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作者简介:

陈花(1994—),女,江西萍乡人,在读硕士研究生,从事油气田开发方面的研究。联系电话:18062448468,E-mail:201771176@yangtzeu.edu.cn。

通讯作者:

关富佳(1978—),男,黑龙江兰西人,副教授,博士。联系电话:18971117517,E-mail:guan_fujia@163.com。

中图分类号:TE31

文献标识码:A

文章编号:1009-9603(2019)02-0087-07

DOI:10.13673/j.cnki.cn37-1359/te.2019.02.012

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目录contents

    摘要

    体积法和重量法是页岩气吸附实验的两种重要方法,相比基于MSB磁悬浮测量的重量法,体积法的实验仪器结构和原理简单,造价也低,是目前中国进行页岩气吸附实验评价的主要手段。然而,体积法的最大问题在于气体压缩因子计算不准确,尤其在高压下,利用该方法对实验数据进行吸附量计算时会产生较大误差。针对体积法在高压下的不适应性,采用编程计算7种复杂气体状态方程压缩因子,并将计算结果与美国NIST数据库中Chemistry 部分计算的甲烷压缩因子进行对比,结果表明:Setzmann方程不论在高压还是低压下气体压缩因子的计算精度均较高,解决了体积法在高压下的不适应性;应用改进前后的体积法分别对实际页岩气等温吸附实验数据进行解释, 当平衡压力超过5 MPa时,两者计算吸附量差异随平衡压力增大而增大,平衡压力为30 MPa左右的极限吸附量相差0.4265 mL/g,可见气体压缩因子计算精度的重要性;应用吸附理论模型对实验测得页岩气吸附等温线进行拟合, 发现Toth吸附模型拟合精度最高。

    Abstract

    Volumetric method and gravimetric method are two important methods for shale gas adsorption experiment. Com- pared with the gravimetric method based on magnetic suspension balance(MSB),the instrument of volumetric method has simple structure and principle,and low cost. The volumetric method is the main means to evaluate the shale gas adsorption experiments in China. However,the biggest problem of the volumetric method is that the calculation of the gas compressibil- ity factor is inaccurate,especially when this method is used to calculate the adsorption amount at high pressure. In order to solve this problem,the compressibility factors of seven complex gas state equations were calculated through programming, and the calculated results were compared with those calculated in the Chemistry part of NIST database in USA. The results show that the gas compressibility factor calculated by the Setzmann equation is accurate at both high and low pressures, which solves the shortage of the volumetric method at high pressure. The actual shale gas isotherm data was interpreted by the volumetric method and the improved volumetric method respectively. The result shows that when the equilibrium pres- sure exceeds 5 MPa,the adsorption amount difference increases with the increase of equilibrium pressure,the maximum ad- sorption amount difference is 0.4265 mL/g at equilibrium pressure of about 30 MPa,which indicates the importance of ac- curate calculating the gas compressibility factor. The adsorption isotherm of the shale gas was fitted by the adsorption theo-ry model. The results show that Toth adsorption model has the highest accuracy.

  • 近年来,页岩气作为常规油气资源的重要补充,是油气开发领域的热点,引起了中外的广泛关注,中国的页岩气勘探开发亦取得了突破性进展[1-3]。目前评价页岩气藏吸附量方法很多,且不局限于实验的方法。但应当指出,构成等温吸附曲线的数据点并不是实验仪器直接测定,而是根据实验的有关数据资料利用理论计算方法求得[4]。胡微雪等通过参考煤层气含气量测定标准进行计算,分析不同时刻产气速率,得出温度和压力对页岩气吸附量的影响[5],但是考虑到煤层气和页岩气储存条件和含气量的不同,该方法并不能准确应用于页岩气吸附量的计算。李武广等通过实测多组等温吸附数据,利用变差函数的球状模型对数据进行拟合,推导得到吸附量与温度、总有机碳含量、成熟度之间的关系式,获得吸附量随多因素变化模型[6],但是该方法在进行吸附量公式拟合时,仅仅是通过公式的相关系数来表征相关性,没有给出具体的理论模型,随机性较大。邢翔等以单层气-固Langmuir吸附理论为基础,运用球状模型进行数据拟合,获得等温吸附特征曲线[7]。由于该方法在原有实验数据基础上进行了2次数据拟合,导致误差变大。曾鑫等通过对实验数据进行拟合,分析温度、压力、总有机碳含量、成熟度、孔隙度和含水量对页岩吸附量的影响[8],但是在数据拟合过程中仅通过数值特征直接给出相应的公式,通过统计学的相关系数表示公式可靠性,没有给出明确的理论模型,公式不具备普遍性,同时,对于吸附量的具体测定也未能详细陈述。陈磊等通过对川西坳陷新页HF-1井须五段泥页岩吸附强度具体分析,总结比表面积、孔隙度、密度、成熟度和湿度等对吸附量的影响强度及影响规律,并给出相应的理论依据[9],其计算新模型对页岩气含量主控因素分析详细,但对于吸附量定义不明确,且未提供吸附量的具体测定方法,只是针对某一具体区块进行实验分析。王建国等在Langmuir单层吸附理论的基础上,拟合球状模型,获得总有机碳含量与吸附量之间的定量关系公式[10],但研究的影响因素单一,同时对于计算所得的吸附量数据未能进行详细说明。为此,笔者通过分析现有吸附量测量方法存在的问题,通过VB编程实现了复杂气体状态方程求解甲烷压缩因子,利用改进后的体积法精确计算了页岩气吸附量,并与改进前的计算结果进行对比,分析可知改进后的计算新方法解决了体积法在高压下的不适用性,具有更高精度。

