Differences and controlling factors of pores structure between coal and shale in Longtan Formation from western Guizhou Province

  • Mengjiang ZHANG , 1, 2 ,
  • Zhaobiao YANG , 1, 3 ,
  • Wei GAO 2 ,
  • Jun JIN 2 ,
  • Xiwei MU 2 ,
  • Dan LU 4 ,
  • Hailong LI 4
Expand
  • 1. China University of Mining & Technology,Xuzhou 221008,China
  • 2. Coal Geology Bureau of Guizhou,Guiyang 550008,China
  • 3. Key Laboratory of CBM Resources and Dynamic Accumulation Process,Ministry of Education of China,China University of Mining and Technology,Xuzhou 221008,China
  • 4. Guizhou Shale Gas Exploration and Development Co. ,Ltd. ,Guiyang 550008,China

Received date: 2024-08-08

  Revised date: 2024-10-23

  Online published: 2024-11-04

Supported by

The National Natural Science Foundation of China(42272195)

the Science and Technology Plan Project of Guizhou Province, China (Grant Nos. Qian Kehe Rencai-CXTD[2022]016, Qian Kehe Jichu-ZK[2024]Yiban687, Qian Kehe Zhanlue Zhaokuang [2022]ZD001-1)

Abstract

Transitional facies with high-frequency cycles of coal-shale-sandstone assemblages are widely developed in the Upper Permian Longtan Formation, western Guizhou, exhibiting significant pore structures contrasts between coal and shale reservoirs. Based on coal rock samples from six typical coal bearing gas wells in Guizhou, a comparative study was conducted on the differences in pore structure between coal from the Longtan Formation in western Guizhou and adjacent shale using analytical techniques such as geological analysis, scanning electron microscopy observation, and low-temperature liquid nitrogen adsorption. The research results show that the specific surface area of coal is 44.2-168 m2/g,and the total pore volume is 0.024-0.065 cm3/g, mainly composed of semi closed pores and slit-shaped pores. The volume and specific surface area of micropores (<2 nm) have absolute advantages, and are positively correlated with the degree of thermal evolution mainly micropores, which are related to gas generation process. Macropores (>2 nm) exhibit strong heterogeneity, linked to differences in microscopic components. The specific surface area of shale is 43.2-66.6 m2/g, and the total pore volume is 0.032-0.059 cm3/g, mainly composed of inkbottle pores. The distribution of micropores and mesopores is relatively uniform, and the pore size distribution curve shows a bimodal distribution (3 nm and 30 nm). Although there are differences in pore distribution between coal and shale, pores of <10 nm are the main contributors to the specific surface area. The extractable asphalt has a significant impact on the pore space of coal, and the pore volume of different pore sizes increases significantly after extraction. The degree of thermal evolution and organic matter content of coal are the main influencing factors on pore structure, while the organic matter content and mineral type of shale are the main factors affecting pore structure, and the thermal maturity exerts limited influence. The findings provide critical constraints for co-exploration of coalbed methane and shale gas in coal-measure systems in western Guizhou Province.

Cite this article

Mengjiang ZHANG , Zhaobiao YANG , Wei GAO , Jun JIN , Xiwei MU , Dan LU , Hailong LI . Differences and controlling factors of pores structure between coal and shale in Longtan Formation from western Guizhou Province[J]. Natural Gas Geoscience, 2025 , 36(5) : 973 -984 . DOI: 10.11764/j.issn.1672-1926.2024.10.010

0 引言

煤层气和页岩气是我国非常规天然气的重要组成部分,近年勘探开发取得了突破性进展1-2。煤系中页岩与煤互层,煤层气与页岩气共采成为非常规天然气产业的重要发展方向3-5。煤层气和煤系页岩气赋存特征与产气规律受自身孔隙结构及连通性影响6-7。其中,煤储层由于有机组分占据绝对优势,发育<3 nm的纳米级有机质孔隙,气体主要以吸附态为主赋存于煤中8。煤系页岩储层孔隙网络较为复杂,由纳米尺度的无机矿物粒间或粒内孔隙、有机孔隙、微裂缝等构成,页岩气常以游离态和吸附态赋存9
黔西上二叠统龙潭组页岩与煤层发育,具有良好的自封盖性,整体含气性较好10。该套煤系地层主要形成于三角洲到潮坪—潟湖环境,有机质热演化程度变化大,从低成熟(R O=0.5%~0.7%)至过成熟(R O>2.0%)均有发育11。龙潭组煤储层有机质微孔对孔容贡献最大,其发育受有机显微组分和成熟度控制,为煤层气提供主要的吸附空间12;煤系页岩孔隙包括粒间孔、粒内孔、微裂隙和有机孔,以无机孔为主,微裂隙提供游离气的容纳空间13
同时,处于低—中成熟度的倾油性海相页岩,生油残留沥青会堵塞储层的孔隙和喉道,制约储集空间和气体赋存特征14。残余沥青对倾气性海陆过渡相页岩和煤样孔隙发育也具有一定的抑制作用,但由于有机孔隙欠发育,其影响机制与海相页岩存在一定的差异15-16。贵州省近年来探索煤系多气开采模式和技术,工程示范取得显著进展17。煤和煤系页岩的孔隙结构差异直接影响煤系气赋存与运移富集规律,是多气共采选层的依据之一。然而,目前黔西煤和煤系页岩孔隙结构差异及其对煤系气赋存规律控制的研究还比较薄弱。
为此,本文选取黔西典型地区龙潭组紧邻层位的不同成熟度煤和页岩样品作为研究对象,采用低压液氮吸附、场发射扫描电镜观察等方法,探讨煤系页岩与煤的孔隙结构差异以及可抽提沥青对孔隙的影响,讨论煤与煤系页岩孔隙结构差异的地质控因,为黔西煤系多气合采提供基础依据。

