Welch William R W, Piri Mohammad
Department of Petroleum Engineering, University of Wyoming, 1000 E University Avenue, Dept. 3295, Laramie, Wyoming, 82071, USA.
J Mol Model. 2016 Jan;22(1):22. doi: 10.1007/s00894-015-2894-8. Epub 2016 Jan 5.
Molecular dynamics (MD) simulations were performed on a hydrocarbon mixture representing a typical gas condensate composed mostly of methane and other small molecules with small fractions of heavier hydrocarbons, representative of mixtures found in tight shale reservoirs. The fluid was examined both in bulk and confined to graphitic nano-scale slits and pores. Numerous widths and diameters of slits and pores respectively were examined under variable pressures at 300 K in order to find conditions in which the fluid at the center of the apertures would not be affected by capillary condensation due to the oil-wet walls. For the bulk fluid, retrograde phase behavior was verified by liquid volumes obtained from Voronoi tessellations. In cases of both one and two-dimensional confinement, for the smallest apertures, heavy molecules aggregated inside the pore space and compression of the gas outside the solid structure lead to decreases in density of the confined fluid. Normal density/pressure relationships were observed for slits having gaps of above 3 nm and pores having diameters above 6 nm. At 70 bar, the minimum gap width at which the fluid could pass through the center of slits without condensation effects was predicted to be 6 nm and the corresponding diameter in pores was predicted to be 8 nm. The models suggest that in nanoscale networks involving pores smaller than these limiting dimensions, capillary condensation should significantly impede transmission of natural gases with similar composition.
对一种烃类混合物进行了分子动力学（MD）模拟，该混合物代表一种典型的凝析气，主要由甲烷和其他小分子组成，重烃含量较少，是致密页岩气藏中发现的混合物的代表。对该流体在本体状态以及限制在石墨纳米级狭缝和孔隙中的情况进行了研究。在300 K的可变压力下，分别研究了多种狭缝宽度和孔隙直径，以找到孔道中心处的流体不会因油湿壁面而受到毛细管凝聚影响的条件。对于本体流体，通过从Voronoi镶嵌获得的液体体积验证了逆行相行为。在一维和二维限制的情况下，对于最小的孔径，重分子在孔隙空间内聚集，固体结构外部气体的压缩导致受限流体密度降低。对于间隙大于3 nm的狭缝和直径大于6 nm的孔隙，观察到了正常的密度/压力关系。在70巴时，预测流体能够在不产生凝聚效应的情况下通过狭缝中心的最小间隙宽度为6 nm，孔隙中的相应直径为8 nm。这些模型表明，在涉及小于这些极限尺寸的孔隙的纳米级网络中，毛细管凝聚应会显著阻碍具有相似组成的天然气的传输。
