A Sudy On Transient Seepage In Complex Fracture System

Recently, fracture problem has become one of research hot topics at home and abroad. Shale gas, tight gas and CBM reservoirs are gas reservoirs which are rich in fractures. The coupling problem between fracture and reservoir is always a complex issue which most scholars at home and abroad use numerical simulation method and semi-analytical method to study. This article uses the mesh-less semi-analytical technique to find the solution in Laplace space. On the one hand, because it is semi-analytical solution, the results obtained are more reliable that some early phenomena can’t be matched by numerical simulation. On the other hand, all models of this article are solved in Laplace space, algorithm will be more simple and quick because we don’t need to consider time variable.Models of this article are relatively comprehensive and includes various models mentioned in the literature, such as horizontal fractures, vertical fractures, ellipse fractures and fracture systems, but there are some results not reported in the literature and there are some models improved from the original models. In terms of vertical fractures, this paper presents asymmetric fractures of finite conductivity in an unconventional gas reservoir, has analyzed the fracture asymmetry influence on typical curves and has carried on the analysis of characteristics of flow. Because face cleats and butt cleats in CBM reservoirs are often anisotropy, we establish mathematical model of CBM reservoir based on it, adopt Angle Matthews and Radial Matthews functions to get the solution of anisotropic model and analyze the influence of anisotropy on typical curve. In terms of horizontal fractures, this paper presents a coupling model between a horizontal fracture and an infinite reservoir, considers the non-darcy effect in the fracture, this article also establishes the typical curve and analyses the influence of non-Darcy factor on type curve. In this paper, the model can be extended to trapezoid fractures, and the coupling solutions of different outer boundaries can be used as a follow-up study of this paper. In the study of the complex fracture system, we first put forward the unsteady mathematical model of a horizontal well with multistage fracturing in a rectangular reservoir and divide the flow section of fractured horizontal well into six sections, they are respectively double linear flow, the first linear flow, the first radial flow, the second linear flow, the second radial flow and boundary flow. The mathematical model of branched fractures is established in carbonate rock reservoir and solved with coupled method, and the model can be simplified into a model with branched fractures in a traditional conventional reservoir which is called Volume Fracturing Model. The result of the four branched fractures and that of the numerical simulation from eclipse are used to verify. Type curve of branched fractures in a carbonate rock reservoir is established. The flow is divided into five regions, they are respectively bilinear flow area, linear flow area, inter-porosity flow area from cave to medium, inter-porosity flow area from substrate to fracture and radial flow area. Finally, we establish the mathematical model of random fractures with finite conductivity, coupled solution of random fracture model is given, and some examples of random fracture are simulated, random fractures will become the main research direction of fracture simulation.