The influence of pressure on hydrocarbon gas generation and carbon isotope of methane from type III kerogen

  • Chun-min HE , 1, 2 ,
  • Jun GAN 3 ,
  • Gang LIANG 3 ,
  • Xing LI 3 ,
  • Xing WANG 1, 2 ,
  • Hui TIAN , 1
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  • 1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
  • 2. University of Chinese Academy of Sciences, Beijing 100049, China
  • 3. Zhanjiang Branch, CNOOC China Ltd. , Zhanjiang 524057, China

Received date: 2020-01-30

  Revised date: 2020-04-30

  Online published: 2020-07-02

Supported by

China National Science and Technology Major Project(2016ZX05026-002-005)

Guangdong Natural Science Foundation(2016A030310119)

Highlights

Samples from transitional source rocks of Qiongdongnan Basin were pyrolyzed under 50 MPa, 100 MPa and 150 MPa, in order to tentatively discuss the effects of pressure on hydrocarbon gas generation and carbon isotope fractionation of methane. The results presented that increasing pressure would retard the hydrocarbon gas yield when pyrolytic temperature was low (EqVR O% less than 2.69%), and when it came to wet gases, the retardation was more significant than methane. However, there would be little influence of pressure at high pyrolytic temperature (EqVR O% more than 2.77%), or even slightly promoting methane generation. Calculated kinetic parameters showed that activation energy and frequency factor increased with growing pressure, as a result of that growing pressure makes the gas in the closed gold tube expand to do more work as well as the reactant concentration enrichment. At the rate of 5 ℃/Ma, pressure increases from 50 MPa to 150 MPa, which will result in the temperature increase of about 10 ℃ for every 50 MPa in the gas window. When temperature is higher than 260 ℃, the effect of pressure is no longer obvious. The experiment results also presented that growing pressure makes methane rich in 12C1. And it could result in 3.2‰ negative shift of carbon isotope values from 50 MPa up to 150 MPa. Meanwhile, the relationships between carbon isotope values and methane conversion were not the same under different pressures, indicating that negative shift of methane carbon isotope value was not only related to the retardation of methane generation, but also related to the slight changes happened to methane generation mechanism, which resulted in the changes in the distribution of the difference of activation energy between 13C1 and 12C1.

Cite this article

Chun-min HE , Jun GAN , Gang LIANG , Xing LI , Xing WANG , Hui TIAN . The influence of pressure on hydrocarbon gas generation and carbon isotope of methane from type III kerogen[J]. Natural Gas Geoscience, 2020 , 31(7) : 931 -938 . DOI: 10.11764/j.issn.1672-1926.2020.05.006

0 引言

压力在有机质热演化过程中扮演的角色一直备受关注[1,2,3]。干酪根生气是一个体积膨胀过程,根据Le Chatelier原理,烃类气体的生成应该受到压力的影响,但基于Arrhenius 方程的经典油气生成理论[4]并没有考虑压力的影响。目前,关于压力在烃类气体生成中的作用仍有不同的观点,主要包括:①压力没有或只有很微弱的影响[1,5,6];②压力抑制烃类气体生成[2,7];③压力对烃类气体生成的影响与压力范围有关,在低压范围内压力增大抑制烃类气体生成,大约在40~60 MPa后压力增大则会促进烃类气体生成[8,9,10],或者相反[11,12,13],这可能与不同压力下活化体积(活化体与反应物体积之差)不同有关[10,14];④压力增大会促进烃类气体生成[15]
在实际地质条件下,有机质通常在埋深大于4.5 km后进入生气阶段[16],因此研究压力超过45 MPa后对烃类气体生成的影响更具实际意义。郝芳等[17,18]、HAO等[19]根据盆地中超压与有机质热演化关系,认为形成于生气阶段之前的超压会抑制烃类裂解,从侧面说明压力增大会抑制烃类气体的生成;CARR等[2]也从理论上解释了压力增加对有机质热演化的影响,即烃类气体膨胀克服压力做功会增大反应活化能;同时,压力增大降低了产物分子的无序度,减小了反应熵变,从而使得频率因子降低。此外,压力作用也会使得甲烷碳同位素值发生正偏[9,10]、负偏[20]或基本保持不变[13]
琼东南盆地是南海北部重要的四大含油气盆地之一,并且盆地超压发育广泛。前人研究表明,琼东南盆地超压体系主要形成于更新世[21,22],而琼东南盆地深水区气田的烃类气主要与凹陷深部烃源岩上新世或更新世以来的生气作用有关[23,24],超压形成时间与天然气大量生成时间基本一致。鉴于此,本研究以琼东南盆地崖城组海陆过渡相烃源岩为例,对其开展了不同压力下(50 MPa、100 MPa和150 MPa)的封闭体系黄金管热模拟实验,以期研究压力在超过50 MPa后对烃类气体生成及甲烷碳同位素的影响,相关结果可对琼东南盆地天然气的进一步勘探提供有益支撑。

