Models and Methods for Understanding of Early Acid Breakthrough Observed in Acid Core-floods of Vuggy Carbonates
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AbstractPreviously, we have studied the acidization of vuggy carbonates with acid core flood experiments in 4-inch diameter by 20-inch long cores, high resolution computerized tomography imaging, image processing and geostatistical characterization. The obvious major finding from these tests is that acid propagates wormholes through vuggy carbonates much more rapidly than occurs in more homogeneous rocks. Acid-created wormholes were observed to break through to the end of the cores an order of magnitude more rapidly than occurs in more homogeneous cores, highlighting the necessity of understanding the flow and transport in vuggy carbonates. The fact that acid channeled through the vugular cores, following the path of the vug system, was underlined with computerized tomography scans of the cores before and after acid injection. This observation proposes that local pressure drops created by vugs are more dominant in determining the wormhole flow path than the chemical reactions occurring at the pore level. Following this idea, we are presenting a modeling study in order to understand flow in porous media in the presence of vugs. Use of coupled Darcy and Stokes flow principles, known as the Darcy-Brinkman formulation, underpins the proposed approach. The power of the Darcy-Brinkman formulation lies in its natural ability to represent both porous media and tube flow in a single equation with variable coefficients.The methods presented here shows that acid injected into vuggy limestones follows a preferential pathway guided by the vug network. The PVbt (pore volumes to break through) for vuggy limestone correlates inversely with the fraction of total porosity comprised by vugs the higher the vuggy fraction of porosity, the lower the pore volumes to breakthrough. Local pressure drops created by vugs are more dominant in determining the wormhole flow path than the chemical reactions occurring at the pore level. The results from the developed model demonstrate that the total injection volume to breakthrough is affected by the spatial distribution, the amount and the connectivity of vuggy pore space.
