Experimental study of acid fracturing behavior in carbonate reservoirs with different fracture-cavity development

Heterogeneity in carbonate formations due to discontinuities (e.g., fractures and cavities) will bring about distinctive acid stimulation effects. However, the differences in fracturing behavior between homogeneous and heterogeneous carbonate formations remain unclear, complicating the optimization of acid fracturing strategies. In this paper, full-diameter carbonate rock samples with different degrees of discontinuity development are selected to investigate the fracturing behavior under different fluid types and injection schemes. Advanced techniques, including 3D CT scanning and 3D laser scanning, are employed to analyze fracture morphology and etching characteristics, respectively. Experimental results show that the coupled hydraulic-chemical effects play different roles in fracture induction between fracture-cavity developed and undeveloped carbonate rocks. Acid-fracturing stimulation consistently induces multiple types of complex fractures in fracture-cavity carbonate rocks, whereas it results in a single artificial fracture in less fracture-cavity carbonate rocks. Furthermore, localized etching patterns are prevalent in most fracture-cavity carbonate rocks, whereas homogeneous carbonate rocks exhibit regional or global etching characteristics. In both carbonate rocks, the stimulation effect of guar fluid is inferior to that of gelled acid but comparable to self-generating acid. Further findings are that alternating fracturing with guar and acid fluids in fracture-cavity carbonate rocks can sustain or even increase the injection pressure, facilitating the formation of new or depth-penetrating fractures. This phenomenon, however, is not observed in fracture-cavity undeveloped carbonate rocks. Potential interaction modes between induced fracture and natural fractures/cavities under different injection conditions are also identified. Finally, preferred fracturing schemes applicable to different carbonate formations are recommended based on the area, number and roughness of the induced fractures.