Research On Transient Seepage Theory Of Fractured Wells With Complex Percolation Mechnisims In Multiscale Shale Gas Reservoir

Shale gas is a kind of self-sealing and self-souring, unconventional gas which is mainly stored in clay shale and its interlayers with free or adsorbed state. The reserve of shale gas in china is tremendous, so how to efficiently develop it has become increasing important to guarantee the energy safety, improve energy structure and keep sustainable economic development.After decades of technique research for shale gas exploitation and development in North American, although the horizontal well drilling and massive hydraulic fracturing have been unprecedented development, the gas engineering of shale gas efficient development, especially for the special seepage law and its complex mechanism of gas flow lag far behind the field practices. The existed theoretical models can not accurately recognize the fracture characteristics, optimize the fracture parameters, evaluate the productivity after fracturing and determine the physical parameters et al., which will affects the efficient and scale development of shale gas reservoir.As a special kind of gas reservoir, shale gas reservoir has significant difference with conventional gas reservoir in accumulation, occurrence and percolation mechanism. Shale gas reservoir has complex, multimodal pore-size distributions and unique storage properties, causing shale gas to be transported by multiple flow mechanisms. In addition, the complex fracture network will be formed during the massive hydraulic fracturing, which is very difficult to be described in the mathematical models. In a word, the development of shale gas reservoir is much more difficult comparing to the conventional gas reservoir, and the development theory and method cannot directly apply in the shale gas reservoir. So, we should establish the corresponding mathematical models according to the characteristics of shale gas reservoir. Based on the literature review, this thesis carried out an in-depth study on the transient seepage theory of fractured well in shale gas reservoir and the major contents of this dissertation are summarized as follows:(1) Based on the research of pore types and structures, gas storage types as well as gas transport mechanisms, the proper mathematical models are selected to describe the gas flow in multi-scale pore media.(2) The pseudo-dual porosity model and pseudo-triporosity models are established by considering the gas follows steady diffusion; the improved pseudo-dual porosity model and the improved pseudo-triporosity models are established by considering the gas diffusion satisfy the Fick law. Considering the gas flow in the nano porous media is the combination of multiple seepage mechanisms including Knudsen diffusion, viscous flow and slippage effect, et al. the comprehensive mathematical model to describe the gas flow in natural fractures, macro pore and nanoscale pore in matrix system is established.(3) Based on the Green and Source function, the mathematical models of fractured vertical and horizontal wells in circle and rectangular closed boundary reservoir are established and the corresponding solutions are obtained for the five different microscopic seepage mechanism model. By the method of numerical simulation, the pseudopressure and production curves are plotted and the effects of gas desorption properties, the related parameters of reservoir and fracturing technology on pressure and production curves are analyzed.(4) The transient seepage mathematical models of fractured vertical and horizontal well in circle and rectangular reservoir are established with the composite medium models which taken into account the effects of fracture network. The corresponding solutions of all kinds of models are solved by the Green and Source function and Boundary Element method. Using the numerical inversion method, the dimensionless pseudopressure and production curves are obtained and the effects of fracture network fracture and reservoir properties on well behaviors are analyzed.(5) The effects of fracture network on well production performance are analyzed with the theoretical models, and the results show that the adsorption gas is mainly affect the well production in middle and later period and the early production is mainly determined by free gas; the number of fractures and the scale of fracture network has a limited effects on the well production performance which has a optimization value; the shale gas production not only depends on the three key technologies, but also depends on the quality of reservoir characteristics, so we should give a priority to develop the sweet area of shale gas reservoir.(6) Field data is interpreted to verify the correctness and utility of the theoretical models developed in this thesis.The research and the results obtained in this paper can provide theoretical guidance to efficient and scale development for shale gas reservoir.