1 样品与实验方法

1.1 样品信息

研究样品采自琼东南盆地浅水区的YC13-1-2井崖城组海陆过渡相烃源岩,原岩和对应的干酪根样品信息见表1。原岩TOC值为11.2%,I H指数为118.7 mg/gTOC。经盐酸/氢氟酸处理得到的干酪根样品TOC值为64.4%,I H指数为127.0 mg/gTOC;干酪根样品显微组分以镜质组为主,含量约为85%,惰质组含量约为5%,沥青质体和壳质组含量均为5%左右,样品有机质类型为III型干酪根。原岩样品实测镜质体反射率为1.05%,原岩用稀盐酸去除碳酸盐岩之后测得的有机质碳同位素值为-27.3‰,干酪根碳同位素值为-27.5‰,平行样测定误差为± 0.2‰。
表1 YC13-1-2井崖城组泥岩及对应干酪根样品基础地球化学参数

Table 1 Geochemical parameters of mudstone and corresponding kerogen samples from Yacheng Formation of Well YC13-1-2

样品类型 深度/m 层位 沉积相

TOC

/%

S 2/(mg/g)

I H

/(mg/gTOC)

T max

/ºC

R O/% δ13Corg/‰

干酪根

类型

泥岩 3 989~4 014 崖城组 海陆过渡相 11.2 13.3 118.7 461 1.05 -27.3 III
干酪根 3 989~4 014 崖城组 海陆过渡相 64.4 81.8 127.0 462 - -27.5 III

1.2 热模拟实验与气体分析

本研究热模拟实验采用封闭体系的黄金管热模技术[6,7,25]。对实验过程简要说明如下。首先,将10~100 mg干酪根样品封装入金管中(50 mm × 4 mm i.d.),之后充入氩气以驱除金管中残留的空气,并在氩气环境中焊封。将封装好的金管放入高压釜中,以水为介质对金管分别施加50 MPa、100 MPa和150 MPa的压力并保持恒定,压力误差在5 MPa之内。将高压釜放入反应炉后,分别以2 ºC/h与20 ºC/h的升温速率从250 ºC加热到600 ºC,温度误差在2 ºC以内。从380 ºC到600 ºC,每隔20 ºC取出一个高压釜,采用淬火方式迅速将温度冷却至室温。
金管中的气体首先在真空装置收集,然后采取外标法用安捷伦6890N-Wason 气相色谱进行定量分析,具体实验方法见PAN 等[26]。甲烷碳同位素在Delta Plus II XL型气相色谱—质谱联用仪上完成,结果采用VPDB标准进行换算,误差为± 0.3‰。

1.3 生烃动力学及甲烷碳同位素动力学参数

鉴于离散活化能模型较适用于煤及III型干酪根,且能够精细地刻画热解曲线[27],因此本研究采用离散活化能模型计算C1-5生烃动力学参数,计算软件为劳伦斯利弗莫尔国家实验室开发的KINETICS 2.41版。已有研究表明,不均匀分布的离散型活化能分布在计算甲烷碳同位素动力学参数时会产生环形振荡,最好的办法是采用连续的活化能分布模型[28,29]。为简化计算,本研究采用了高斯分布模型及单一频率因子进行计算,计算软件采用美国加州理工学院环境和能源中心开发的GOR-Isotope 2015版。该软件的理论基础是13C112C1活化能之差ΔEa与12C1生成活化能的关系呈S型分布,通过拟合测量的甲烷碳同位素值,能够分别计算出13C112C1的活化能,详细方法见TANG等[29]

