Determination of gas content in coal reservoir conditions

摘要

The gas content is one of the most key factors to evaluate the development potential of coalbed methane resource.However, it is difficult to determine the gas content in the reservoir conditions.Commonly, the GB/T 19559-2008 (Chinese national standard) is took as a reference to acquire the actual measurement methane content, and some prediction methods are also presented, i.e., coal rank-ash content-gas content analogue method, gas content gradient method, comprehensive analysis method considering the geological conditions, isothermal adsorption method, relative flux method and different mathematical models prediction methods.In this paper, a new gas content prediction method based on Henry's law and isothermal adsorption characteristics is presented to acquire the gas content in the reservoir conditions.The fractures and pores in the coal reservoir are filled with water, and the free gas is less.For the under-saturation coalbed methane reservoir, the adsorbed phase gas is the main type for the methane content.In the underground water, the salt ions (Na+ 、Cl 、Mg2+) do not have Laser Raman activity, and they could not produce Laser Raman spectroscopy (LRS).While, the molecule of methane dissolved in the underground water could produce LRS.Therefore, the ratio of peak area between methane and water in the reservoir conditions is used to be the indicator of the methane concentrations, the methane solubility in the underground water could be tested, and the methane partial pressure could be calculated with the help of Henry's law.According to the Langmuir isothermal adsorption curve, the adsorption methane content with the given methane partial pressure could be calculated.