Molecular Dynamics simulations of multiphase flow in nanopores with organic and inorganic substrates

由于全球能源密集型生活方式的快速发展（Mokhatab, 2006） （Wang et al., 2016），对化石燃料的需求不断增加，这就要求开发更多的油气储层，这些储层中的碳氢化合物被困在地下的多孔或裂缝岩层中。油气藏可分为常规油气藏和非常规油气藏。随着常规油气资源的迅速减少，非常规资源之一的页岩气在油气工业中发挥着重要作用（Civan, 2010; Javadpour, 2009； Fathi等）。， 2013)，特别是在美利坚合众国。2009年，美国天然气消费量的87%是国内生产的。根据美国能源信息署2011年年度能源展望，美国页岩气的潜在天然气资源量为2552万亿立方英尺（Tcf），足以为美国提供至少100年的天然气消费量，每天671亿立方英尺（EIA， 2017年8月）。在英国，根据英国地质调查局和英国石油和天然气管理局的数据，英国东南部Wessex地区和Weald盆地侏罗纪页岩的页岩油储量估计为55亿桶。据估计，苏格兰中部谷地的页岩气储量分别为60亿桶和1329万亿立方英尺，英国中部部分地区位于西部的雷克瑟姆和布莱克浦之间，东部的诺丁汉和斯卡伯勒之间。页岩油气产自富含有机物、细粒、低渗透沉积岩的页岩区（美国能源部，2009年）。页岩基质中的孔隙被认为是纳米级的（Loucks et al., 2009; Javadpour et al., 2009）。在这个尺度下，研究人员报道了显著的表面吸附和不均匀分子分布（Li et al., 2014）。对于页岩储层中的油气来说，表面吸附非常重要，因为它严重阻碍了碳氢化合物的流动性。由于基于达西定律和Navier-Stokes方程的宏观/连续体方法在纳米尺度上的有效性有限，因此对页岩中的流动和输运进行分析需要特别注意（Bitsanis et al., 1988）。因此，对特低渗透油藏提高油气采收率的认识还不完全清楚。提高采收率（EOR）的过程——例如通过注入盐水或二氧化碳——强调了在纳米尺度上理解多相输运现象的必要性。由于在EOR中注入的流体旨在取代碳氢化合物，因此影响该过程效率的重要因素是流体的混相水平、粘度、接触角和表面张力。在本研究中，分子动力学（MD）被用于深入了解这些复杂的孔隙尺度传输过程。MD是一种强大的方法，使我们能够在纳米尺度上理解物理和化学过程，例如各种流体在有机和无机基质上的吸附（Jin et al., 2016）。MD具有模拟物相、成分和几何非均质性的能力。因此，该研究项目旨在了解纳米级孔隙中的流动现象，以提高采收率。

The increasing demand for fossil fuels - driven by a fast proliferation of energy-intensive lifestyles over the globe (Mokhatab, 2006) (Wang et al., 2016) - requires the unlocking of ever more oil and gas reservoirs where hydrocarbons are trapped in porous or fractured rock formations in a subsurface. Hydrocarbon reservoirs can be divided in conventional and unconventional reservoirs. With the rapid decline in conventional hydrocarbon resources, one of the unconventional resources, shale gas, plays an important role in the oil and gas industry (Civan, 2010; Javadpour, 2009; Fathi et al ., 2013) especially in United States of America. In 2009, 87% of the consumption rate of natural gas in the United States was produced domestically. Potential natural gas resources from shale play in the United States amount to 2,552 trillion cubic feet (Tcf) according to EIA Annual Energy Outlook 2011, enough to provide the US for at least 100 years with natural gas consumption rate of 67.1 billion cubic feet per day (EIA, August 2017) .In Britain, according to British Geological Survey and UK Oil &Gas Authority, central estimation of shale oil in place from Jurassic shale of the Wessex area and the Weald basin in south-east England is 5.5 billion barrels (bbl) in total. The central estimations of shale gas in place are 6.0 billion bbl and 1329 trillion cubic feet from the midland valley of Scotland and Bowland shale gas in part of central Britain in an area between Wrexham and Blackpool in the west, and Nottingham and Scarborough in the east.Shale oil and gas is produced from shale plays which are organic-rich, fine-grained, low-permeability sedimentary rocks (US DOE, 2009). Pores in shale matrix are considered to be of nanometer-scale (Loucks et al., 2009; Javadpour et al., 2009). At this scale, significant surface adsorption and inhomogeneous molecular distribution are reported (Li et al., 2014). The surface adsorption is important for oil/ gas in place in shale reservoirs as it strongly hinders the mobility of the hydrocarbons. Analysis of flow and transport in shale requires careful attention since macroscopic / continuum approaches based on Darcy's law and the Navier-Stokes equations have limited validity at the nano-scale (Bitsanis et al., 1988). As a result, enhanced hydrocarbon recovery in ultra-low permeable reservoirs is not fully apprehended. The process of enhanced oil recovery (EOR) - for instance through injection of brine or carbon dioxide - accentuates the need for understanding multiphase-transport phenomena at the nano-scale. Since in EOR the injected fluid is intended to displace hydrocarbons, important factors influencing the efficiency of the process are the levels of miscibility of the fluids, their viscosities, the contact angle and surface tension. In this research, Molecular Dynamics (MD) is used to achieve insight in these complex pore-scale transport processes. MD is a powerful method that enables us to understand physical and chemical processes at the nano-scale such as the adsorption of various fluids on organic and inorganic substrates (Jin et al., 2016). MD has the capacity of modelling heterogeneity in phase, composition and geometry.This research project thus is about developing an understanding of flow phenomena in nano-scale pores for EOR purpose.