  • 1 计算吸附量存在的主要问题

  • 目前,用于页岩吸附实验的方法应用最为广泛的为体积法和重量法[11]。其中,基于体积法的吸附量计算方法有两种思路,一种是不考虑吸附相体积的体积法,另一种是考虑吸附相体积的体积法。 GB/T19560—2008[12] 中规定的体积法即是基于第1种思路的计算方法,但其是错误的[13]

  • 1.1 不考虑吸附相体积

  • 不考虑吸附相体积算法的基本原理[14-15] 如图1所示。平衡前,样品罐中未被页岩固体占据的自由空间体积为 V1,压力为 p1,参考罐中的气体体积为 V2,压力为 p2(图1a);平衡后,样品罐中自由气体积仍为 V1,压力为 p3,参考罐中的气体体积仍为 V2,压力为p3(图1b)。

  • 图1 不考虑吸附相体积的体积法示意

  • Fig.1 Schematic of volumetric method without considering adsorption phase volume

  • 应用气体状态方程,计算平衡前后气体的物质的量,减少的那部分气体的物质的量即为吸附量。等温吸附平衡前后气体的物质的量分别为:

  • n1=p1V1Z1RT+p2V2Z2RT=1RTp1V1Z1+p2V2Z2
    (1)
  • n2=p3V1+V2Z3RT
    (2)
  • 等温吸附平衡前后气体的物质的量的差值即为吸附量,其表达式为:

  • Δn=n1-n2=1RTp1V1Z1+p2V2Z2-p3V1+V2Z3RT
    (3)
  • 1.2 考虑吸附相体积

  • 考虑吸附相体积算法的基本原理[14-15] 如图2所示。平衡前,样品罐中未被页岩固体占据的自由空间体积为 V1,气体密度为 ρ1,参考罐中容积为 V2,气体密度为ρ2(图2a);平衡后,样品罐中自由气体积为V1V,ΔV 为吸附相体积,气体密度为 ρ3,参考罐容积不变,仍为 V2,气体密度为 ρ3(图2b)。可见,平衡后样品罐中自由空间体积变小。

  • 图2 考虑吸附相体积的体积法示意

  • Fig.2 Schematic of volumetric method considering adsorption phase volume

  • 由物质平衡原理可知,等温吸附平衡前后气体质量不变,平衡方程为:

  • ρ1V1+ρ2V2=ρ3V1+V2-ΔV+ρsΔV
    (4)
  • 将(4)式进行变形,得到吸附相体积表达式为:

  • ΔV=ρ1-ρ3V1+ρ2-ρ3V2ρs-ρ3
    (5)
  • (5)式中平衡前后气体密度表达式为:

  • ρ=mV=nMCH4V=pVZRTMCH4V=pMCH4ZRT
    (6)
  • 1.3 吸附量计算存在的主要问题

  • 由(3)式和(6)式可以明显看出,决定体积法吸附量计算精度的是甲烷压缩因子计算的准确程度。现有的压缩因子计算模型均是根据STANDING和KATZ在1941年发表的甲烷压缩因子与压力、温度的关系曲线拟合而来的,只是所采用的方法不同,导致适用条件与计算精度也各不相同[16],但多数模型仅适用于常温常压条件。对此有学者进一步展开了研究,DRANCHUK与ABUKASSM建立了用于高压下计算甲烷压缩因子的DAK模型[17];李相方等以STANDING和KATZ发表的甲烷压缩因子图表为基础,采用拟合方法,建立了高压高精度甲烷压缩因子的解析模型(LXF模型)[18]

