Investigation on acid fracturing treatment in limestone formation based on true tri-axial experiment

Acid fracturing treatment is the most effective method for stimulating limestone formations, which are abundant in the northeast Sichuan basin of China. The effective acid etching improves the fracture width, resulting in high fracture conductivity, while fracture propagation behavior is always affected by discontinuities and the fractured vuggy structure. Hence, the fracture geometry differs from that in homogeneous formations. However, the fracture propagation mechanism remains unclear. In order to understand the process more clearly, a series of large-scale, true tri-axial simulation experiments were conducted for the first time to investigate the fracture initiation and propagation in limestone formations under acid fracturing treatment. Moreover, the effects of multiple factors on fracture propagation were discussed, and a 3D scanner was used to depict and describe the fracture surface etching feature. The experimental results demonstrated that natural fractures had a serious impact on the fracture initiation and propagation. The induced fracture initiated from the open-hole section to form a transverse fracture, or from the natural fractures to form a longitudinal or an inclined fracture. The fracture surface was rougher in the acid fracturing than in hydraulic fracturing, owing to the non-uniform acid fluid dissolution. The black-brown remains on the fracture surface were the result of acid etching, which could be used to judge the fracture propagation direction and area. However, the roughness degree of the fracture surface near the wellbore was larger than that far from the wellbore. Furthermore, HCL fluid was likely to etch the fracture surface within a short propagation distance, while clean-acid fluid promoted induced fractures to propagate at a long distance. Furthermore, a slow decrease in the extension pressure in the fracture curve indirectly indicated that the acid fluid reacted with the matrix and propagated slowly, which was a sign of successful acid fracturing treatment. According to the experimental results, the acid etched width and fracture propagation distance are assumed to be two important factors for estimating the acid fracturing treatment performance.