高压条件下页岩过剩吸附量与绝对吸附量的确定及吸附机理研究

Absolute sorption capacity of shales is one of key parameters in determining the shale gas capacity. Due to the physical and chemical properties of methane, the excess sorption and absolute sorption of shales tend to be significantly different along with the increase of pressure, and therefore to accuratily determine the absolute sorption capacity of shales is essential for the evaluation of deep buried shale gas. The most of Paleozoic shales in Southern China experienced much deeper burial history and characterized by high maturity and buried more than 3000 meters deep, to study their excess and the absolute sorption capacities under the ultra-high pressure (greater than 20 MPa) is important to the proper potential evaluation of shale gas in these deep buried Paleozoic shales. Therefore, this study aims to determine the excess sorption capacity of representative Paleozoic shales under high (and ultra) high pressure (greater than 20 MPa)，to investigate and set up the correlation and conversion model between the absolute sorption and excess sorption capacity of shales, and therefore to be able to calculate the absolute sorption capacity precisely and to provide reliable assesment parameter for gas potential evaluation of these deep buried Paleozoic shales in South China. On the other hand, the study on the ultra-high pressure methane sorption of shales is also very important to the developemnt of supercritical methane sorption theory under ultra high pressure.

页岩的绝对吸附量是决定页岩含气性的关键参数之一。由于甲烷的物理化学性质，页岩的过剩吸附量与绝对吸附量随着压力的升高会出现很大的差异，而确定页岩的绝对吸附量对于大埋深页岩气的评价至关重要。我国南方古生界页岩大都经历过较大的埋深，具有成熟度高、 现今埋藏深普遍超过3000米等特点， 研究其(超)高压条件下（大于20 MPa）的过剩吸附量与绝对吸附量的相关性对南方古生界深大埋深页岩气资源潜力评价有重要意义。本申请拟通过模拟吸附实验分析测试典型古生界页岩在高压条件下（大于20 MPa）的过剩吸附量，探索和建立高压条件下页岩的过剩吸附量与绝对吸附量的换算模型，准确计算高压下页岩的绝对吸附量，为南方古生界页岩气资源勘探评价提供科学依据。同时，页岩超高压甲烷吸附研究在国际上属于前沿性的研究课题，有助于完善甲烷超临界超高压吸附理论。

根据我国南方古生界寒武系、志留系等代表性页岩的基础地质分析，如有机碳含量、粘土矿物组成测定、孔隙度测定，SEM测定、比表面积与孔径分析等，并测试了0-20MPa下的甲烷吸附容量。综合分析研究了有机碳含量、粘土矿物含量、孔隙结构与页岩含气性的相关关系。相关研究可知：1）中国南方古生界页岩甲烷吸附容量与其有机碳含量之间具有整体性的正相关关系；2）页岩吸附容量与页岩比表面积呈正相关关系；3）页岩中粘土矿物与其甲烷吸附量没有明显的相关关系，显示了粘土矿物不是页岩吸附作用的主要载体；4）寒武系页岩的吸附热为8.46 kJ mol-1，志留系页岩吸附热为4.29 kJ mol-1，显示志留系页岩比寒武系页岩更容易吸附甲烷气体，二者标准吸附反应熵分别为- 37.39和- 21.74 J mol-1 K-1，也反映了志留系页岩有更好的吸附性能。关于页岩高压吸附研究得到以下成果：1）确定了页岩在高压吸附测试的计算模型；2）寒武系和志留系页岩的20MPa、30MPa、40MPa、50MPa 动力学吸附曲线，形态相似，即气体先扩张，压力变形先下降再微上升后逐渐吸附平衡；3）页岩高压吸附的模型选用Langmuir模型。在低压下，Langmuir模型适用较好，当过剩吸附量达到最大值后，误差越来越大。4）确定了高压下页岩的绝对吸附量和过剩吸附量的转化模型，利用直接测的过剩吸附量和NIST 数据库，可以计算出绝对吸附量。