  • 由图3可以看出:DAK和LXF模型在计算甲烷压缩因子时均存在误差,DAK模型计算的压缩因子相对误差随着压力升高而逐渐增大,对比温度为2.6时最大相对误差超过2%;LXF模型虽然在一定程度上减小了高压下压缩因子的计算误差,却不适用于低压条件,对比温度为2.0时最大误差也超过2%。而两模型最大的优点是模型为解析式,不需要复杂的迭代计算。

  • 图3 DAK和LXF模型甲烷压缩因子计算精度

  • Fig.3 Calculation accuracy of methane compressibility factor of DAK and LXF models

  • 2 引入复杂气体状态方程计算甲烷压缩因子

  • 当考虑吸附相体积的体积法计算甲烷吸附量时,甲烷密度需要通过状态方程计算压缩因子后求得,而不同状态方程计算的压缩因子差别较大。由于压缩因子的微小偏差会使甲烷吸附量计算结果产生较大的误差,因此,为了实现甲烷吸附量的精确计算,不能片面地寻求计算公式的简便性,要确保方法的精确程度,计算过程的复杂性可以通过编程实现,然后将计算程序嵌入吸附量解释软件中即可。

  • 利用VB语言,对Vanderwaals方程[19]、RK方程[20]、SRK方程[21]、PR方程[22]、PRSV1方程[23]、 PRSV2方程[24]、ESD方程[25] 和Setzmann方程[26] 等复杂气体状态方程进行编程,计算不同温度、0~50MPa下的甲烷压缩因子,与美国标准局(NIST)数据库中Chemistry部分计算的甲烷压缩因子进行对比,并计算相对误差。

  • 由图4可知,在实验温度为360K的条件下, Setzmann方程的计算结果与NIST数据库中Chemis⁃ try部分计算的甲烷压缩因子相对误差最小,平均相对误差为0.006%,其他气体状态方程按平均相对误差由小到大排序依次为RK方程、ESD方程、SRK方程、PRSV2方程、PRSV1方程、PR方程和Vander⁃ waals方程,其值分别为0.527%,1.765%,2.541%, 3.266%,3.284%,3.414%和5.136%。Setzmann方程对甲烷压缩因子的计算精度最高,因此,选用Setzmann方程计算的甲烷压缩因子来改进体积法吸附量计算方法,并将编制的计算程序嵌入吸附实验解释软件系统,实现实时测试,实时解释。

  • 图4 360K下各气体状态方程计算的甲烷压缩因子及其与NIST数据误差

  • Fig.4 Methane compressibility factors calculated by different gas state equations at 360K and their errors compared with NIST database

  • 3 页岩气等温吸附实验及吸附量计算结果对比

  • 为了对比吸附量计算方法改进前后的精度,应用页岩露头样品进行等温吸附实验获取直接的实验数据,选用Setzmann方程计算的甲烷压缩因子改进体积法吸附量计算方法,分析吸附量计算方法改进前后吸附等温线的差异性,并应用吸附理论模型对吸附等温线进行拟合。

  • 实验岩样 实验所用岩样取自川东龙马溪组页岩露头,海拔为1 231m。采用线切割方式,将样品制作成直径为2.54cm、长度为5cm的圆柱体,将岩样进行烘干、称重、孔隙度物性测试以及页岩气等温吸附实验。实验用页岩样品质量为12.56g,孔隙度为4.27%。

  • 实验仪器 实验仪器采用湖北创联石油科技有限公司生产的页岩气高压等温吸附仪。

  • 实验方法 实验温度选取360K(与涪陵页岩气藏温度相当),测试平衡压力为0~30MPa,在进行抽真空和自由空间体积测试后,进行页岩气等温吸附实验,具体实验操作流程参考GB/T19560— 2008[12] 进行。

