We investigated the performance of viscoelastic surfactant (VES) solution when applied in treatment on the uninvaded matrix using core flooding tests to analyze the impact of VES/CaCl2 concentration on fluid viscosity. In this paper, core samples from Tahe carbonate reservoir, with an average permeability less than 0.02 × 10−3 μm−2 and a small average porosity in the range of approximately 0.04−5.24% are used in the experiments. Computed tomography (CT) scanning is used to provide a detailed description of inner structure variation of cores after the acid system treatment. The results confirmed that a large pressure difference contributed to fracture propagation and the relative permeability of water increased significantly after the treatment. It was also found that higher concentrations of VES and/or Ca2+ induced higher viscosity and a stronger fracturing effect, while a lower concentration improved the reaction rates and etching effect, generating small worm holes inside the core. Foam in-situ produced during the etching process is the major contributor to the fluid viscosity enhancement. The permeability of fracture formed on the surface of the core is more sensitive to the confining pressure. These findings can help better understand the rheological properties of the acid system and etching and fracturing mechanisms during acid treatment, and which provides instructions for field implementation.