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作者简介:

朱艳(1992—),女,湖北荆州人,硕士,工程师,从事CO2地质封存与页岩气开采研究方面的工作。联系电话:13007115805,E-mail:yanzlyf333@126.com。

通讯作者:

李义连(1965—),男,湖北潜江人,教授,博导。联系电话:15671693076,E-mail:yl.li@cug.edu.cn。

中图分类号:TE32+8

文献标识码:A

文章编号:1009-9603(2019)06-0129-07

DOI:10.13673/j.cnki.cn37-1359/te.2019.06.017

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目录contents

    摘要

    面对全球能源短缺问题,页岩气作为一种新兴的非常规资源成为研究热点。以鄂尔多斯盆地延长组页岩为例,采用数值模拟手段建立三维地质模型,研究不同CO2或CO2/N2混合气体注入速率及混合气体中N2占比对页岩气产量的影响。研究结果表明,以0.05,0.5和1 kg/s的速率注入CO2或CO2/N2混合气体均能提高页岩气产量,注入混合气体效果优于单纯的 CO2。仅以 0.05 kg/s的速率注入 CO2,30 a后甲烷生产井中不会出现气体突破现象,页岩气产量可提高13.21%。以0.05 kg/s的速率注入CO2/N2混合气体,甲烷生产井中会出现N2突破现象,N2质量分数越大, 突破时 N2含量越多。实际工程中为了防止甲烷生产井中气体突破含量超过 10% 的限制值,需要严格控制 CO2或 CO2/N2混合气体的注入速率以及混合气体中N2的质量分数。

    Abstract

    Facing global energy shortage,shale gas has become a research hotspot as new unconventional resources. A 3D geological model is established to analyze the effect of injection rate of CO2 or mixture of CO2 and N2 as well as percentage of N2 in mixed gas on shale gas production by numerical simulation of the Yanchang Formation shale in Ordos Basin. Re- sults show that the injection of CO2 or mixture of CO2 and N2 at the rate of 0.05,0.5 and 1 kg/s can increase shale gas pro- duction,and the injection of mixed gas obtains better recovery than injection of CO2 only. When CO2 is only injected at the rate of 0.05 kg/s,shale gas production is increased by 13.21% after 30 years with no CO2 breakthrough in methane produc- tion wells. When the mixture of CO2 and N2 is injected at the rate of 0.05 kg/ s,N2 will occur in the methane production wells. The ratio of N2 is larger,the content of N2 is higher after breakthrough. However,in order to avoid gas content in the methane production well exceeding the 10% when the breakthrough occurs,it is necessary to strictly control the injection rate of CO2 or mixture of CO2 and N2 as well as percentage of N2 in the mixed gas.

  • 面对全球能源短缺问题,页岩气作为一种新型清洁能源将打破传统的能源利用格局,开启全球 “页岩气革命”时代[1-2]。美国作为全球页岩气勘探开发最早最成功的国家,早在2000年就实现了页岩气的商业化开采,2008年页岩气产量为599×108 m3,仅占美国天然气产量的10.5%[3];2016年页岩气产量达4 820.1×108 m3,占美国天然气产量的64.3%[3]。美国已经逐步摆脱天然气长期依赖进口的局面,由天然气进口国转变为出口国[4-5],据英国石油公司 (BP)预测,2015—2035年美国页岩气将成为全球天然气供应增长的主要来源[6]。中国页岩气勘探开发虽然起步较晚,但据美国能源信息署(IEA)发布的数据,中国页岩气技术资源可采量却居全球首位,其值为36×1012 m3,约占全球页岩气总量的20%[3]。国家能源局发布的《页岩气发展规划(2016年— 2020年)》指出,中国2020年有望实现页岩气产量300 × 108 m3,2030年实现页岩气产量800 × 108~1 000×108 m3[37-9]。从长远看,页岩气资源也将改变中国能源利用结构,摆脱长期依赖煤、石油等常规资源的局面。

  • 目前全球页岩气可采资源量非常可观,但页岩的低孔低渗透特性导致实际开采过程却不容乐观。开采工程面临压裂技术不成熟、资金不足、环境影响等一系列难点问题[10-13]。因此,为了持续顺利开发页岩气资源,必须在页岩气开采技术上寻求新的突破。近年来兴起的CO2地质封存联合页岩气开采技术,可以同时满足页岩气开采和CO2地质封存,解决了新能源开发和温室气体减排两大世界性难题。中外已有学者研究证实CO2或N2注入确实有助于提高页岩气的产量[14-20],但大部分研究仅限于室内实验探究和单组分气体模拟。因此,在已有研究的基础上,以鄂尔多斯盆地富县区延长组页岩为例,采用数值模拟方法研究CO2或CO2/N2混合气体注入对页岩气产量的影响,为以后联合技术的持续发展提供更多科学依据,也为实际工程应用提供更多技术支撑。

