The impact of effective tortuosity on carbonate acidizing and the validation of Damköhler and Péclet dimensionless phase space

Abstract

Acid stimulation is one of the main methodologies to enhance well productivity and increase permeability lost by near-wellbore formation damage through matrix acidizing. The desired outcome of acidizing carbonate rocks is to form deep conductive flow channels called “wormholes”. The productivity enhancement depends on how deep these wormholes penetrate into the reservoir. The porous medium is characterized by three fundamental parameters which are anisotropic or isotropic porosity, permeability, and percolation. Here we test the hypothesis that effective tortuosity - a key parameter for microstructure characterization in porous media - has a significant effect on wormhole formation. Significant progress has been made by empirical work on carbonate acidizing to increase the stimulation efficiency, but further improvements are possible by developing a deeper understanding of the impact of effective tortuosity on wormhole initiation and development. We use Indiana limestone core-flooding experiments to inject diluted hydrochloric acid (HCl) at different acid concentrations and injection rates. Effective tortuosity was calculated for the tight Indiana limestone and was compared with an earlier study on the highly porous Mount Gambier limestone to evaluate the effect of tortuosity on wormhole formation. Our results suggest that the inverse Damköhler and Péclet phase space, traditionally used for evaluating criteria for wormhole formation, needs to be extended by tortuosity as a third dimension to capture the effect of microstructure for the initiation of wormhole phenomenon. Incorporating this extension will improve the prediction of wormhole initiation conditions for all carbonate rock types by using a universal dimensionless phase space.