2 实验结果

2.1 不同压力下烃类气体产量

为方便描述生烃演化及甲烷碳同位素值与成熟度之间的关系,本文采用TANG等[28]的等效镜质体反射率 (EqVR O) 模型将不同升温速率下温度与成熟度关联。图1给出了不同压力下崖城组干酪根C1-5、C1和C2-5烃类气体的产量。从图1(a)可知,该样品C1-5最大生气量在165 mL/gTOC左右。在2 ºC/h升温速率下,大约在520 ºC(EqVR O≈2.77%)以前压力增大抑制了烃类气体生成,并且抑制作用随着压力增大而增强,在温度超过520 ºC(EqVR O≈ 2.77%)之后,压力增大反而会略微增加烃类气体的产率。在20 ºC/h的升温速率下,压力对烃类气体生成作用基本一致,只是转折点对应的温度升高至560 ºC(EqVR O≈2.69%)左右。从图1(b)可以看出,压力对甲烷生成作用的影响整体上与C1-5总烃类气体一致,只是抑制作用相对减弱。从图1(c)可以看出,随着压力增大,干酪根生成的C2-5重烃气产量明显下降,最大产量也下降。HILL等[13]采用黄金管在400 ºC温度下热解原油72 h,结果也显示从60 MPa增加到200 MPa,C2-5重烃气产量下降;MI等[8]的实验结果也显示不同压力下C2-5重烃气最大产量不同。C2-5重烃气产率下降相对明显,说明压力对干酪根生成C2-5重烃气的抑制作用强于甲烷。
图1 不同压力下YC13-1-2井崖城组海陆过渡相干酪根生成烃类气体演化

Fig.1 Hydrocarbon gas yields of transitional kerogen from Yacheng Formation of Well YC13-1-2 under different pressure

2.2 不同压力下甲烷碳同位素值

不同压力下YC13-1-2井崖城组干酪根生成的甲烷的碳同位素演化特征如图2所示。可以看出,在相同温度下,随着压力的增大甲烷碳同位素值变低。50 MPa下甲烷碳同位素值最低为-40.7‰,最高为-26.7‰;100 MPa下甲烷碳同位素值最低为-41.1‰,最高为-27.7‰;在150 MPa下甲烷碳同位素值最低为-42.1‰,最高为-27.2‰。以2 ºC/h升温速率下的甲烷碳同位素为例,从50 MPa增加到100 MPa,甲烷碳同位素值在440 ºC(EqVR O≈1.68%)时变轻幅度最大,为1.9‰;从50 MPa增加到150 MPa时,在460 ºC(EqVR O≈1.93%)时变轻幅度最大,为3.2‰; 从100 MPa增大到150 MPa,在460 ºC(EqVR O≈1.93%)时变轻幅度最大,为1.6‰。
图2 不同压力下YC13-1-2井崖城组海陆过渡相干酪根生成甲烷碳同位素值演化

Fig.2 The evolution of carbon isotope values of methane generated from transitional kerogen from Yacheng Formation of Well YC13-1-2 under different pressures

2.3 生烃动力学与甲烷碳同位素动力学参数拟合

图3给出了YC13-1-2井崖城组干酪根在不同压力下生成烃类气体的动力学参数。可以看出,当压力从50 MPa增大到150 MPa时,频率因子分别为9.9×1011 s-1、4.1×1012 s-1、1.79×1013 s-1,活化能加权平均值分别为57.39 kcal/mol、59.68 kcal/mol、62.23 kcal/mol。频率因子及活化能均随着压力增大而增大。
图3 不同压力下YC13-1-2井崖城组海陆过渡相干酪根C1-5烃类气体生成的活化能分布与频率因子

Fig.3 The frequency factors and distribution of activation energy for C1-5 generation from the transitional kerogen of Yacheng Formation of Well YC13-1-2 under different pressures

表2给出了不同压力下YC13-1-2井崖城组干酪根生成甲烷的碳同位素动力学参数,其中13A/12A频率因子之比设定为1.02,接近理论计算范围中间值[29]。甲烷生成母质的碳同位素值为-27.5‰,与干酪根碳同位素值(-27.5‰)一致。在压力增大过程中,12C113C1活化能之差最小值与最大值的变化并不固定,只有活化能差值ΔEa与12C1活化能之间的S型分布函数的均值μ随着压力增大而增大。
表2 不同压力下YC13-1-2井崖城组干酪根生成甲烷碳同位素动力学参数

