More than half of the world's oil reserves are hosted in limestone reservoirs. Oil recovery from these reservoirs remains a challenge for the petroleum industry due to lack of understanding of the multiphase fluid flow in fractured reservoirs and estimation of production potential. Wettability of these reservoirs to oil is contrary to water-wet reservoirs, such as sandstones. In fact, oil naturally adheres to limestone surfaces, preventing it to be recovered preferentially over water. Further, the presence of fractures makes it even more difficult for oil to flow as there exists different flow regimes: flow in a matrix, matrix-fracture interface, and fracture. Therefore, issues associated with flow processes in different regions need further studies. In this study, laboratory-scale models are made from polyethylene beads (oil-wet material) with fractures oriented at various angles to the direction of fluid flow. Multiple phases (oil and water) are injected under both imbibition and drainage conditions while maintaining a steady state. These models are used to visualize flow processes in various flow regions and develop models for relative permeability curves. In addition, laboratory-scale limestone block samples are cut from Mount Gambier outcrop and fractures are integrated in different inclination (dip) and azimuth angles (from the north). Samples are aged in oil to develop oil-wet reservoir characteristics prior to running the displacement experiments. A numerical code is developed and validated against experimental data. The numerical model is then used to generate saturation profiles and estimate relative permeability to oil and water. Results show that fractures act as a highly permeable pathway for fluids and cause early breakthrough by providing the least resistive path for non-wetting fluid (brine) to flow. This effect is more dominant as the number of fractures increase, which increase the unswept area near fractures, thereby reducing mobile oil saturation. As fracture moves away from the direction of the flow, an increase in resistance to fluid flow is observed, which causes more oil to be swept by the brine phase. On the other hand, fractures with a dip angle provide additional gravity-assisted fluid flow and cause water relative permeability (k(rw)) to reach its maximum value with increase in water saturation. Moreover, aging of limestone samples has resulted in reducing rock consolidation, forming an oil-wet mixture of rock particles, crude oil, and precipitated salt, thereby clogging pore spaces. This revealed a major cause of low recoveries from limestone reservoirs and hence opens a new venue for future investigations. The study also provides new relative permeability data based on fractured limestone samples, which would be very useful in understanding characteristic shape of relative permeability curves with respect to fracture orientations.
