52 / 2023-05-15 20:51:01

A new numerical calculation method of gas flow model and gas permeability coefficient in single-porosity coal seam

Coal seam,Gas flow,Single-porosity,Gas permeability coefficient,Radial flow method

China is rich in coal seam gas resources, safe and green mining and utilization of coal seam gas resources is a very far-reaching and valuable research topic. The theoretical modeling and calculation of gas flow in coal seams are very important for gas extraction and coalbed methane resources exploitation. The traditional single-porosity gas flow model considers that coal seam is a kind of uniform continuous medium and gas migrates in fractures mainly under the action of pressure gradient and it is mainly built on the theoretical basis of Darcy's law. In coal mine site, engineering calculation requirements are generally simple and convenient. Many scholars established a gas flow model in uniform continuous medium by mathematical equation, combined with the nature of coal and its influencing factors, and obtained the gas emission law of borehole by solving the single-porosity gas flow model. In order to get the analytical solution of the gas flow equation, predecessors simplified it, and the result is equivalent to simplifying the gas content in coal seam as proportional to the square of gas pressure. On this basis, the dimensionless correlation formula of borehole gas emission and coal seam permeability coefficient is obtained through dimensionless analysis, which is also the key step of traditional borehole radial flow method to determine the coal seam permeability coefficient. The above treatment benefits from the convenience of calculation after model simplification, but ignores the deviation of calculation results. With the development and popularization of numerical calculation methods, gas flow and gas emission can be calculated accurately without simplifying the gas flow model. In view of the above situation, this paper firstly established the single-pore gas flow model of coal seam based on the relationship between gas content and pressure under Langmuir's and simplified expressions respectively, then carried out dimensionless processing on it, and carried out numerical iterative solution through independently developed program code. The deviation of the calculation result caused by the simplified formula is described quantitatively from the perspectives of gas pressure and content distribution as well as gas gushing velocity and cumulative gushing amount of borehole. The simplification of Langmuir formula into parabolic formula does not substantially simplify the calculation. The Langmuir formula should be used for coal seam gas content. There are essential differences between constant coefficient model and Langmuir model which cause inestimable errors in the calculation of borehole gas emission characteristics and coal seam permeability coefficient. Then a new numerical calculation method for coal seam permeability coefficient was established on the basis of Langmuir formula, and a specific expression for calculating coal seam permeability coefficient is given. Compared with the traditional radial flow method, the advantages of the new calculation method for coal seam permeability system are analyzed in detail. The new expression of coal seam permeability coefficient can correct the calculation error of traditional method from the root, simplify the tedious trial calculation process, and ensure the rationality of the calculation process. The research content can provide basic support for coal seam gas migration theory modeling, key flow parameter determination, mine gas prevention and utilization and other projects.