| Fractured-vuggy reservoirs are rich in resources all around the world, and the prospect of exploiting this kind of reservoir is bright. However, with the massive fractured-caves and multiple media in the formation, the big range of their sizes and the complicated flow mechanism of the fluid in them, from macroeconomic viewpoint, it's impossible to overlook the fact that fluid flows in different directions within caves and cracks, which eliminates the feasibility of applying multi-media model to describe the flow in fractured-vuggy reservoirs. Since it's inaccessible to use existing models to simulate or predict the seepage mechanisms and flow regimes in fractured-vuggy reservoirs, a new applicable flow-simulating theory is in need. This paper has built several network models (including models of single-phased seepage in matrix, single-phased seepage in cracks, and single-phased free flow in caves) of discrete media network for fractured-vuggy reservoirs, with their unique geologic features specifically considered. In the model, the formation was divided into three systems:matrix, crack and cave. The crack system was dimensionality-deducted; the matrix and crack systems were deemed seepage regions while the cave system was deemed a free flow region. the noted slip boundary condition of velocity raised by Beavers-Joseph-Saffman was adopted at the coupling interface of seepage flow and free flow fluids. A discrete media network model for biphase seepage flow in fractured-vuggy reservoirs was built then, in which caves were considered as cross flow terms when coupled with biphase seepage flow in reservoirs with large scale. After building the models, the matrix and crack systems were discretized by triangle elements and line elements, and then their finite element interpolation functions deduced. Weighted residual method was used to obtain the "weak" form of the equivalent integral of pressure and saturation equation simulated by discrete fracture network, the unit feature matrix and array. Galerkin finite element was then built and used to deduce the numerical calculation format of single-phased seepage flow, single-phased free flow and biphase seepage flow. The results have the merits of maintaining high precision and low grid orientation in simulating flows. The traditional finite element method for free flow region was abandoned in this paper, because the equations in the model are nonlinear, which requires the using of Newton's iteration method to solve them, yet the solving process is usually slow with the final outcomes prone to diverge. However, adopting Characteristic-Based Split method can prevent this from happening. Characteristic-Based Split method adopts linear interpolation functions, makes the orders of the differential terms in the functions uniform, and clears the problems that should have encountered later in numerical methods coupling. Meanwhile, only one linear equation set to deal with at each step makes it faster to get to the end of solving and the answers more accurate than the ones that would have diverged using Newton's iteration method instead. Specific to solve the problem that stiffness matrix misshapes in finite element calculation, this paper has adopted the preconditioned conjugate gradient algorithm to solve the linear equation set and optimized the process of finite element numerical calculation. Moreover, on the basis of the proposed discrete media network model of fractured-vuggy reservoirs and the finite element numerical calculation format, this paper has programmed the "DFVNSIM numerical simulator for discrete fractured-vuggy network", the simulating result on the theoretical model of which conformed well with the result that Eclipse simulating gave in a test. The triangle network was proved to be well applicable in simulating flows, which confirmed the correctness of the theoretical basis and the deduced result in this paper. According to case study, the direction and the length of the fractures and the size of the caves all have significant effect on the flow regime of the fluid in the formation, so it's critical to acknowledge the distribution of the fractures and caves before exploiting the reservoir, deciding the well pattern. This paper has established a theory and a method of simulating the flow regime in fractured-vuggy reservoirs, which is aimed at helping make the exploiting of this kind of reservoirs more efficient... |
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