页岩气输运机理的微纳尺度力学研究

发布时间：2018-01-04 19:25

本文关键词：页岩气输运机理的微纳尺度力学研究　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：随着人类社会的快速发展,传统的煤和石油资源急剧消耗。这些化石燃料的过度使用,已严重污染了全球环境。在这种情况下,非常规油气资源日益受到关注和重视。页岩气以其分布范围广、资源储量大、污染低等优点,成为当前油气勘探开发的热点。页岩气是从页岩层中开采出来的天然气,主要成分是甲烷。不同于常规油气藏,页岩基质含有大量的微纳米孔隙,具有低孔隙度和低渗透率的特点。实际地层条件下,页岩气具有多样的储存方式(吸附、游离、溶解等)和多尺度的渗流方式(解吸附、扩散、非达西流、达西流等)。相应地,页岩气的勘探开发涉及一系列多尺度多学科交叉的问题。当前,页岩气的勘探开发仍存在以下几个关键问题。吸附是页岩气的主要储存方式之一,但对纳米孔隙中甲烷吸附机理和吸附结构的认识还不够全面。实际地层影响吸附的因素很多,还没有准确表征页岩吸附能力机理的理论模型。由于大量的甲烷吸附于孔隙中,开采时主要为吸附的逆过程解吸附。对于观测到的解吸附滞后现象,仍未得到合理的解释。受解吸附滞后现象的影响,传统的降压开采方式效率低下。工程上引入注气驱替的新方法,但具体的驱替机理和驱替过程尚不明确。此外,微纳米孔隙中甲烷的流动存在微尺度效应,需建立准确描述其流动行为机理的模型。针对这些问题,本文采用蒙特卡洛和分子动力学方法,开展了系统的研究。研究发现,由于壁面与甲烷存在分子间的相互作用力,纳米孔隙的存储容量高于相同体积的自由态。当甲烷吸附于壁面时,其势能降低。随着孔隙宽度的增加,吸附态甲烷的结构从单个吸附层转变为四个吸附层。对比不同孔隙对应的等温吸附线发现,低压时小孔隙反而能存储更多的甲烷,这表明小孔隙的吸附作用更强。开采时主要为吸附的逆过程解吸附,工程和实验上通常可以观察到明显的解吸附滞后现象。目前解释此现象的机制主要有两种:毛细凝聚和孔喉尺寸变化,本文对这两种机制分别进行了模拟和分析。结果表明:毛细凝聚引起的解吸附滞后主要发生在甲烷的临界温度以下,高温高压的地层条件下不会发生毛细凝聚现象。进一步研究发现,甲烷的吸附会导致页岩基质的肿胀,继而引起孔喉尺寸的收缩。解吸附时,吸附的甲烷需要更高的能量才能通过缩小的孔喉,部分甲烷被困在孔隙中从而引起滞后。基于此机制,研究了压强和温度对解吸附滞后的影响规律。为驱替吸附的甲烷并提高开采效率,工程上通常采用注入气体的方法,常见的注入气为二氧化碳或氮气。模拟研究了这两种气体的驱替机理:二氧化碳可以直接替换出吸附的甲烷而氮气通过降低甲烷的分压促进其解吸附。进一步,对比了相同工况下这两种气体的驱替过程:注入二氧化碳时,突破时间长,驱替面急剧。而注入氮气时,突破时间短,驱替面平缓。页岩基质含有大量的微纳米孔隙,甲烷在这些孔隙中流动时存在微尺度效应。首先,利用平衡分子动力学方法计算了受限孔隙内甲烷的密度分布和自扩散系数。研究发现:吸附层中甲烷的密度较大,自扩散系数较小。在此基础上,利用非平衡分子动力学方法模拟了宽度从1到10 nm孔隙中甲烷的流动行为。结果表明:甲烷的流动速度分布与孔隙宽度有关。随着宽度的减小,速度曲线由抛物线转变为直线。在相对较大的孔隙中,抛物线流可用滑移边界修正的Navier-Stokes方程描述。相应地,计算了有效粘度和滑移长度。随着孔隙宽度的减小,表面扩散机制逐渐变得显著。在小孔隙中,中心位置速度是均匀的而壁面附近速度线性增加。为描述这种流动规律,采用分段多项式进行拟合。对比发现,这种情况下表面扩散机理能显著增加总流量。本文利用分子模拟揭示了页岩气关键问题的微观机理,得到的结果对页岩气的勘探开发具有重要意义。
[Abstract]:With the rapid development of human society, the traditional coal and petroleum resource. The excessive use of fossil fuels, the global environment has been seriously polluted. In this case, unconventional oil and gas resources has been paid more and more attention. With its wide distribution of shale gas resources, storage capacity, low pollution, become the current hot oil and gas exploration. Shale gas is mined from shale gas, mainly methane. Different from the conventional oil and gas reservoir, shale matrix contains lots of micro nano pores, with low porosity and low permeability characteristics. The actual reservoir conditions of shale gas has a variety of storage methods (the adsorption of free dissolved, etc.) and multiscale flow pattern (desorption, diffusion, non Darcy flow and the Darcy flow etc.). Accordingly, the exploration and development of shale gas involves a series of multi scale and multi discipline cross problem. At present, shale The exploration and development of gas still has the following key issues. The adsorption is one of the main storage methods of shale gas, but the understanding of methane adsorption and adsorption mechanism of nano pore structure in the actual formation is not comprehensive enough. Many factors affecting the adsorption mechanism, theoretical model is not accurately characterize shale adsorption ability. Due to the large amount of methane adsorption on the pore, mining mainly for the inverse process of adsorption desorption. The desorption observed with hysteresis, has not been properly explained. Influenced by solution adsorption hysteresis, the traditional way of depressurization efficiency. On the introduction of new methods for the construction of gas injection, but the specific mechanism of flooding and flooding for the process is not clear. In addition, the micro pores of the methane flow in micro scale effect, need to establish an accurate description of its flow behavior mechanism model. To solve these