石墨狭缝中异形孔洞内甲烷赋存状态分子动力学模拟

页岩气主要以吸附、游离和溶解的方式存在于页岩中,而页岩中有机质孔隙结构的多样性又使得页岩气的赋存状态存在具体的差异,同时也受到地质条件的影响。利用Materials Studio软件构建代表有机质的石墨孔隙模型,在石墨层中建立两种带有异型孔洞(矩形和三角形)的狭缝。在345 K,38 MPa的特定温压条件下,采用分子动力学方法研究狭缝内甲烷吸附状态和吸附密度。模拟发现在矩形孔洞最底部甲烷的密度值可达0.599 g/cm3,在三角孔洞底部顶角位置密度值可达0.456 g/cm3,均超出或接近液化甲烷密度。同时发现在孔洞内的甲烷径向分布函数值远大于狭缝界面处的值。最后将现场采集的含气页岩样本进行扫描电镜成像,识别有机质孔隙边界,依据模拟结果,确定第1层和第2层吸附边界,并赋予模拟结果计算出的密度值,最终计算出所研究层位页岩有机质内甲烷储集能力和可能含气量。

Molecular dynamics simulation of existing state of methane in graphite slit special-shaped holes

Shale gas mainly exists as a state of absorbed and free in the pores of shale.The multiformity of structures of organic pores makes it difference in the existing state of shale gas.The graphite model was used as shale organic matrix in Materials Studio and two types of holes (rectangle and triangle) in graphite slit were built.Under the condition of 38 MPa and 345 K,the methane densities in different holes and different locations were calculated and the absorption state of methane was studied by using molecular dynamic method.The maximum density of rectangular holes was high up to 0.599 g/cm3 and the value of density was high up to 0.456 g/cm3 in the bottom of the triangle vertex angle,where the densities were above or close to the density of liquid methane.At the same time,the value of radial distribution function in pore was higher than the interface of slit.Finally,the shale samples collected in the field were imaged by scan electron microscope,and then the organic matter boundary can be identified clearly in the SEM images.Then the first and second absorbed layers were outlined in the image of the studied area.Based on the simulation results of density distributions,the possible shale gas content was calculated.