超临界条件下煤层甲烷视吸附量、真实吸附量的差异及其地质意义

Differences between apparent and true adsorption quantity of coalbed methane under supercritical conditions and their geological significance

煤层甲烷高压等温实验一般已属于超临界条件,由其吸附数据计算出的视吸附量不能反映真实吸附量,两者存在差异,因而由视吸附量建立的煤层气产能评价及含气性评价需要重新进行厘定。为深入研究这一差异性,基于前人成果并结合一般气体状态方程,给出了甲烷视吸附量和真实吸附量在不同压力点下的关系式,对具体等温吸附数据进行了计算。结果显示:真实吸附量和视吸附量差值随压力增大而增大,煤储层吸附性越强差值越大;同时,以视吸附量代替真实吸附量求取的临界解吸压力和实测饱和度均要大些。据此认为,依据视吸附量预测深部含气量会远远低估深部煤储层的含气性,超临界条件下,深部游离气含量数值可能要远远大于以往的认识。该结论对于重新认识煤储层真实吸附性及含气性具有重要意义。

High pressure isothermal experiments of coalbed methane are generally carried out under supercritical conditions,thus the apparent adsorption quantity(AAQ) calculated from adsorption data can not reflect the true adsorption quantity(TAQ) and there exist differences between them.Therefore,previous evaluations of coalbed methane productivity and gas content based on the AAQ need to be reevaluated.Based on previous study results and in combination with the general equation of gas state,the relational expressions of the AAQ and the TAQ under different pressures are established for calculating their differences by using the true adsorption isotherm data.The results show that the difference between the AAQ and the TAQ become enlarged with the increasing of pressure points and with the enhancing of adsorbability.Both the critical desorption pressure(CDP) and measured saturation calculated by using the AAQ are relatively higher than that calculated by using the TAQ.It is therefore believed that the CBM content of deep coal reservoirs may be significantly underestimated when the AAQ is used in calculation.The content of free gas in deep coal reservoirs is much higher than that from our general perception.These understandings are helpful to the reevaluation of true adsorbability and gas content of coal reservoirs.