  • 1 研究区概况

  • 富县区位于鄂尔多斯盆地东南部,坐标为东经108°40′~109°50′和北纬35°40′~36°05′,位于陕西省延安市富县及其邻县境内[21];构造上属于陕北斜坡东南部,整个区域地层平缓,地层倾角小于1°。富县区发育中生界延长组页岩,有机质含量高,成熟度较高,分布范围广,具有良好的勘探开采前景[22]。经现场钻孔揭示,延长组从新到老依次为:长1油层组、长2油层组、长3油层组、长4+5油层组、长6油层组、长7油层组、长8油层组以下未钻穿[23]

  • 2 数值模拟方法

  • 2.1 页岩储层中气体的流动特性

  • 由于页岩本身的致密性且发育很多不均匀的裂隙,通常不能采用常规的等效连续介质来刻画,因此采用经典的双重孔隙介质(裂隙系统和基质系统)来刻画页岩模型(图1)。页岩被多组相互垂直的裂隙切割,被分割的中间六面体为页岩基质,由于页岩基质致密且发育大量孔隙,因此基质孔隙度高、渗透率低,而裂隙孔隙度低、渗透率高。页岩气开采过程中全局流动仅发生在与井相连的裂隙系统中,而基质和裂隙间的物质、能量交换靠两者间的压力差来实现。

  • 图1 实际页岩储层和双重孔隙介质页岩模型

  • Fig.1 Diagram of shale reservoir and shale model with dual porous medium

  • 目前普遍认为页岩气流动过程为:解吸—扩散—渗流[24-25]。开采页岩气首先采出的是裂隙中的游离气;随着开采的进行,裂隙中的压力减小,裂隙和基质间的压力差会使基质表面的吸附气解吸出来流入裂隙网络;基质内部的气体会在浓度差的作用下扩散到基质表面,再通过解吸流入裂隙中;最终裂隙网络中的气体通过渗流作用流入井筒。

  • 2.2 页岩储层中气体的吸附特性

  • 模拟采用的软件TOUGH+,是一款由美国劳伦斯伯克利国家实验室研发的非等温多介质多组分多相流体及热量运输模拟软件[26]。与常规的TOUGH家族软件相比,TOUGH +可以刻画CH4, CO2,O2,N2等12种真实气体在低渗透裂隙岩层中的运移过程,其在TOUGH2的基础上增加了气体吸附模型,可以用于模拟页岩气的开采过程。

  • CO2和N2注入到页岩中将会和原位的CH4发生竞争性吸附,使页岩基质上吸附态的CH4解吸变为游离态,流入裂隙网络中易于被产出。因此,准确描述气体吸附特性对页岩气开采尤为重要。页岩对气体的吸附特性采用普遍适用的Langmuir等温吸附模型进行刻画,其表达式为[27-28]

  • C=CLppL+p
    (1)
  • 郭平等对50℃不同压力下鄂尔多斯盆地延长组页岩对CH4,N2,CO2的吸附特性进行了室内测试,得出了不同压力下气体的吸附量[29]。采用Lang⁃ muir等温吸附方程对实验数据进行拟合。拟合结果(图2)显示3条曲线的相关系数分别为0.999 9,0.92 3 7和0.999 9,说明Langmuir等温吸附方程能非常好地描述延长组页岩对CH4,N2,CO2的吸附特性。根据拟合结果可知,CH4,N2,CO2的最大吸附量分别为3.119,2.223和13.187m3/t,Langmuir压力分别为2.064,6.733和1.496MPa。

  • 图2 延长组页岩对CH4,N2,CO2的吸附特性曲线

  • Fig.2 Adsorption characteristics curves of CH4,N2 and CO2 in Yanchang Formation shale

