Numerical and experimental investigation of acid wormholing during acidization of vuggy carbonate rocks

Abstract

Both small- (micron) to large- (centimeter) scale heterogeneities in carbonates cause the injected acids to propagate very differently than that predicted by a homogeneous model. Very few studies, either theoretical or experimental, address the effect of large scale heterogeneities (vugs) on matrix acidizing. This study explores the effects of heterogeneity on vuggy carbonate acidizing with high resolution computerized tomography imaging, image processing, geostatistical characterization, acid core-flood experiments (with 4-inch by 20-inch vuggy cores), and numerical simulations. We observed that acid propagates wormholes through vuggy carbonates much more rapidly than those in homogeneous rocks. In fact, an order of magnitude early acid breakthrough observed in the experiments highlighted the necessity of understanding the flow and transport in vuggy carbonates. The fact that acid channeled through the vugular cores and following the path of the vug system, is underlined with computerized tomography scans of the cores before- and after acid injection and with the connected component labeling (CCL) algorithm. This observation suggests that the local pressure drops created by vugs are more dominant in establishing the wormhole flow path than the chemical reactions occurring at the pore level. Following this idea, we present a modeling study to understand the flow in porous media in the presence of vugs. Use of coupled Darcy and Stokes flow principles, known as Darcy–Brinkman formulation (DBF), underpin the proposed approach. The results demonstrate that the total injection volume to breakthrough is affected by spatial distribution, and the amount and connectivity of vuggy pore space. Much deeper penetration of wormholes is predicted for the vugular rocks with low pore volumes to breakthrough or PVbt ranging from 0.04 to 0.15, compared to the homogeneous cases with PVbt close to one.