Influence of fractures and heat transmission on wormhole propagation in fractured carbonate rocks

Abstract

Carbonate reservoirs exhibit significant heterogeneity and often contain natural fractures. Matrix acidizing is a commonly used technique in fractured carbonate formations. Currently, the influences of natural fractures and the acid-rock molar reaction heat on wormhole propagation are rarely considered in acidizing models. In this paper, the two-scale continuum model, the embedded discrete fracture models (EDFM), and the heat transfer model are combined to develop a computational model of acidic fluid reactive flow in carbonate rocks with a complex fracture network that takes into account temperature variations. The model considers not only the temperature variation caused by the acid-rock molar reaction heat, but also simulates carbonate acidizing with any complex fracture network. Numerical simulations of the model were carried out to obtain the wormhole propagation patterns and the breakthrough curves under different conditions, and the simulation results were in good agreement with those of previous study. In particular, the sensitivity of fracture permeability, acid concentration, acid temperature and matrix porosity heterogeneity during acidizing is studied. The simulation results show that fractures can significantly alter the structure of wormholes and decrease the pore volume of acid required to reach the breakthrough (). However, fractures have minimal impact on the growth structure of wormholes prior to encountering fractures. During the reaction, the highest temperature was observed at the front end of the wormhole, and it was found that the optimum injection rate increased with the increase of the injected acid temperature. Lowering the temperature of the acid can enhance the efficiency of acidizing in fractured carbonate formations, particularly when applied at an optimal injection rate. When carbonate formations have well-developed fractures, the structure of the wormholes is insensitive to the heterogeneity of the matrix porosity, and they tend to grow along the fractures.