Wormholing behavior in matrix acidizing in vuggy, naturally fractured carbonate reservoirs

Abstract

In vuggy, naturally fractured carbonate reservoirs, hydrocarbon is mainly stored in vugs and natural fractures (NFs). To enhance oil production in such reservoirs, matrix acidizing is used to generate wormholes to connect the wellbore with NFs and vugs. Due to the simultaneous existence of NFs and vugs, the acid flow and interaction with rock in acidizing is complicated, and the wormhole propagation behavior needs to be clarified. To this end, the pore structure characterization method accounting for the matrix, vugs, and NFs is established based on geostatistics methods, which is then combined with the two-scale wormhole model to simulate wormhole propagation in the vuggy, naturally fractured carbonate reservoirs. The effects of geological factors and engineering factors on wormholing behavior are analyzed through extensive numerical simulations. The research shows that both the dimensionless correlation length and the areal proportion of vugs affect wormhole propagation. There is an optimal combination of the dimensionless correlation length and the areal proportion (evaluated by comprehensive influence coefficient), rendering more vugs being connected with wormholes. NFs have a dominant effect on wormhole propagation. The optimal natural fracture density and dimensionless length of NFs exist for a given formation with the same vug distribution. The distribution of the NFs and the vugs has a dominant effect on wormhole pattern and vugs that can be connected. In engineering practice, retarded acid system with low diffusion coefficient of hydrogen ions (H+) is recommended to obtain longer wormholes so as to increase the possibility of connecting remote vugs with the wellbore. Meanwhile, for vuggy, naturally fractured carbonate reservoirs, increasing the injection rate promotes acid flow in NFs, facilitating wormhole propagation and connecting with more vugs with the same injection volume as long as the pumping pressure is lower than the formation breaking pressure. This research provides theoretical basis for matrix acidizing in vuggy, naturally fractured carbonate reservoirs.