Table 2 Various kinetic paramters of methane carbon isotope fraction for kerogen from Yacheng Formation of Well YC13-1-2 under different pressures

压力/MPa 13A/12A βL/(cal/mol) βH/(cal/mol) μ/(kcal/mol) Σ/% δ13Cinit/‰
50 1.02 25 64 48.686 4.0 -27.5
100 1.02 15 70 50.686 7.0 -27.5
150 1.02 24 65 53.501 6.46 -27.5

注: 13A/12A为频率因子之比;βL为最小活化能差值;βH为最大活化能差值;μ为S型活化能差值的均值所对应的甲烷活化能;σ为S型活化能差值的方差;δ13Cinit为甲烷母质碳同位素值 (详见TANG等[28]

3 讨论

3.1 压力对烃类气体生成作用的影响

实验结果表明,在热解温度低于520~560 ºC(EqVR O≈2.69%~2.77%)时压力增大会抑制烃类气体生成,超过这一温度后压力对烃类气体生成作用影响较小,对甲烷的生成略有促进作用[图1(a)],但对湿气仍表现为抑制作用[图1(c)]。此外,压力增大在增大反应活化能的同时也提高了反应的频率因子(图3)。本研究中压力对活化能的影响与CARR等[2]的观点一致,但压力对频率因子的影响却完全相反。UGUNA等[11,12]认为,不同实验结果得到压力对有机质热演化作用不一致的主要原因可能与实验装置或方法的不同有关。CARR等[2]及UGANA等[11,12]采用的是定体积水热增压装置,在整个反应过程中实验体系体积保持不变,水压增大并不会影响反应物浓度,并且水的存在阻碍了反应物分子之间的相互接触,故而反应频率因子降低。而封闭金管实验中,由于气体的生成造成黄金管内生压力越来越大,整个反应体系体积也在不断膨胀。压力增大抑制了烃类气体膨胀,整个反应体系的体积减小,反应物浓度增大,分子碰撞频率增大,故反应频率因子增大。同时,压力增大使得气体膨胀做功增加,因而反应所需活化能增大,故低温下压力对烃类气体的生成有抑制作用。但是,当热解温度高于520 ºC时(EqVR O>2.77%),活化能之间的差异对反应速率的影响逐渐变小,此时频率因子增大的作用逐渐显现出来,进而压力增大开始略微促进甲烷的生成。
图1(b)可以看出,压力对甲烷生成的影响与C1-5烃类气体相似,但影响相对较小,而压力增大却强烈抑制了C2-5重烃气的生成[图1(c)],这可能与不同烃类气体活化能的大小不同有关。有机质在热裂解过程中,长链自由基通过β-断裂形成更小的自由基及烯烃分子[30,31]。甲烷的形成源自甲基自由基,而甲基自由基相对于C2-5自由基而言并不稳定,因此β-断裂给了甲基自由基更高的活化能,故形成甲烷需要的活化能明显高于C2-5重烃气[32]。因此,压力增大甲烷活化能的增幅相对较小,进而对甲烷生成的影响也较小。然而C2-5由于活化能相对较低,因此压力增大C2-5的活化能增幅明显,故压力对C2-5重烃气生成的抑制作用也更强。至于不同压力下C2-5重烃气最大产量不同,则与湿气组分裂解有关[8]。当热解温度超过湿气组分裂解温度后,湿气组分开始裂解,C2-5重烃气产量不再增加,因此热解温度升高至湿气组分裂解温度时的C2-5产气量为最大产量,该最大产量可能不是实验观测到的最大产量。鉴于压力增大抑制了C2-5湿气组分的生成,故高压下C2-5重烃气最大生气量降低。
利用图3给出的生烃动力学参数可将实验结果外推到地质条件。由于琼东南盆地具有较高的地温梯度[33],本文选择5 ºC/Ma的升温速率来计算地质条件下烃类气体的生成特征。从图4可以看出,若以生气转化率0.1为生气窗门限[34],则50 MPa下YC13-1-2井崖城组干酪根大约在160 ºC时进入生气窗,100 MPa下则大约在170 ºC时进入生气窗,而在150 MPa下则大约在180 ºC时进入生气窗。从50 MPa增大到150 MPa,压力每增大50 MPa进入生气门限温度大约升高10 ºC。而当温度上升至260 ºC以后,压力对干酪根生气作用几乎没有影响。
图4 地质条件下(5 ºC/Ma)压力对YC13-1-2井崖城组海陆过渡相干酪根生气的影响

