Visualization study of viscous fingering and its implications in acid fracturing with multistage alternating injection

Multistage alternating acid fracturing enhances non-uniform acid etching and fracture conductivity by promoting viscous fingering, thereby improving carbonate reservoir productivity. However, the evolution of viscous fingering under varying injection conditions remains insufficiently understood, limiting the optimization design of injection parameters. This study employs a visualized experimental setup to examine the effects of viscosity contrast, injection rate, fracture width, and the number of alternating stages on viscous fingering morphology and acid-etching patterns. Quantitative metrics, including fractal dimension and fingering area, were utilized to characterize non-uniform etching induced by fingering. Results indicate that higher viscosity contrast suppresses piston-like displacement and promotes fingering bifurcation, increasing etching non-uniformity and fracture conductivity. Injection rate and fracture width jointly affect the inlet acid velocity. Fingering complexity initially increases with acid velocity but declines beyond a critical threshold, suggesting the existence of an optimal injection rate for a given treatment. Increasing the number of alternating stages deepens etched channels and enhances fingering complexity, improving the fracture conductivity. However, with a fixed acid volume, more stages reduce per-stage acid volume, shortening the effective penetration distance. Thus, a balance between acid penetration and non-uniform etching is critical for optimal multistage alternating acid fracturing design.