Fracture and vug effects on wormhole pattern during acidizing of triple porosity carbonate rocks

Abstract

Carbonate reservoir acidization is performed to mitigate formation damage and create desirable and highly permeable pathways for the movement of oil and gas. The most significant characteristic of carbonate reservoirs is the presence of fractures and vugs within their structure, which contribute to their triple porosity. The presence of fractures and vugs complicates the flow equations in the medium, which can significantly impact the success or failure of the acidizing. Therefore, understanding their role in the behavior of fractured carbonate reservoirs is a challenging and important issue. Comprehensive studies investigating the simultaneous effect of fractures and vugs on the acidizing in these types of reservoirs have not been reported. Therefore, this study provides a detailed and more precise examination of the acidization process within a triple porosity carbonate media. To simulate the acidizing, a two-scale continuum model was coupled with the pseudo-fracture method in a triple porosity carbonate media. The flow within the matrix and the vug were modeled using Darcy's law, while the flow within the fracture was modeled using the cubic law (free flow). This modeling approach accurately simulated the overall flow of the acidizing fluid. Generally, increasing the length and decreasing the width of a single horizontal fracture improves the acidizing, whereas a vertical one causes a delay at average injection rate. The presence of cross-fractures enhances acidization at all injection rates, and when a horizontal fracture is present at the system's outlet, it also significantly reduces the PVbt. Conversely, the presence of multiple vertical ones within the reservoir delays the acid reaction time, Increasing the number and size of vug improves the acidizing. When fractures and vugs coexist within the rock, it is the type and location of these fractures that control the wormhole propagation pattern and the extent of variations in porosity inside the vug.