Fluid Flow Law In Fractured Vuggy Media

Carbonate reservoirs have made important contributions to oil and gas production in the world. It is very difficult to understand the fluid flow law in this kind of reservoir because of high heterogeneity. Fluid flow in fractured vuggy media was studied systematically by physical simulation, numerical simulation and theretical analyzation. The main results are as follows: (1) At the beginning of this study, reservoir space of fractured-vuggy reservoirs was divided into six types: generalized matrix, independent fracture network, independent vug, fracture network system, fractured-vuggy system and vug system on the basis of fractured-vuggy in Tahe Oilfield. (2) The Forchheimer number has been used as a criterion to identify the beginning of non-Darcy flow in fractured-vuggy media. And the formula of critical velocity from Darcy flow to non-Darcy was developed based on this criterion. Characteristics of single-phase critical velocity variation in different structural fractured-vuggy models had been analyzed systematically. In the range of experimental studies, single-phase critical velocity increases with fluid viscosity and fracture density (or connectivity, vug density and vug porosity). (3) According to characteristic parameter and shape, relative permeability curves had been divided into six types: water-phase top concave, water-phase straight line, water-phase below concave, water-phase top convex, water-phase chair and humpback.①In the range of experimental displacement rate, most of relative permeability curves in fractured-vuggy models are belong to water-phase below concave due to early breakthrough of injected water.②Residual water saturation is small, and oil relative permeability descended quickly at the beginning and its decline speed gradually slow with water saturation increasing. However, water relative permeability curve showed below concave clearly, water relative permeability increased slowly at high water saturation, and maximum of water relative permeability was at residual oil.③Reduction of vertical fractures perpendicular to fluid flow or augment of vertical fractures parallel to fluid flow will increase the residual oil saturation and decrease the water saturation of isoperm point.④ Existence of vugs will increase the residual oil saturation.⑤Two-phase critical velocity is not a fixed value. (4) Single-phase fluid flow in fractured-vuggy media had been simulated by finite element method.①Effect of gravity on fluid flow decreases with the decrease vug diameter in the vug and can be neglected in fracture.②When velocity increases to a certain numerical, there is similar"jet"phenomenon in vug. (5) Characteristics of two-phase fluid flow under gravity in different vug diameter, vug position and pore throat position models had been numerical studied by level set method coupling with N-S equation at the first time.①Final recovery is lower in fractured model with bigger vug and residual oil presents in the top space of vug.②Whether oil in the vug, where is only one port connected with the main fracture, can be taken out depending on the location of the vug. If fracture is above the vug, oil in the vug can be taken out by the density difference between oil and water. On the contrary, only a little part of oil in the vug can be flooded out.③If there is only one pore throat connected with vugs, the oil in vugs can't be recovered regardless of orientation of fracture connected with pore throat. The research achievements improved the mechanism of fluid flow in fractured-vuggy media and provide a theoretical basis to further understand the fractured-vuggy reservoir and enhance oil recovery in this kind of reservoir.