Acidizing of naturally-fractured carbonate formations
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The lab experimental studies of acidizing of fractured carbonate cores show three etching patterns under different conditions: wormholing, channeling and surface etching. Based on the observations in experiments, a mathematical model of acidizing of naturally-fractured carbonates is developed. Rough-surfaced fractures are numerically generated in the model. The model predicts the same acid etching patterns with the same dependencies on fracture properties as observed in the experiments. When large single fractures are implemented into the model, the acid etching and penetration in single fracture as well as the effects of fracture properties are observed. The studies of acid behavior at intersection points of fracture networks indicate that acid flow and transport is concentrated in a main flow path in the network, which is formed by a system of intersected fractures. A method is developed to directly numerically generate the main flow path without generating the network. The model is combined with the generated main flow path to predict the result of acidizing natural fracture networks. A leakoff model that includes the effects of both matrix leakoff and the leakoff from the tail fractures is integrated into the model. The simulation results illustrate the acid penetration and the etching of the fracture walls along the main flow path and the effects of the network properties. The model predicts deeper acid penetration in acidizing of naturally-fracture carbonate formations than would be possible with only matrix flow.