The Research On The Evaluation Methods Of CBM Saturation

The coal reservoirs in China are characterized by the feature of "three low and one high"——low porosity, low permeability, low gas saturation and high heterogeneity. Currently, there have appeared abundant research results on the porosity and permeability of the coal reservoirs. But, the theoretical researches on the coal gas saturation and the results of practical results for guiding the CBM (Coal Bed Methane) development are deficient. The gas-bearing property of coal reservoirs is not only one of the basic elements of CBM accumulation, but also the important basis to make a decision on the CBM exploration and development. However, the gas saturation of the coal reservoirs is considered to be one of the most important factors for the gas-bearing evaluation. Then it is inevitable and necessary to do intensive researches on the gas saturation of the coal reservoirs.The value of the CBM saturation is calculated by the ratio of the actual gas content to the theoretical saturated adsorbing capacity corresponding on the absorption isotherm which is measured under the condition of the coal reservoir pressure. Among them, the actural gas content is measured by the gas content test technology ( the USBM direct method is still the most common method until now), while the saturated theoretical saturated adsorbing capacity is gained by the isothermal adsorption test. Therefore, the accuracies of both the measured actual gas content and the saturated adsorbing capacity will directly make an influence on the calculated result of the CBM saturation. As a result, it is necessary to make further standards for these two test technologies. And, based on this, the more reasonable evaluation system on the CBM saturation of the main CBM basins in China can be initially built.According to the research on the every factors influenced the accuracies of the measured actual gas content and the saturated absorbed capacity tests, we can propose some improvements on these two test technologies. The factors, which can make influences on the accuracy of the measured gas content, mainly include: the choice of the calculated methods for the lost gas, the coal sample type with different structures, the choosing of the tested coal samples, the temperature and the pressure, milling time and other external influence factors. Based on the data from desorption tests, there are some ways that can effectively improve the accuracy of actual gas content test, which contain reducing the exposure time of coal samples, choosing the different desorption time and calculated methods according to the coal samples with different structures, choosing the reasonable milling time and drilling speed, etc., and making the desorption test under the condition of the coal reservoir temperature. On the other hand, the factors, influenced the accuracy of the isothermal adsorption test, mainly include the test temperature, the determination of the equilibrium moisture content, gas components, test pressure and so on. Through the research, to improve the accuracy of the saturated absorption capacity measurement, we can make the test under the condition of the reservoir temperature, improve the accuracy of the equilibrium moisture content determination, choose the actual gas components for the test, and optimize the maximum pressure according to the absorption capability of the coal sample.Based on the detailed study on the gas saturation, according to the collected and collated data from the main CBM basins, such as Qinshui Basin, Ordos Basin and so on, and also combined with the desorption and absorption test data, we can make a further macroscopical evaluation on the CBM saturation, and then initially built the gas saturation evaluation system for the main CBM basins in China. The scale of gas saturation in the CBM target areas of China is among 20～91%, with the average value at 45%. According to the distribution situation of the average gas saturation in the main gas-bearing areas, the CBM saturation can be divided into four ranks, namely, >75%, 60～75%, 45～60% and <45%.