Abstract: Acid fracturing technology is an important method for stimulating carbonate reservoirs and increasing production. Acid etching of fractures significantly enhances fracture conductivity between the wellbore and the reservoir. However, rough-walled fractures in carbonate reservoirs feature extremely complex geometries, resulting in the difficulty to accurately evaluate the dissolution pattern and the fracture conductivity in the process of acid stimulation. Existing studies have not fully considered the effect of fracture roughness on the evolution of dissolution morphology, as well as the effect of nonlinear flow behavior in acidized fractures on the fracture conductivity. Therefore, the effects of fracture roughness, acid injection rate, and acid concentration on the dissolution and conductivity properties of rough fractures are investigated. A dissolution transition model to quantify different dissolution patterns along a rough-walled fracture is established, the effect of dissolution patterns on the nonlinear flow coefficient of fractures was quantified, and a method to interpret the conductivity of the acidized fracture considering non-linear flow effects is further proposed. Three different dissolution patterns are determined by characterizing the acid dissolution morphology along fracture surfaces and the acid breakthrough time, namely, uniform dissolution pattern, channel dissolution pattern and surface dissolution pattern. A channel dissolution coefficient is then defined to capture different types of dissolution patterns. The degree dissolution is deepened as the channel dissolution coefficient increases, and the dissolution patterns change from uniform dissolution pattern to channel dissolution pattern, and then to surface dissolution pattern. When the acid concentration increases, the boundary channel dissolution coefficient that dominates the transformation of dissolution patterns increases accordingly. By comparing the nonlinear flow properties and conductivities of acidized fractures under different dissolution patterns, it is found that the channel dissolution pattern and the surface dissolution pattern feature relatively high conductivity. Considering the influence of factors such as the total acid injection amount and the reservoir stress environment, the channel dissolution pattern is preferred to achieve higher acidizing efficiency.

Key words: carbonate rock, rough-walled fractures, dissolution pattern, conductivity, acid fracturing