MODELING FLOW AND TRANSPORT IN FRACTURED VUGGY FORMATION

The existence of vugs or cavities in naturally fractured formation, such as Karst aquifers, has long been observed. Even though these vuggy formations can be largely attributed to reserves of oil, natural gas, and groundwater, few investigations of fractured vuggy rock can be found in the literature. To investigate flow behavior in such vuggy fractured reservoirs, we present a multi-continuum conceptual model, which is based on geological data and core observations from carbonate formations. The conceptual model considers vuggy fractured rock as a triple-continuum medium, consisting of (1) highly permeable fractures, (2) low-permeability rock matrix, and (3) various-sized vugs. The matrix system may contain a large number of small or isolated cavities (of centimeters or millimeters in diameter), whereas vugs are larger cavities, with sizes from centimeters to meters in diameter, indirectly connected to fractures through small fractures or separated by rock matrix. Similar to the conventional double-porosity model, the fracture continuum is responsible for the occurrence of global flow, while vuggy and matrix continua, providing storage space, are locally connected to each other (and interacting with globally connecting fractures). In the numerical implementation, a control-volume, integral finite-difference method is used for spatial discretization, and a first-order finite-difference scheme is adapted for temporal discretization of governing flow equations in each continuum. The resulting discrete nonlinear equations are solved fully implicitly by Newton iteration. The numerical scheme is verified against an analytical solution.