Method for calculating gas content of thin seam

Abstract

The invention relates to a method for calculating gas content of a thin seam. The method comprises the following steps: 1) collecting basic data of a coal seam to be measured and basic data of a plurality of coal cores; 2) calculating the critical desorption pressure Pcd of each coal core sample according to the basic data of each coal core in the step 1); 3) characterizing the ash content Aad byusing a compensation density logging curve with good lithology sensitivity and a natural gamma logging curve; 4) characterizing the Langmuir volume, the Langmuir pressure and the critical desorption pressure by using the ash content calculated in the step 3) and the corrected burial depth; and (5) substituting the ash content, the Langmuir volume, the Langmuir pressure and the critical desorptionpressure obtained in the step (3) and the step (4) into an equation, solving a regional empirical coefficient, and calculating the current gas content of the thin coal seam of the target layer. According to the calculation method, on the premise that reasonable geological significance and calculation accuracy are guaranteed, the formula form and parameters are simplified, and the wider and wider application range is achieved.

Description

technical field [0001] The invention relates to the field of coal bed gas exploration and development, in particular to a method for calculating the gas content of thin coal seams. Background technique [0002] Coalbed methane is an important unconventional clean energy source. Accurate prediction of gas content in coal reservoirs is of great significance in the safety production of coal mines and the evaluation of coalbed methane resource exploration and development. For decades, predecessors have done a lot of research on the quantitative prediction of coal seam gas content. Among them, the most accurate is the coal core desorption test, but the cost is high and the test horizon is limited, which often cannot meet the needs of exploration and development evaluation. The indirect evaluation method of gas content using well logging and physical parameters is practical, low-cost and effective. The higher resolution can make up for the lack of coring and coal core analysis da...

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Problems solved by technology

[0005] The isothermal adsorption method is to use the Lan's equation to calculate the gas content at different temperatures and pressures. The research shows that the adsorption capacity of coal to methane is related to temperature and pressure, but in actual research, the Lan's equation calculates the gas content. It is the theoretical maximum adsorption amount, which is higher than the actual desorption amount, and requires various corrections. However, most of the various deformation correction formulas of the Lange equation are cumbersome, require many types of parameters, and have limitations in application.

[0006] The nonlinear prediction method is to use mathematical methods to express the nonlinear relationship among various complex factors that affect the coal seam gas content. These relationships are even random and fuzzy. Commonly used algorithms include BP neural network, gray system theory, support Vector machines, etc. These methods have high accuracy, but the data volume required for modeling is large, and the scope of application of the model is small

[0007] In addition, the above commonly used indirect evaluation methods have good application effects in medium-thick and above coal seams, but for thin coal seams with a thickness of less than 1m, or even extremely thin coal seams less than 50cm, the quantitative prediction effect on gas content is not good. Ideal, especially in complex formations where thin coal seams and sandy mudstone are inter-deposited. Compared with thin coal seams, many logging curves have low vertical resolution and curve characteristics are greatly affected by surrounding rocks, and are not sensitive to the response characteristics of thin layers. Seriously affected the accuracy of gas content prediction in thin coal seams

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