Development And Applications Of Numerical Simulation Methods Of Flow In Three-dimensional Fracture Network Based On Embedded Discrete Fractures

The development of unconventional oil and gas reservoirs is the key point in the strategic decision-making in the field of energy in China.Horizontal well fracturing technology is the core technology in the development of such reservoirs.The complex fracture network formed after fracturing and the tight matrix of the reservoir constitute the main seepage medium of unconventional oil and gas reservoirs.Although many numerical simulation methods of flow in this kind of medium have been established,there are still some problems such as low model dimension to describe the three-dimensional distribution of fracture network,low calculation accuracy,poor generality to complex geological conditions and seepage model,etc.Therefore,it is of great significance to establish a three-dimensional fracture network flow numerical model with high efficiency,high accuracy and good generality to seepage model for the development of unconventional oil and gas reservoir.First of all,this paper puts forward the parameterized representation method of fracture surface in three-dimensional(3D)reservoirs,gives the method of obtaining the distribution of fracture cells,the geometric parameters of cells and the connections between cells,and establishes an efficient and rigorous pre-processing algorithm of 3D embedded discrete fractures that can be applied to any-shape inclined fracture surface in theory,which provides a preprocessing basis for the subsequent development of highprecision and efficient numerical models based on embedded discrete fractures.Secondly,the Green element method(GEM)based on two sets of nodes and mimetic GEM are developed,in which the mimetic GEM can be used to solve the second-order elliptic or parabolic partial differential equations(including typical mass and heat transfer equations)accurately and stably.Combined with the basic idea of GEM,this paper deduces the unsteady approximation scheme of the transfer flux between matrix cell and fracture cell in multiphase flow,and establishes the 3D mixed Green element EDFM,which improves the early simulation accuracy of multiphase flow in fractured horizontal wells.Furthermore,multi-layer virtual cell EDFM is proposed,which utilizes the intermediate results of embedded discrete fracture preprocessing and avoids the complexity of preprocess brought by local grid refinement(LGR)technique,and achieves the purpose of significantly improving the early simulation accuracy in the case of more extensive seepage model,and more simple and practical.Finally,In view of the poor seepage-model generalization of classical EDFM,this paper analyzes the problems of dimension upgrading and simulation accuracy of the original projection-based embedded discrete fracture model(p EDFM),and puts forward the corresponding modifications for these problems,and proves the equivalence theorem which shows that fracture-projection configuration being physical is equivalent to the fracture-projection configuration being topologically homemorphic to the fracture,and proposes to "cut" the reservoir by flexibly adding flow barriers,so as to realize the efficient processing of complex boundary shape and interlayers,thus establishing The 3D p EDFM with high efficiency,accuracy and practicability can be applied to extensive reservoir development models.A series of the numerical simulators of flow in three-dimensional fracture network based on embedded discrete fractures independently developed in this paper are applied to study depletion development of tight condensate gas reservoir,influence of water invasion layer on water injection huff and puff and CO2 geological storage,and realizes the efficient and high-accuracy simulation of flow in 3D fracture network under the condition of 3D Cartesian grid,and has a wide range of seepage model adaptability,which provides an important numerical simulation tool for the development of unconventional oil and gas reservoirs.