problems, this paper uses the mask. Carlo and molecular dynamics methods, systematic studies were carried out. The study found that due to the wall and the methane interaction force between molecules, the storage capacity is higher than that of the same volume of nano pore free state. When the methane adsorbed on the wall surface, the potential energy decreases. With the increase of pore width, structure of adsorbed methane from a single the adsorption layer into four layers. The adsorption isotherm of different pore corresponding, but small pores can store more methane pressure, which indicates that the stronger adsorption of small pores. When mining mainly for the inverse process of adsorption desorption, engineering and experiment can usually be observed desorption hysteresis there are two main mechanisms. At present this phenomenon: capillary condensation and pore size changes, the two mechanisms were simulated and analyzed. The results showed that capillary condensation caused by solution Adsorption hysteresis occurs mainly in the following critical temperature of methane, high temperature and high pressure formation does not occur capillary condensation. Further study found that the adsorption of methane will lead to shale matrix swelling, followed by pore size shrinkage. Desorption, adsorption of methane requires more energy to pass through the pore throat narrowing. Some methane trapped in the pores causing lag. Based on this mechanism, studied the influence of pressure and temperature on the desorption hysteresis. The displacement for the adsorption of methane and improve the efficiency of mining, gas injection method is usually used in engineering, the common injection gas as carbon dioxide or nitrogen. Displacement mechanisms were studied the two gases: carbon dioxide can directly replace the adsorption of methane and nitrogen by reducing methane partial pressure to promote its desorption. Further, compared to the same condition of the two gases The displacement process: injection of carbon dioxide, the breakthrough time is long, sharp and surface displacement. When nitrogen is injected, the breakthrough time is short, flat surface displacement. The shale matrix contains lots of micro nano pore, micro scale effect of methane flow in these pores. First, the density distribution of limited pore methane and self the diffusion coefficients were calculated by equilibrium molecular dynamics method. The study found: methane adsorption layer in high density, self diffusion coefficient is smaller. On this basis, the flow behavior of width from 1 to 10 nm in the pore of methane was simulated using non-equilibrium molecular dynamics method. The results show that the flow velocity distribution and pore width of methane with the decreasing of the width, velocity curve by parabolic into line. In a relatively large pore, parabolic flow Navier-Stokes equation with slip boundary correction. Accordingly, effective viscosity calculation And the slip length. With the decrease of pore width, surface diffusion mechanism becomes more significant. In small pores, the center position of velocity is uniform and near wall velocity increases linearly. In order to describe the flow pattern, the piecewise polynomial fitting. By comparison, in this case the surface diffusion mechanism can significantly increase the total flow. The micro mechanism of key problems of shale gas is revealed by molecular simulation, the result has important significance for the exploration and development of shale gas.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TE37

【参考文献】