含水煤层中气体吸附、解吸-扩散的分子模拟研究

发布时间：2018-01-10 23:14

本文关键词：含水煤层中气体吸附、解吸-扩散的分子模拟研究　出处：《西南石油大学》2015年博士论文　论文类型：学位论文

【摘要】：煤层气储层通常饱和着大量地层水,水的存在会对气体的吸附和扩散产生影响。为了提高煤层气的产量,压裂成为主要的增产措施,而压裂液的适用性是决定煤层气井压裂效果好坏的关键因素之一。因此,本文针对含水煤层中气体吸附和扩散时的特征以及活性水压裂液和泡沫压裂液对CH4扩散的影响,利用分子模拟方法和实验手段展开如下研究：(1)煤层气储层具有非均质性,所以采用Fuchs煤化学结构模型构建煤分子狭缝孔模型。在干燥煤的气体吸附研究中,采用蒙特卡罗法模拟了不同温度和孔径下H2O、CO2和CH4的吸附特征；在含水煤的气体吸附研究中,则分别考虑了单组分CO2和CH4以及双组分CH4/CO2在不同温度、孔径、含水率和摩尔比下的吸附特征。(2)通过分子动力学方法分别模拟计算了单组分CO2和CH4以及双组分CH4/CO2在不同温度、压力、孔径、含水率和摩尔比下的扩散系数、分子平均自由程和等势而分布等参数,揭示了气体在含水煤中的扩散规律。(3)利用分子模拟方法分别建立了泡沫压裂液中起泡剂十二烷基磺酸钠、十二烷基硫酸钠、十二烷基苯磺酸钠和甜菜碱以及稳泡剂聚丙烯酰胺(PAM)、聚乙烯醇(PVA)、聚乙二醇(PEG)和羧甲基纤维素钠(CMC)的分子结构模型。通过分子动力学方法分别模拟计算了不同起泡剂和稳泡剂与煤、水和含水煤之间的相互作用能和泡沫压裂液影响下CH4的扩散系数,并与实验测定出的饱和泡沫压裂液煤粉的CH4有效扩散系数进行了对比,揭示了泡沫压裂液对煤层CH4扩散的影响规律。(4)通过分子模拟方法分别研究了KCl对水分子在煤表面吸附的影响以及钾离子含量的多少对助排剂(十二烷基硫酸钠和十二烷基苯磺酸钠)分子在煤表面吸附的影响。利用分子动力学方法模拟计算了活性水压裂液影响下CH4的扩散系数,并与实验测定¨的饱和活性水压裂液煤粉的CH4有效扩散系数进行了对比,揭示了活性水压裂液对煤层CH4扩散的影响规律。在上述的研究工作中,,吸附和扩散规律总体归纳起来主要体现在以下几方面：(1)在干燥煤中气体的吸附强弱为H2OCO2CH4;温度的升高对水分子吸附的影响程度大于CO2和CH4；孔径的增大使水的吸附量逐渐增大,而C02和CH4则呈现出相反的趋势；水因氢键和范德华势的共同作用使其在煤狭缝中的吸附热高于CO2和CH4的吸附热；水在狭缝孔中吸附时密度均匀分布,而CO2和CH4的密度分布集中在壁面。(2)含水煤中,含水率的增大、温度的升高和孔径的增大会使单组分CO2和CH4的吸附量降低。双组分CH4/CO2也有类似结论,但当温度逐渐升高和逸度逐渐增大时,CO2和CH4的绝对吸附量曲线和过剩吸附量曲线均出现交错区；孔径的增大有利于CH4绝对吸附量的增加,但无论是在哪种孔径下CO2的过剩吸附量始终大于CH4的过剩吸附量；CO2摩尔含量的增大使CO2的过剩吸附量曲线逐渐出现峰值、开阔区和交错区；孔径的增大使高摩尔含量CO2的过剩吸附量骤减程度逐渐增大；在大孔径下,当CH4/CO2摩尔比小于1且压力较高时,CO2在竞争吸附中处于劣势。(3)含水煤中,温度的升高、压力的降低、孔径的增大和含水率的降低会促进单组分CO2和CH4在煤狭缝孔中的扩散,使扩散系数增大。双组分CH4/CO2也有类似结论,当CH4/CO2的摩尔比逐渐减小时,CO2的扩散系数逐渐增大,而CH4的扩散系数仅在0.35×10-8m2/s左右变化,可见不管CO2的摩尔含量是否占优势,CH4的扩散过程始终处于劣势。虽然影响因素不断变化,但气体扩散始终以分子扩散和Knudsen扩散为主。(4)起泡剂和稳泡剂分子虽然会吸附于煤分子表面,但不同起泡剂和稳泡剂分子的吸附性能有较大差异；与无起泡剂时CH4的扩散系数对比可知,起泡剂的加入会使CH4的扩散系数显著降低,造成CH4从煤基质微孔向裂缝的扩散受阻；与仅加入起泡剂时CH4的扩散系数对比可知,稳泡剂的加入大大降低了CH4的扩散系数,大分子聚合物型稳泡剂会对煤层带来极大的伤害,显著降低CH4的扩散速度。(5)活性水压裂液中KCl的存在可减少水分子和助排剂分子在煤表面的吸附；因钾离子有效干扰了水分子占据煤表面的吸附位,减缓了水分子吸附引发的煤基质膨胀效应,所以2%KCl水溶液影响下含水煤层中CH4的扩散系数稍大于水影响下的扩散系数,而因助排剂分子与煤表面发生了吸附,所以活性水压裂液影响下的扩散系数最小。综上,本文完成了含水煤层中气体吸附扩散的分子模拟研究,揭示了单组分和双组分气体的吸附和扩散规律以及泡沫压裂液和活性水压裂液影响下煤层CH4的扩散规律,取得研究手段和结论认识的突破。
[Abstract]:Coalbed gas reservoir is usually saturated with lots of formation water, the water would gas adsorption and diffusion effect. In order to improve the gas production, fracturing has become the main measures to increase, and the applicability of the fracturing fluid is the key factor to determine the quality of the CBM well fracturing effect. Therefore, this paper according to the characteristics of gas water coal seam in the adsorption and diffusion of active water fracturing fluid and the effect of CH4 foam fracturing fluid diffusion, simulation method and experimental method is studied as following: (1) the use of molecular CBM reservoir heterogeneity, so the chemical structure model of Fuchs coal slit pore model. The coal molecular adsorption on coal gas drying in the simulation, different temperature and different aperture H2O by Monte Carlo method. The adsorption characteristics of CO2 and CH4; adsorption on coal gas water, were considered the single component and CH4 and CO2 Two component CH4/CO2 in different