页岩气超临界状态吸附模型及其地质意义

Supercritical adsorption model of shale gas and its geological significance

为了表征各类型页岩孔隙吸附机理,合理解释页岩气超临界吸附特征,建立了DubininAstakhov和Langmuir-Freundlich(简称D-A和L-F)超临界吸附模型.采用等温吸附实验、低温液氮吸附实验和压汞实验进行模型验证和页岩孔隙分布特征研究,进而依据D-A和L-F模型阐明页岩气超临界吸附特征及意义.结果表明:D-A和L-F模型能够同时表征超临界下微孔充填式吸附和中、大孔单层吸附机理,合理解释页岩气高压负吸附现象;基于吸附速率压力敏感性可将页岩吸附分为4个阶段,各阶段具有不同的地质意义;过剩吸附量与绝对吸附量差异随埋深增大愈为显著,绝对吸附量评价吸附气含量更符合实际;微孔吸附能力约为中、大孔的2倍;灰分和水分对吸附起负作用,且对各类型孔隙的影响程度相当.

The Dubinin-Astakhov and Langmuir-Freundlich（D-A and L-F）supercritical adsorption model was introduced to present the mechanisms of various types of pores and explain the supercritical adsorption characteristics of shale gas.By isothermal adsorption experiment,low temperature N2 adsorption experiment and high-pressure mercury intrusion experiment,the model was verified and the pore distribution features were analyzed.And then the characteristics and significance of shale gas supercritical adsorption were clarified according to the D-A and L-F model.The results show that the D-A and L-F model can characterize the filling adsorption features of micropores and the monolayer adsorption features of mesopores and macropores under supercritical conditions,and can also reasonably explain the high pressure negative adsorption phenomenon of shale gas.Based on the pressure sensitivity of adsorption rate,the adsorption process can be divided into 4stages,of which each stage has different geological significances.The variance between excessive adsorption capacity and absolute adsorption capacity grows more and more significant with the depth increases,while the latter is more feasible in the evaluation of adsorption gas content.The adsorption capacity of micropores is about twiceas large as that of the mesopores or macropores.Both ash and moisture have negative effects on adsorption,and exert equal influence on various types of pores.