Research on the Multi-Modal Flow Mechanism of Oil and Water in Fractured Carbonate Reservoirs under Multiple Coupling Effects

Abstract

In a fracture-cavity carbonate reservoir, a myriad of irregular cavities is distributed, serving as primary reservoirs for oil storage. Simultaneously, fractures play a pivotal role by establishing efficient pathways for the movement of oil and water between these cavities, thereby facilitating fluid migration. Consequently, gaining a comprehensive understanding of the connectivity between the cavities and fractures within the reservoir is crucial for optimizing and implementing efficient development strategies. However, the fluid flow behavior within the cavities and fractures, under the influence of multiple coupled fields, remains highly complex, and previous studies have yet to fully elucidate this intricate phenomenon. This study addresses the knowledge gap by employing numerical simulations to investigate the interactions between cavities and fractures, as well as the fluid flow patterns within them. The study utilizes porous media permeability based on Darcy's law to characterize fluid flow within the matrix, fracture flow described by the Brinkman equation for fluid movement within fractures, and free flow based on Navier-Stokes equations to depict fluid motion within solution cavities. Building upon this foundation, and applying the theory of multi-field coupling, different flow models were tailored for the cavities, fractures, and rock matrix, taking into account key factors such as fracture angle, stress state, and fracture connectivity. The simulation results provide valuable insights, from which we draw the following conclusions: (1) When the maximum principal stress direction is perpendicular to the fracture direction, the fractures experience compression perpendicular to their normal direction, leading to a tendency of closure and consequently reducing the efficiency of oil migration within the fractures. Conversely, when the maximum principal stress is parallel to the fracture direction, the fractures undergo tension along their normal direction, causing them to open up and thereby enhancing the efficiency of oil migration within the fractures. (2) With an increasing fracture angle, the angle between the fractures and the principal stress increases. As a result, the fractures experience an increased compressive stress component, leading to a decrease in their conductivity and reducing the efficiency of fluid migration between different cavities. (3) Increasing the tortuosity of the fractures reduces the flowability of the fluid within the fractures. The larger the tortuosity of the fractures, the poorer the conduit capacity of the fractures for the oil phase, resulting in a decrease in the efficiency of oil migration between different cavities.