页岩气在孔隙表面的赋存状态及其微观作用机理

Occurrence State and Micro Mechanisms of Shale Gas on Pore Walls

吸附态是页岩气的主要赋存状态之一,对吸附气含量的准确评价是页岩气勘探开发中的重要环节.在页岩吸附气含量评价过程中,所选用的等温吸附模型是否遵循页岩气的赋存状态及其微观作用机理,是决定模型适用性的关键所在,也是决定吸附气含量评价准确性的重要因素.因此,需要对页岩气在孔隙表面的赋存状态及其微观作用机理开展深入研究,为科学地优选或建立吸附气评价模型提供理论依据.利用巨正则蒙特卡洛(Grand Canonical Monte Carlo,简称GCMC)法分别模拟甲烷在有机质和伊利石孔隙中的吸附特征并得到分子构型,并进行分子动力学(MD)模拟使体系达到充分平衡.在此基础上,根据气体浓度分布、密度场分布以及分子间相互作用等特征阐明页岩气在孔隙表面的赋存状态及其微观作用机理.研究表明,页岩气在孔隙表面的吸附作用并非单层吸附,吸附相可划分为强吸附层、弱吸附层和二者之间的吸附层波谷.强吸附层主要受到矿物表面的吸附作用;吸附层波谷与弱吸附层既受到矿物表面的吸附作用,又受到不同吸附层之间的吸附作用.Langmiur模型与BET模型的假设条件与此机理不严格相符,可能对模型评价精度造成一定影响.对页岩气在孔隙表面赋存状态及其微观作用机理的研究,有望为吸附模型的优选或建立提供理论依据.

The adsorption gas is one of the main occurrence states of the shale gas,so the accurate evaluation of the adsorption gas amount is important for exploration.The key for the applicability of the models lies in whether the adsorption model obeys the occurrence state and its micro mechanisms of shale gas on the pore walls,which is the critical factor influencing the accuracy of the evaluation of the adsorption gas amount.The occurrence state and its micro mechanisms of shale gas on the pore walls are investigated in this study,aiming at optimizing the adsorption model.By the Grand Canonical Monte Carlo(GCMC)method,the adsorption behavior of CH4 in both organic and illite pores was simulated,and the molecular dynamic(MD)was performed on the molecular structure at a certain temperature and pressure.Based on the simulations,the gas distribution,the interaction distribution and density field were investigated to document the occurrence state and its micro mechanisms of shale gas on pore walls.It is found that the adsorption of shale gas on pore walls is not the single layer adsorption,and the adsorption phase can be divided into the strong adsorption layer,the weak adsorption layer and the trough of adsorption layer.The strong adsorption layer is mainly adsorbed by the pore walls,while the weak adsorption layer and the trough of adsorption layer are not only adsorbed by pore walls,but also adsorbed by the adsorption layers.This is inconsistent with the hypothetical condition of the Langmiur model or the BET model,so the reasonability of the models and the accuracy of the evaluating results would be limited.The occurrence state and its micro mechanisms of shale gas on pore walls revealed in this study can provide theoretical basis for optimizing and establishing adsorption models.