Experimental Study on Fracture Propagation and Seepage Variation Law under Alternating Injection Acid Fracturing Technology

This study targets the ultradeep, high-temperature, and high-pressure carbonate reservoirs in the Penglai Gas Area, Sichuan Basin. The existing acid fracturing technologies have ambiguous stimulation mechanisms and are difficult to regulate accurately. We conducted a series of physical experiments on the alternating injection of gelled acid and drag-reducing acid, aiming to optimize acid fracturing techniques and improve the reservoir stimulation performance. Dolomite cores collected from the Dengying formation were used as rock samples. With a true triaxial hydraulic fracturing simulation system, we designed four groups of alternating injection cycles and different mixing ratios of the gelled acid and drag-reducing acid. Combined with three-dimensional (3D) laser scanning technology to characterize the morphology of acid-etched fractures, parameters such as fracture roughness (JRC) and etching volume were quantitatively analyzed to explore the laws of fracture propagation, etching characteristics, and seepage variation under alternating injection conditions. The experimental results indicate that the second alternating cycle achieves the best acid etching and permeability improvement performance. By contrast, the fourth cycle produces insufficient etching and thus reduces the fluid flow capacity. The permeability enhancement capacity is the best when the ratio of gelled acid to drag-reducing acid is 7:3, and the excessive or insufficient proportion of gelled acid will reduce the etching heterogeneity and the permeability enhancement effect. Fracture morphology, etching degree, and JRC are the core factors determining seepage and permeability enhancement capacity. The tortuous (vertical + transverse) fracture has the optimal permeability enhancement potential, and the riverbed/groove etching morphology with a high JRC corresponds to higher seepage and permeability enhancement effects. This work clarifies the rules of fracture propagation and fluid flow evolution during alternating injection of acid fracturing. It provides experimental data and theoretical references for optimizing high-efficiency acid fracturing technologies for ultradeep carbonate reservoirs.