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The determination of dynamic reserves is important for tight sandstone gas reservoirs in production. Based on the geological and gas data of the Yan’an gas field, the influence of pressure on the properties of natural gas is studied by mathematical methods. At the same time, the modified flowing material balance equation is established considering the changes in gas viscosity and compressibility. The result shows that (1) the viscosity of natural gas increases rapidly with pressure; (2) the deviation factor decreases with pressure (P < 15 MPa) and then increases (P > 15 MPa) with temperature; (3) the compressibility decreases rapidly with pressure and increases with temperature; (4) compared with the results of the material balance method, the average error of the flowing material balance method is 33.95%, and the accuracy of the modified flowing material balance method is higher with an average error of 1.25%; and (5) a large change in the production will affect the accuracy of the modified flowing material balance method, especially a shut-in for a long time before the pressure drop production is calculated at a certain time, so data points that are relatively consistent should be selected as far as possible to calculate the dynamic reserves. The findings of this study can help in the accurate evaluation of dynamic reserves of the tight gas reservoir in the Yan’an gas field and are an important guide for the formulation of a rational plan for the gas reservoir and its economic and efficient development.

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The determination of dynamic reserves of gas well is an important basis for rational production allocation and development of a single well. The commonly used flow material balance method (FMB method) uses the slope of the curve of wellhead pressure and cumulative production after stable production of gas well to replace the slope of the curve of average formation pressure and cumulative production to calculate the controlled reserves of single well. However, based on the theoretical calculation, the FMB method ignores the change of natural gas compression coefficient, viscosity and deviation coefficient in the production process. After considering these changes, the slope of the curve of the relationship between bottom hole pressure and cumulative production and the slope of the curve of the relationship between average formation pressure and cumulative production are not equal. In order to solve this problem, the influence of pressure on each parameter is considered, and the equation of modified flowing material balance method is derived. The application of Yan'an gas field in Ordos Basin shows that: compared with the results of the material balance method, the result of the flow material balance method is smaller, and the maximum error is 58.816%. The consequence of the modified mobile material balance method is more accurate, and the average error is 2.114%, which has good applicability. This study provides technical support for an accurate evaluation of dynamic reserves of tight gas wells in Yan'an gas field, and has important guiding significance for economic and efficient development of gas reservoir.