  • 计算结果 由图5可以看出,当平衡压力大于5MPa时,随着压力的增大,改进前后计算得到的吸附量相差越来越大,且改进前的吸附量计算结果偏大。这是因为,页岩气-固吸附过程中,随着平衡压力的增大,吸附量和吸附相体积增大,而改进前的计算方法未考虑吸附相体积,故改进前的吸附量计算模型求得的吸附量偏大。当平衡压力为29.702MPa时,改进前后计算的吸附量分别为3.389 3和2.962 8mL/g,相差0.426 5mL/g,进一步说明,改进前未考虑吸附相体积且高压下甲烷压缩因子计算不准确,从而影响了吸附量的计算精度。

  • 图5 改进前后甲烷压缩因子计算结果对比

  • Fig.5 Comparisons of methane compressibility factors before and after improvement

  • 吸附等温线的模型拟合 中外学者建立了许多吸附模型,一般主要用于拟合页岩对甲烷的气固吸附过程的模型为Langmuir方程[27]、Toth吸附模型[28]和Langmuir-Freundlich(L-F)吸附模型[29],其表达式分别为:

  • Vm=abp1+bp
    (7)
  • Vm=aKbp1+Kbpn1n
    (8)
  • Vm=aKbp1n1+Kbp1n
    (9)
  • 利用OriginLabPro软件和这3个吸附模型对实验吸附等温点进行拟合。结果表明,不管是改进前还是改进后,Langmuir方程的拟合精度均不高,其改进前后相关系数分别为0.998 48和0.997 75(图6a),尤其在高压下,改进后的吸附等温点与拟合线偏差较大。Toth吸附模型改进前后相关系数分别为0.999 79和0.999 43(图6b)。L-F吸附模型是由Langmuir方程和Freundlich方程结合得到的,其改进前后相关系数分别为0.999 34和0.998 64(图6c)。综上可知,Toth吸附模型改进前后拟合线均能较好地拟合吸附等温点,拟合精度最高,适用于川东龙马溪组页岩吸附等温线的模型拟合。这是因为: Langmuir方程只有Langmuir吸附常数 a 和Langmuir压力常数b这2个参数,不能较好地研究页岩吸附特性[30],更多的是定性地判断页岩吸附能力的大小[31];Toth和L-F吸附模型为3参数模型,引入了结合常数 K b以及与温度和页岩孔隙分布有关的模型参数n,综合考虑了吸附相表面能量不均匀性,以及被吸附相分子之间的作用力等复杂因素。

  • 图6 吸附等温点的Langmuir方程、Toth和L-F吸附模型拟合

  • Fig.6 Fitting of Langmuir equation,Toth adsorption model and L-F adsorption model at adsorption isotherm point

  • 4 结论

  • 与美国NIST数据库对比,Setzmann方程计算的甲烷压缩因子精度较高,解决了体积法计算吸附量的高压不适应性,同时考虑了吸附相体积,从而改进了基于体积法的吸附量计算方法,提高了计算精度。

  • 选用川东龙马溪组页岩露头岩样进行页岩气等温吸附实验,应用改进前后的吸附量计算方法与实验数据求得等温吸附曲线的数据点,结果表明,甲烷压缩因子在高压下的计算误差会对页岩气吸附等温线产生较大影响。同时,应用吸附理论模型对实验数据进行拟合,发现Toth吸附模型拟合精度最高。

  • 符号解释:

  • V1V2 ——样品罐自由空间体积和参考罐容积,cm3p1p2p3 ——平衡前样品罐和参考罐及平衡后两罐压力, MPa;n1n2 ——等温吸附平衡前后气体的物质的量,mol; Z1Z2Z3 ——平衡前样品罐和参考罐及平衡后两罐气体压缩因子,f;R ——摩尔气体常数,J/(mol·K),其值为8.314; T ——温度,K;Δn ——吸附量,mol;ρ1ρ2ρ3 ——平衡前样品罐和参考罐及平衡后两罐气体密度,kg/m3;ΔV ——吸附相体积,cm3ρs ——甲烷吸附相密度,一般取定值,其值为0.424g/cm3ρ ——气体密度,g/cm3m ——气体质量,g; V ——气体体积,cm3n ——气体的物质的量,mol;MCH4 —— 甲烷气体的摩尔质量,g/mol;p ——气体平衡压力,MPa; Z ——气体压缩因子,f;Vm——吸附量,cm3/g;a ——Langmuir吸附常数,cm3/g;b ——Langmuir压力常数,是温度的函数,且与吸附热有关,1/MPa;K b——结合常数;n ——与温度和页岩孔隙分布有关的模型参数。

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