Fracture and vug interaction behavior considering acid etching effect

Fractured-vuggy carbonate reservoirs are characterized by low porosity， low permeability， and strong heterogeneity. Acid fracturing is the primary technical means to increase the production of this kind of reservoir， and the key to fracturing reconstruction is to communicate fractured-vuggy reservoir efficiently. However， due to the influence of heterogeneity and randomly distributed vugs， the fracture propagation paths are complicated during acid fracturing， and the law of fracture-vug interaction is not clear. Therefore， laboratory experiments were carried out to study the changes in porosity， permeability， and mechanical properties of carbonate rocks before and after acid etching. The experimental results showed that acid-rock reaction significantly influences rock mechanics parameters. With a 15% mass fraction of gelled acid as an example， core porosity and permeability increase； the compressive strength and elastic modulus of cores decrease by 42% and 60%， and Poisson’s ratio increases by 25% after acid etching. Then， the rock’s physical and mechanical evolution equation was introduced into the classical cohesive model， and a fracture propagation model considering acid-rock reaction was established. The model was compared with the conventional hydraulic fracture propagation model and the laboratory physical simulation experiment of acid fracturing， and the effectiveness and correctness of the model were verified. Finally， the model was used to study the effects of horizontal in-situ stress difference， vug size， and injection displacement on fracture propagation path. The simulation results showed that ① Artificial fractures tend to penetrate small vugs， bypass medium vugs， and communicate with large vugs. ②It is helpful for the communication between fractures and vugs when the horizontal stress difference exceeds 5 MPa and the injection displacement exceeds 0.057 m3/s. ③ The acid-rock reaction can significantly improve the fractured-vuggy communication efficiency， and the fracture propagation area by acid fracturing is twice that by conventional hydraulic fracturing when the injection displacement is 0.1 m3/s. Therefore， in order to improve the fractured-vuggy communication efficiency and acid fracturing effect， the injection displacement should be appropriately increased when stimulating the carbonate reservoir with high stress difference.