While flowing in the fracture, acid may leak off through the wormholes and appear a finger-like pattern during the process of acid fracturing, which is commonly called "acid fingering". Acid fingering is an unsteady hydrodynamic phenomenon that happens when low viscous acid displaces more viscous pad fluid. Many researchers have studied the acid leak-off and viscous fingering, however, there are still many unsolved problems so that the acid fingering has to be ignored, while describing the acid flowing and reaction process.

To solve the discrepancy, a comprehensive study on the moving interface of acid fingering has been conducted with both experimental study and numerical method. Firstly, the laboratory experiments were performed using a self-developed large-scale visual physical device with the structure of three-plates and double-slits, and it perfectly simulated the phenomenon of acid fingering. Secondly, the Volume of Fluid (VOF) equation based on previous large-scale visual physical simulation device has been established and been set up with the similar sizes of it as well. Thirdly, the orthogonal design has been used to analyze the morphologies of acid fingering under the different influence factors, which includes acid type (acid concentration and pad fluid/acid viscosity ratio), acid concentration and acid injection flow rate.

The reasonable agreements between the experimental and numerical results indicate the capability of the finger interface in simulating acid fluid and pad fluid. On the one hand, the acid fingering in the fractures would strengthen the uneven surface etched by contacted fresh acid. On the other hand, this phenomenon will reduce the acid leakage as well as the reaction rate between acid and carbonate rock, consequently improving the effect of acid fracturing. Besides, the acid is more inclined to the flow through lower area under gravity, and the acid fingering would be more significant. Consequently, the finger branches increase, so does the degree of bifurcation.

A combined experimental and numerical approach is adopted to investigate acid fingering in carbonate. Acid fingering could be successfully simulated by large-scale visual laboratory experiment. And numerical method provides a simple but effective way for the operators to predict finger interface under different flow patterns because of low CPU demands and minimal experimental materials requirement of this model, which might reconcile the discrepancy between laboratory study and production practice.

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