Numerical simulation of large-scale acidification in fractured carbonate reservoirs based on a step-by-step algorithm

Carbonate reservoirs are highly heterogeneous, and acid-etched wormholes are often formed in matrix due to acidification. As the dominant channels, wormholes can effectively communicate the fracture systems and increase the effective distance of acidification. Limited by computational accuracy and convergence conditions of models, most scholars often conduct small-scale research of acidification in fractured formations on the centimeter scale. However, the effective distance of acidification, especially the deep acidification, is usually on the meter level. Therefore, a high-efficiency simulation method of acidification applicable for large-scale formations is needed in the oilfield. Based on the classical two-scale continuum model, the study solves the problem of wormhole propagation in radial flow fractured formations using the discrete fracture network model. The proposed step-by-step algorithm ignores the process of dissolution and reaming in the near-borehole acidified area. It only needs to consider the fractures that actually affect the wormholes in different acidification stages, and track the growth dynamics of the dominant wormholes. Studies have shown that the finite boundary of computational domain will cause a big error in the simulation results, and the growth process of wormholes in infinite formations can be effectively simulated by setting a certain seepage resistance zone outside the acidification zone. Moreover, the flow field analysis shows that there is a local control domain around the fracture, and the size of the control domain has a relation with the projection of fracture in the direction of acid injection, and has nothing to do with the fracture position. The growth trajectory of wormholes is determined by the fractures whose control domains cover the tips of wormholes, unrelated to other fractures. There is a linear relationship between the inlet pressure and the acid concentration of the dominant wormholes and the logarithm of the acid injection time. The proposed step-by-step algorithm has high computational efficiency and can progressively simulate the growth process of wormholes in large-scale and complex fracture formations. © 2020, Editorial Office of ACTA PETROLEI SINICA. All right reserved.