Production from carbonate reservoirs can be significantly improved through acid fracturing. Creating acid fractures with high conductivity is a key factor in enhancing production. 3D scanning technology enables the digital study of fracture conductivity. It is unclear that how to select the suitable resolution during 3D scanning. This study aims to obtain the conductivity optimal correlation resolution (COCR) for acid fracture research. The 3D scanning result obtained at COCR exhibits the optimal correlation with the fracture conductivity. Experiments involving 3D scanning, fracture conductivity tests, and roughness evaluations were conducted in this research. The experimental results showed that: (1) Acid etching would enhance fracture roughness and conductivity. The strength of the fracture surface would decrease with acid etching, which leads to the significant decreasing of the fracture conductivity under high closure pressure. (2) The roughness evaluation based on Gaussian curvature demonstrated advantages, which can evaluate the roughness from a local view and build a global evaluation method with the fracture conductivity. (3) In acid fracture research, scanning at the resolution of 0.1 mm maximized the roughness-conductivity correlation, which can be defined as COCR. These conclusions provide a reference for selecting 3D scanning resolution in digital studies of acid fracture conductivity.