Acidization of carbonate rocks is a common practice to reduce the formation damage near wellbore. In this process, an acidic solution is injected to dissolve some of the rock that creates conducting channels called wormholes. These wormholes facilitate the flow of hydrocarbons to the wellbore. In the literature, there are several theoretical and experimental studies performed to understand this process. However, there are very few 3-D numerical studies that can predict quantitatively the experimental results such as the optimum injection rate. An accurate prediction of the optimum acid injection rate is necessary for field scale operations. Therefore, the main objective of this study is to quantitatively predict the experimentally observed acidization curve (amount of acid required to breakthrough versus acid injection rate), so that the validated model could be used for field scale simulations.
In this study, we present 3-D simulation of carbonate acidization with HCl using a two-scale continuum (TSC) model. The model describes the reactive transport at Darcy scale and retains all pore scale physics through structure-property relations. Unlike previous studies, we use a new two-parameter (pore-broadening and pore-connectivity) structure-property relation to describe the change in permeability, pore radius and interfacial area per unit volume with porosity. We predict quantitatively the experimentally observed acidization curve for HCl-limestone system and show the existence of a critical heterogeneity (that corresponds to minimum amount of acid required to breakthrough). We also present scaling criteria to estimate the wormhole tip diameter and optimum acid injection rate, for vuggy and non-vuggy carbonates. Finally, we present the flow dynamics of acid inside the wormhole.
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