Fig.4 Influence of pressure on hydrocarbon gases yield from transitional kerogen of Yacheng Formation of Well YC13-1-2 under geological condition (5 ºC/Ma)

3.2 压力对甲烷碳同位素的影响

实验结果表明,从50 MPa增加到150 MPa,甲烷碳同位素值最大可减小3.2‰(图2),说明压力增大会使得甲烷碳同位素组成变轻。利用表2给出的甲烷碳同位素分馏动力学参数,图5给出了地质条件下(5 ºC/Ma)压力对甲烷生成及其碳同位素值的影响。可以看出,压力增大抑制了甲烷的生成,且大约在R O=3.0%之后抑制作用才基本消失。在R O值介于1.0%~3.0%之间时,压力增大可导致甲烷碳同位素组成变轻。但总体来看,压力对甲烷碳同位素值影响较小,不如成熟度明显。
图5 地质条件下(5 ºC/Ma)压力对YC13-1-2井崖城组海陆过渡相干酪根甲烷生成与同位素的影响

Fig.5 Influence of pressure on the evolution of methane yield and carbon isotope value of methane generated from transitional kerogen of Yacheng Formation of Well YC13-1-2 under geological condition (5 ºC/Ma)

值得注意的是,压力在造成甲烷同位素组成变轻的同时也会抑制甲烷的生成。根据瑞利分馏原理,甲烷转化率变低时其碳同位素组成也会变轻。因此压力增大造成甲烷碳同位素组成变轻可能与甲烷生成受到抑制有关。为此,本研究计算了甲烷转化率与其碳同位素值之间的关系(图6)。结果表明,在甲烷转化率为0.05~0.95的范围内,不同压力下甲烷碳同位素演化曲线并未重合,说明压力增大导致甲烷碳同位素组成变轻并非完全源自压力对甲烷生成的抑制作用,这与SHUAI等[20]的看法一致。此外,不同压力下甲烷碳同位素分馏的动力学参数也存在一定差异(表2),这可能与压力增大后甲烷的生成机制发生了轻微变化有关,即高压下更易发生分子重排(如甲基转移)形成异构烷烃[35],通过支链烷烃与直链烷烃裂解形成甲烷的比例发生了轻微变化,使得整体13C112C1活化能差值的分布形态也发生了轻微变化,最终导致不同压力下的甲烷转化率与碳同位素关系不一致。
图6 不同压力下YC13-1-2井崖城组海陆过渡相干酪根甲烷转化率与碳同位素关系

Fig.6 The relationships between methane carbon isotope values and conversion under different pressures

4 结论

以III型成熟干酪根为研究对象,利用黄金管封闭体系热模拟实验装置开展了50 MPa、100 MPa和150 MPa下的烃类气体生成热模拟实验,以期探讨压力对烃类气体生成的影响。取得的主要认识如下:
(1)对于C2-5重烃气的生成,压力在整个热模拟温度范围内均表现出强烈的抑制作用;对甲烷而言,在热解温度较低时压力增加表现为抑制作用,但当温度升高到520~560 ºC(R O=2.69%~2.77%)之后,压力增大对甲烷产率略有促进作用。压力增大对不同烃类气体的影响效果不同可能与其活化能大小有关,甲烷具有较高的活化能,压力增大导致的甲烷活化能增幅较小,故影响也较弱;而C2-5烃类气体活化能较小,压力增大能明显增大其活化能,故而影响较明显。
(2)相同温度下,压力增大导致甲烷碳同位素组成变轻,同位素值降低最大幅度可达3.2‰。但总体来看,甲烷碳同位素的分馏主要受成熟度控制,压力的影响较小。此外,压力对甲烷碳同位素的影响不仅与其抑制甲烷的生成有关,还可能与压力增大改变了甲烷的生成机理,使13C112C1生成的活化能差值的分布发生轻微变化有关。
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Outlines

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