temperature, pore size, adsorption characteristics of water content and molar ratio. (2) simulated single component CO2 and CH4 and two-component CH4/CO2 in different temperature, pressure, pore size was calculated by molecular dynamics method, the diffusion coefficient of water content and the molar ratio of the molecular mean free path and etc. the potential distribution parameters, reveals the law of gas diffusion in coal in water. (3) were established for foaming agent twelve sodium dodecyl sulfate foam fracturing fluid using molecular simulation method, twelve sodium dodecyl sulfate, twelve sodium and beet alkali and foam stabilizing agent of polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene glycol (PEG) Suo Jiaji (CMC) and cellulose molecular structure model. To simulate different foaming agent and foam stabilizing agent and coal by the method of molecular dynamics, the interaction between water and water content of coal and foam fracturing fluid. Sound diffusion coefficient CH4, the saturation of foam fracturing fluid and pulverized coal and experimental determination of the effective diffusion coefficient of the CH4 were compared to reveal the influence of coal seam fracturing fluid of CH4 diffusion. (4) the KCl of water molecules on the surface of coal adsorption and the influence of the number of rows of potassium ion content to help agent were studied by molecular simulation methods (twelve sodium dodecyl sulfate and twelve sodium) molecules in the influence of coal surface adsorption. Using the molecular dynamics simulation of the diffusion coefficient of liquid crack under the influence of the activity of CH4 hydraulic calculation, and experimental measurement of saturated hydraulic fracturing fluid. The activity of pulverized coal CH4 effective diffusion coefficient were compared, revealing the active water fracturing fluid influence law of coal seam CH4 diffusion. In this research work, adsorption and diffusion, summed up the general law mainly reflected in the following aspects: (1) in dry coal gas The adsorptive body is H2OCO2CH4; the rise of temperature influence on the adsorption of water molecules is greater than CO2 and CH4; the diameter increases the absorption capacity of water increases gradually, while C02 and CH4 showed the opposite trend; the adsorption heat water by interaction of hydrogen bonds and Fan Dehua potential the heat of adsorption in slit in coal higher than that of CO2 and CH4; the water adsorption in slit pores in the uniform density distribution, while the CO2 and CH4 density distribution concentrated in the wall. (2) water content in coal, the increase of the water content, temperature and pore size increases with single component CO2 and CH4 adsorption amount decreased. Two component CH4/CO2 has a similar conclusion, but when the temperature rises gradually and fugacity increases gradually, the absolute adsorption curves of CO2 and CH4 and the excess adsorption curves were ecotone; pore size increased CH4 absolute adsorption, but no