Naturally fractured reservoirs contain a significant amount of the world's oil reserves. The behaviour of fractured reservoirs during production is a problem of much current interest. Oilrecovery by gravity drainage in a fractured reservoir strongly depends on the capillary height of the porous medium. Capillarity and gravity forces are usually the major driving forces in fractured reservoirs.
The G-O gravity drainage experiments in fractured models show that during the early stages of the process, there is no gas entering in the matrix, implying that gas only invades the fractures due to their lower resistance to flow. Once the driving forces in fracture and matrix equalize, the gas phase enters into the matrix as well as the fracture, which causes oil drainage from the matrix. The oil production rate from fracture medium at the time of gas invasion into the matrix is called the "characteristic drainage rate". Experiments show that the characteristic rate depends just on the dimensions of the fracture and properties of the test fluid, and not on the properties of the matrix.
A series of flow visualisation experiments were performed using unconsolidated packed models of rectangular geometry with two fractures on the side. Parametric sensitivity analyses were performed considering the effect of different system parameters such as fracture aperture, matrix height and permeability, and fluid viscosity on the liquid drainage rate. These experiments enabled us to capture some aspects of the flow communication between matrix block and fracture during gravity drainage. After analyzing our experimental data, we found that the rate of liquid flowing from matrix to fracture is proportional to the difference of liquid levels in the matrix and in the fracture. In addition, the characteristic rate and the maximum liquid drainage rate from these fractured models are determined for such a stable gravity-dominated process. The experiments show that the presence of fracture is more pronounced in lower matrix permeability systems. For a given fracture-matrix systemwith different initial liquid saturation conditions, it is found that the production history can be correlated by plotting the fraction of recoverable liquid as a function of time. Furthermore, ultimate recovery factor can be correlated using dimensionless numbers such as the Bond Number and the dimensionless time.
Multiphase flow in fractured reservoirs is very different from that in conventional un-fractured reservoirs. The presence of fractures with permeability values that are orders of magnitude higher than that of the porous matrix provides a different mechanism of oil displacement and production in naturally fractured reservoirs.
Gravity drainage plays an important role in oil production from fractured reservoirs. This mechanism will be highlighted especially in the cases in which a gas cap (initial or secondary) is present in direct contact with the oil zone. Capillary continuity is perhaps the most important parameter affecting the performance of gravity drainage process [1–3]. Oil recovery by gravity drainage in a fractured reservoir strongly depends on the total height of capillary continuity.