Study On The Flow Characteristic Of GAS In Tight Reservoir

Tight sandstone gas and shale gas are important unconventional gas resources for their wide distribution and huge developing potentials. In these low permeability reservoirs the microscopic pore-throat structures and migrations exhibit typical characteristics of "unconventional gas reservoir". Several transport mechanisms exist in gas migration, mainly including molecular diffusion, Knudsen diffusion and convection and the traditional Darcy Law is not fully applicable. Therefore, it is necessary to conduct study of transmission mode and capacity of gas in tight porous media. Dusty-Gas Model considers the molecular diffusion and advection process in multi-component system and the interaction between molecules and porous media wall with the introduction of Knudsen diffusion term. Moreover, Dusty-Gas Model and pore network simulation model could better describe the complex flow characteristics of gas in tight reservoirs and analyze the results with classical continuum flowing theory. This thesis focuses on the study of the process simulation and mechanism of gas flow in tight reservoirs.Firstly, the flowing models of gas in tight reservoirs are compared and selected and based on this three classical pore network models are established as simulation platform for evaluation of fluid flow in porous media:(1) with regular cube as the basic structural unit, the irregular topology pore network model is constructed by randomly setting the coordination number; (2) based on the digital core of real rock obtaining from micro-CT scanning, the realistic pore network model is constructed by directly extracting the pore space topology structure and geometric parameters with the maximum ball algorithm; (3) the random pore network model generation algorithm of equivalent topology is established, and the random network model equivalent to realistic pore network is constructed based on the pore-throat structural parameters and pore space topology structure of real rocks.Next, the pore network model simulating the flowing of gas in tight reservoirs is established with the constructed pore network models as the migration flow channel and the Dusty-Gas Model to describe the gas transmission and migration in porous media. The model consists of gas flow equations, control equations and auxiliary equations in tight reservoirs. The discrete form of equations is established with pore network node as the describing object and finally solved by the numerical solution of nonlinear equations to realize the gas flow simulation in micro-scale pore network. Then a new method for evaluating gas flow parameters is proposed.Based on the established evaluation principles and methods of flow parameters, the method for determining model equivalent permeability and diffusion coefficient is presented. And the impact factors of flow parameters and the representative elementary volume are evaluated with the regular pore network. The influences of throat radius, coordination number and throat length on flow are analyzed and the influencing factors and correction principles of apparent permeability are proposed based on the simulation results of single component system and multi-component system.The comparisons of simulation results and experimental data verify the feasibility of the combination of pore network model and Dusty-Gas Model established in this paper. The study here has enriched the microscopic percolation theory and broadened the research scope of percolation theory. The results provide theoretical basis for the parameter determination and research of percolation in tight reservoirs especially in shale gas reservoirs. The method presented is an effective way to solve the seepage problem in shale gas reservoirs.