  • 2.3 地质模型的建立

  • 富县区延长组长7油层组页岩埋藏深度为650~1 200m[30],选取页岩埋藏深度为1 200m建立三维地质模型(图3)。模型XYZ轴方向总长度分别为154,100和90m。模型剖分情况如下:X 轴两侧为对称水力压裂区,压裂范围均为15m,采用对数剖分方式进行网格剖分,中间区域采用均匀剖分方式进行网格剖分;Y轴方向全部进行水力压裂,沿井两侧15m范围采用对数剖分方式进行网格剖分,向外采用均匀剖分方式进行网格剖分;Z 轴方向全部进行水力压裂,采用均匀剖分方式进行网格剖分。

  • 图3 三维地质模型示意

  • Fig.3 Schematic diagram of 3D geological model

  • 水力压裂过程除了会产生渗透率非常大的裂缝外,还会对周围一定范围内的岩层产生扰动,使其水力传导系数变大。据DUAN等的报道,研究区延长组页岩的实际水力裂缝宽度为1.6mm[30],本模型中假定水力裂缝的传导系数为16mD∙m,通过对水力压裂裂缝周边15m范围内的网格的渗透率进行离散,使其更符合实际情况。

  • 2.4 模型参数设置

  • 富县区延长组目标页岩层埋藏深度为1 200m,根据页岩埋深计算可得初始静水压力为11.87MPa。富县区地表平均温度约为12.5℃,地温梯度为2.8℃/100m[31],经计算,延长组页岩初始温度为46.1℃。根据文献[26],页岩孔隙度设置为3.52%,页岩基质渗透率设置为2.53×10-4 mD,天然裂隙渗透率设置为3.33×10-4 mD,气体饱和度设置为65%。其他物性参数设置参考经验值(表1)。

  • 表1 水文地质参数设置

  • Table1 Hydrogeological parameters of numerical model

  • 2.5 模拟场景设置

  • 为研究注入CO2或CO2/N2混合气体对页岩气产量的影响,分别设置以下2种模拟场景。

  • 场景1∶0~30a,井1为甲烷生产井,采用定压2.068MPa方式开采页岩气30a。0~5a,井2为甲烷生产井,采用定压2.068MPa方式开采页岩气5a;5~10a,井2转变为CO2注入井,采用定速方式注入; 10~30a,关闭井2。在该场景中,通过改变CO2的注入速率,探讨其对页岩气产量的影响。

  • 场景2∶0~30a,井1为甲烷生产井,采用定压2.068MPa方式开采页岩气30a。0~5a,井2为甲烷生产井,采用定压2.068MPa方式开采页岩气5a;5~10a,井2转变为CO2/N2混合气体注入井,采用定速方式注入;10~30a,关闭井2。在该场景中,保持气体总的注入速率不变,改变混合气体中N2的质量分数,探讨其对页岩气产量的影响。

  • 3 页岩气增产的影响因素

  • 3.1 CO2注入速率

  • 当CO2注入速率分别为0.05,0.5和1kg/s时,由甲烷生产井中页岩气产量随时间的变化曲线(图4a)可知,当CO2注入后页岩气产量明显增加,CO2注入速率越大,页岩气产量增加的幅度越大。单纯页岩气开采30a产量为1.59×106 kg,当CO2注入速率为0.05kg/s时,30a页岩气产量达1.80×106 kg,产量提高了13.21%。当CO2注入速率为0.5kg/s时,30a页岩气产量为3.75×106 kg,产量提高了135.85%。当CO2 注入速率为1kg/s时,30a页岩气产量达4.61×106 kg,产量提高了191.82%。这是因为页岩基质对CO2的吸附能力强于CH4,当注入大量CO2 后,CO2可以直接置换出页岩基质中的CH4,使其解吸到裂隙网络中变成游离态CH4,从而容易产出。与此同时,CO2注入后会引起储层压力抬升形成巨大的压力差,这种压力差也有助于CH4从页岩基质中解吸出来。

  • 图4 不同CO2注入速率下甲烷生产井中页岩气产量和CO2累积含量随时间的变化曲线

  • Fig.4 Curves of shale gas content and cumulative CO2 content in methane production well with time at different CO2 injection rates

  • 由甲烷生产井中CO2的累积含量随时间的变化曲线(图4b)可知,当CO2注入速率为0.05kg/s时,30a甲烷生产井中没有出现CO2突破现象;当CO2注入速率为0.5kg/s时,30a甲烷生产井中CO2累积含量为1.11×107 kg;当CO2注入速率为1kg/s时,30a甲烷生产井中CO2累积含量为7.38×107 kg。实际工程中规定,甲烷生产井中CO2突破含量超过产气量的10%时就应该闭井处理,避免增加后续处理过程的费用[32]。虽然CO2的注入速率越大,越有利于增加页岩气的产量,但同时CO2的突破时间越早,综合分析得出最优CO2注入速率为0.05kg/s。