matter in which CO2 had left suction aperture The amount of CH4 is always greater than the excess adsorption; increasing the CO2 molar content of the excess adsorption curve of CO2 gradually peak, open area and transitional area; the aperture increases the excess adsorption degree and the content of CO2 was sharply increased gradually; in large aperture, when the CH4/CO2 molar ratio is less than 1 and the pressure is higher. In the competitive adsorption of CO2 at a disadvantage. (3) water content in coal, the increase of temperature, the pressure decreases, the diameter increases and the lower moisture content will promote the diffusion of single component CO2 and CH4 in coal slot Kong Zhong, the diffusion coefficient increased. Two CH4/ CO2 has a similar conclusion, when CH4/CO2 the molar ratio is gradually reduced, the diffusion coefficient of CO2 increases gradually, and the diffusion coefficient of CH4 changes only around 0.35 * 10-8m2/s visible, regardless of whether the CO2 molar content is dominant, the diffusion process of CH4 is always at a disadvantage. Although the influence factors of changing, But the gas diffusion has always been to molecular diffusion and the diffusion of Knudsen. (4) foaming agent and foam stabilizer molecules adsorbed on the surface while the coal molecule, but different adsorption properties of foaming agent and surfactant molecular foam had great difference; and comparing the diffusion coefficient of CH4 foaming agent, foaming agent with the the diffusion coefficient of CH4 decreased significantly, resulting in CH4 blocked from coal matrix to crack the micropore diffusion; diffusion coefficient compared with only adding CH4 foaming agent, adding foam agent greatly reduces the diffusion coefficient of CH4, high molecular polymer foam stabilizing agent will bring great harm to the coal layer, significantly reduce diffusion the speed of CH4. (5) active water fracturing fluid in the presence of KCl can reduce the adsorption of water molecules and help row agent molecules on the surface of coal; due to the adsorption of potassium ion effectively interfere with water molecules and coal surface, slowing the water molecular adsorption caused by coal The matrix expansion effect, so 2%KCl solution under the influence of diffusion coefficient of water in coal seam CH4 is slightly larger than the diffusion coefficient of water under the influence, and because of additive molecules and coal surface adsorption, so the active water fracturing fluid under the influence of the diffusion coefficient minimum. To sum up the molecular simulation study of gas containing water in coal seam the adsorption and diffusion, reveals the single and binary component gas adsorption and diffusion and foam fracturing fluid and active water diffusion in CH4 coal seam fracturing fluid under the influence of research methods and conclusions achieved breakthrough.

【学位授予单位】：西南石油大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TE377

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