页岩气容量法等温吸附实验气体状态方程优选

Selection of shale gas state equation for volumetric isothermal adsorption experiments

页岩气藏资源评价和开发的关键基础资料——等温吸附曲线,通常是通过容量法等温吸附实验获取的,决定其准确与否的关键是选择合适的气体状态方程来计算吸附/解吸气量。为此,针对不同气体状态方程对页岩气相态描述差异较大的情况,设计了简易、方便的气体状态方程优选评价实验装置和实验方案,该方法避开了对气体体积的频繁计算所带来的实验误差,仅通过测试不同状态下的压力即可评价气体状态方程的适用性,其原理和设备操作简单,计算方便。对实验结果的分析结论表明:(1)不同气体状态方程对甲烷的相态描述均有不同程度偏差,且压力越高偏差越大,页岩等温吸附实验中必须优选合适的气体状态方程来计算吸附气量和解析气量;(2)低压下(小于10 MPa)宜选用SRK方程、高压下(大于10 MPa)宜采用RK方程来计算甲烷气田相态变化;(3)总体上,结合我国页岩气藏埋藏较深、压力多在20 MPa以上的实际情况,RK方程能更好地计算出甲烷气田的相态变化特征,从而满足页岩等温吸附实验的需要。

The absorption isotherm, as the critical basic data for the evaluation and development of shale gas resources, is usually ac- quired by means of volumetric isothermal adsorption experiments, so it is critical to select the suitable gas state equation for gas adsorp- tion/desorption volume calculation. When different gas state equations are used to describe the phase state of shale gas, the description is discrepant. In view of this, simple and convenient experiment devices and programs were designed for gas state equation selection and evaluation. Based on this new method, the experimental errors caused by frequent gas volume calculation is evaded and the adaptability of gas state equation can be evaluated only by testing the pressure in different states. This method is characterized by simple theory and operation and convenient calculation. The analysis on experimental results shows that the description of phase state of methane by differ- ent gas state equations is different, and the higher the pressure is, the more obvious the difference is. Therefore, it＇s extremely indispens- able to select the suitable gas state equation for gas adsorption/desorption volume calculation when the isothermal adsorption experiments are performed. Besides, when the phase change of methane is calculated, it is proper to adopt the Soave-Redlich-Kwong （SRK） equation of state for low pressures （〈10 MPa） and the Redlich-Kwong （RK） equation of state for high pressures （〉10 MPa）. Finally, given the fact that the shale gas reservoirs in China are generally buried deep with higher formation pressures （mostly 〉20 MPa）, the RK equation can better reflect the phase change characteristics of methane fields and meet the requirements of shale isothermal adsorption experiments.