An Integrated Theoretical and Experimental Study on the Effects of Multiscale Heterogeneities in Matrix Acidizing of Carbonates
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AbstractCarbonates often have small (micron) to large (centimeter) scale heterogeneities in flow properties that may cause the effects of injected acids to differ greatly from what is predicted by a model based on a homogenous formation. To the best of our knowledge, there are neither theoretical nor experimental studies on the effect of large scale heterogeneities (vugs) on matrix acidizing. The abundance of carbonate reservoirs (60% of the world's oil reserves) and the lack of a detailed study on the effect of multi-scale heterogeneities in carbonate acidizing are the main motivations behind this study.In this work, we first present a methodology to characterize the carbonate cores prior to the core-flood acidizing experiments. Our approach consists of characterization of the fine-scale (millimeter) heterogeneities using computerized tomography (CT) and geostatistics, and the larger-scale (millimeter to centimeter) heterogeneities using a connected component labeling algorithm. In order to understand the connectivity of vugs and thus their contribution to flow, a 2D visualization algorithm, connected component labeling (CCL), was implemented in a 3-dimension system.After multi-scale characterization of the cores, acid core-flood experiments were conducted. 4-inch diameters by 20-inch long cores were used to decrease the geometric effects on the wormhole path. The post acid injection porosity distribution and wormhole paths were visualized after the experiments.The results demonstrate that acid follows not only the high permeability paths but also the spatially correlated ones. While the connectivity between the vugs and size of the core is the predominant factor, spatial correlation of the petro-physical properties has less pronounced effect on wormhole propagation in acidization of carbonates.