  • 3.2 CO2/N2混合气体注入方式

  • 在保证气体突破可能性最小的前提下,选取CO2/N2混合气体的注入速率为0.05kg/s。改变混合气体中N2的注入质量分数依次为0%,20%,50%和80%,得到甲烷生产井中页岩气产量随时间的变化曲线(图5)。从图5可以看出,注入CO2/N2混合气体比单独注入CO2更有利于提高页岩气产量,混合气体中N2质量分数越大,页岩气增产效果越明显。当单独注入CO2时,页岩气最高产量为1.80×106 kg;当注入质量分数分别为20%,50%和80%的N2时,页岩气最高产量分别为2.04×106 ,2.43×106 和2.74×106 kg。这是因为CO2和N2同时注入页岩中,N2的迁移速率比CO2快,能在更短的时间迁移到甲烷生产井附近形成压力差,这种压力差有助于CH4从页岩基质中解吸出来。

  • 图5 甲烷生产井中页岩气产量随时间的变化曲线

  • Fig.5 Curves of shale gas content in methane production well with time

  • 为更加清晰地观察CO2/N2混合气体注入后沿水平方向的迁移情况,选取甲烷生产井和气体注入井所在平面(Z=-45m)进行气体饱和度刻画。由各时段气体饱和度分布情况(图6)可知,N2的迁移速率比CO2快,30a后N2均能从注入井迁移到甲烷生产井中发生突破,而CO2均未迁移到甲烷生产井附近。混合气体中N2的质量分数越大,甲烷生产井中发生气体突破的时间越早,最终突破的N2含量越大。

  • 图6 30a后(Z=-45m)平面上气体饱和度分布情况

  • Fig.6 Gas saturation distribution on plane(Z=-45m)after 30years

  • 从不同N2注入质量分数下甲烷生产井中N2累积含量随时间的变化曲线(图7)可以看出,当注入质量分数为20%的N2时,30a甲烷生产井中N2的累积含量达0.050 4×106 kg,为页岩气产量的2.47%;当注入质量分数为50%的N2时,甲烷生产井中N2的累积含量达0.484×106 kg,为页岩气产量的19.92%;当注入质量分数为80%的N2时,甲烷生产井中N2的累积含量达1.34×106 kg,为页岩气产量的48.91%。结果显示,混合气体中N2的质量分数为50%和80%时,甲烷生产井中N2的累积含量均超过了10%的页岩气产量,不满足实际工程的需求。因此,工程实践中需要严格控制混合气体中N2质量分数,来达到提高页岩气产量的目的。

  • 图7 甲烷生产井中N2累积含量随时间的变化

  • Fig.7 Curves of cumulative N2 content in methane production well with time

  • 4 结论

  • 注入CO2有利于提高页岩气的产量,CO2注入速率越大,页岩气产量增加幅度越大。在0.05,0.5和1kg/s这3种速率下,仅以0.05kg/s的速率注入CO2, 30a后甲烷生产井中不会出现气体突破现象,此时页岩气产量可提高13.21%;以0.5和1kg/s的速率注入CO2,30a后甲烷生产井中突破的CO2含量均超过页岩气产量的10%,不符合实际工程规定。

  • 同等注入速率条件下,注入CO2/N2混合气体比单独注入CO2更有利于提高页岩气的产量,混合气体中N2质量分数越大,页岩气产量增加的幅度越大,但同时甲烷生产井中越容易发生气体突破现象。当混合气体中N2质量分数为20%,50%和80%时,甲烷生产井中均会出现气体突破现象,30a后N2 突破含量百分比分别为2.47%,19.92%,48.91%,仅当N2质量分数为20%,甲烷生产井中突破的N2含量未超过产气量的10%。

  • 注入CO2或CO2/N2混合气体均能提高页岩气的产量。但出于安全和成本考虑,工程实践中应注意合理设计气体注入速率和混合气体中N2质量分数来达到提高页岩气产量的目的。

  • 符号解释

  • C ——气体吸附量,m3/t;C L ——Langmuir体积,代表最大气体吸附量,m3/t;p ——当前压力,MPa;p L——Langmuir压力,为气体吸附量达到最大吸附量50%对应的压力,MPa。

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