Multiscale flow and transport in highly heterogeneous carbonates
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Vuggy carbonates often exhibit multiscale features that make rock characterization a challenging problem. Moreover, a thorough understanding of the variability and fluid flow in vuggy carbonates is lacking. Classical theories of flow and transport are not applicable for highly heterogeneous carbonates. Therefore, it is necessary to conduct research on this type of carbonate which is found in many petroleum reservoirs and aquifers. A new method was developed in this research to understand the flow and transport properties of a vuggy carbonate sample. This method combines physical experiments, theoretical modeling, numerical simulations, and high resolution visualizations. The study focused on a Cretaceous carbonate sample containing centimeter-scale vugs. This roughly cylindrical sample, 25 cm in diameter by 36 cm in height, was X-ray CT scanned with a resolution of 0.5 mm. Effective permeability and transport properties were determined from experiments on three sub-samples from the 36-cm high rock. Single phase flow and transport through a computational sub-sample was numerically simulated using the parallel subsurface simulator (Parssim). Flow and transport through a touching-vug network, characterized from CT scan data were also numerically simulated. Seven types of pore space were characterized for the computational sub-sample, whose porosity is 21.6%. The total pore volume in this sub-sample is partitioned into 64% of vugs and 36% of matrix porosity. The vug volume is composed of separate-vugs and a touching-vug network, which account for 21% and 43% of the total pore volume respectively. The touching-vug network is divided into a flowing-vug network and deadend-vugs, which account for 22% and 21% of the total pore volume respectively. Study results show that vugs in this sample are interconnected with relatively narrow throats of a few millimeters or less in diameter. Dispersivity in this sample is a non-trivial convolution of at least three phenomena: advection through a flowing-vug network, mass-transfer between the flowing-vug network and dead-end vugs, and transport through the matrix and non-touching vugs. This miscible flooding behavior cannot be simply explained by classical one dimensional convection-dispersion model.