Characterization of rock pore size distribution and its effects on wormhole propagation
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Copyright 2016, Society of Petroleum Engineers. Carbonate rocks have complex heterogeneities that result from syn and post-depositional stressors. These heterogeneities invariably affect the movement of fluid through the formation. When considering an acid treatment procedure, care must be taken to optimize the acid concentration and pumping schedule to encourage the formation of wormholes. Despite the abundance of carbonate formations (60% of conventional reserves), there is little consensus on the effect of physical formation properties related to acidizing efficiency. This study characterizes the pore size distribution for different carbonate rocks and evaluates how the optimum pore volume to breakthrough, PVbt,opt, and the optimal interstitial flux, Vi,opt, are related to various physical properties of the rock. The pore size distributions evaluated in this study are constructed with micro-computer tomography (CT) imaging, a non-invasive X-Ray imaging technique pioneered in the medical field. CT is improved over medical CT because it can scan at higher energies and higher resolution. In this work, resolution for scanned samples are from 6-8 m/voxel and sample sizes are approximately 1cm3. From the raw data, image processing is used to distinguish pore space from the surrounding matrix. Object counter software is used to identify and measure individual pores, which can then be organized into a pore size distribution. We find that the shape of the pore size distribution is influenced by the type of carbonate rock, where the primary difference between scanned samples is their pore structure. Statistical parameters are calculated by fitting a lognormal distribution function to each sample's pore size distribution.
