectively. The wormhole tortuosity was 1.8 in sample A, compared to 1.4 in Sample B. This work introduces VES systems with superior divergent ability and viscosity measurements at high temperature even when in contact with the other acid additives, which leads to less acid spending and better wormhole structure.