1 样品与方法

1.1 实验样品

黔西地区位于扬子板块,是贵州省主要的含煤地层分布区域,同时也是煤系气资源的主要勘查开发区。研究区内地层从古生界到新生界均有分布。本文样品采自黔西部地区6口井,其构造单元属华南板块—上扬子陆块—威宁隆起区、六盘水断陷槽及黔北隆起区,采样层位为上二叠统龙潭组,采样深度为600~1 000 m(表1)。
表1 煤和相邻页岩样品信息及基本地球化学参数

Table 1 Information and basic geochemical parameters of coal and adjacent shale samples

岩性

样品

编号

采样

地区

井号

采样

深度/m

总有机碳

/%(wt)

镜质组反射率

/%

热解峰温

/℃

氢指数

/(mg/g)

石英含量 /%

黏土矿物

含量/%

其他矿物含量/%

镜质组

含量/%

其他有机

组分/%

GXM-1 六盘水 ZM1 801 50.42 1.05 456 122 9.0 6.0 10.0 55 20
GXM-7 六盘水 ZM1 856 70.84 1.08 454 167 4.5 5.5 5.0 64 21
GXM-10 六盘水 NM1 637 42.00 2.00 526 22 9.0 7.0 10.0 55 19
GXM-11 毕节 BM1 610 63.00 2.20 524 31 5.0 10.0 10.0 53 22
GXM-14 毕节 AM1 668 23.13 2.41 498 32 8.0 20.0 15.0 50 7
GXM-17 遵义 FM1 644 62.01 4.31 605 5 5.0 15.0 5.0 60 15
GXM-21 遵义 FM1 669 65.67 4.42 609 7 8.0 5.0 8.0 65 14
GXM-26 遵义 JM1 927 76.94 3.07 605 5 5.0 5.0 5.0 80 5
GXM-28 遵义 JM1 944 87.94 3.26 605 5 2.0 2.0 4.0 85 7

煤系

页岩

GXM-2 六盘水 ZM1 797 5.34 1.20 453 102 29.3 54.6 16.1
GXM-8 六盘水 ZM1 857 13.49 1.66 464 100 12.8 79.7 9.5
GXM-9 六盘水 NM1 635 5.62 2.10 528 26 15.4 59.5 25.1
GXM-12 毕节 BM1 612 9.84 2.04 534 21 4.7 77.3 18.0
GXM-15 毕节 AM1 657 5.85 1.76 495 44 20.5 58.7 20.8
GXM-16 遵义 FM1 641 7.60 3.20 604 4 9.0 86.7 4.3
GXM-20 遵义 FM1 668 6.98 3.25 604 7 3.8 91.2 5.0
GXM-27 遵义 JM1 912 5.21 2.34 605 9 0.0 96.8 3.2
GXM-29 遵义 JM1 948 6.73 2.77 604 5 4.1 90.5 5.4

注:“—”表示无数据

1.2 基础地球化学参数测试

样品总有机碳含量采用CM250型碳硫分析仪进行测定,依据为国家标准《沉积岩中总有机碳的测定》(GB/T 19145—2003)。煤的有机与矿物组成含量利用光学显微镜鉴定,按照国家标准《煤的工业分析方法》(GB/T 212—2001),在光学视域下运用统计学方法定量分析。
镜质组反射率(R O)由Leica DM4P显微光度系统测定,将煤和页岩块样制成光片,依据石油天然气行业标准《沉积岩中镜质组反射率测定方法》(SY/T 5124—2012)来进行,每个镜质组组分测定50组,按正态分布取值。
样品岩石热解参数(如S 1S 2T maxI H)由Rock-Eval 6热解仪测定,操作过程严格依据国家标准《岩石热解分析》(GB/T18602—2012)。
页岩的矿物组成含量依靠X射线衍射仪(XRD)对200目全岩粉末进行矿物定量分析测定,采用步进扫描,依据石油天然气行业标准《沉积岩中黏土矿物和常见非黏土矿物X射线衍射分析方法》(SY/T 5163—2010)来完成。

1.3 FE-SEM扫描电镜观察

样品高分辨率成像主要采用Hitachi SU8010型扫描电镜完成。所有样品都制作成单面光滑的小块样,经氩离子抛光仪反复研磨抛光,使其足够平整光滑。然后,在表面镀上一层10 nm金膜提高导电性,尽可能使图像清晰。

1.4 低温液氮和CO2吸脱附实验

低温液氮和CO2吸脱附实验采用ASAP2020型吸附仪,依据国家标准《气体吸附BET法测定固态物质比表面积》(GB/T 19587—2017)进行。
液氮吸附在-196.15 ℃温度下进行,相对压力(P/P 0)为0.05~0.998;CO2吸附则在-0.15 ℃温度下,相对压力(P/P 0)范围为0.000 1~0.032。氮气吸附—脱附曲线应用BET和BJH理论进行分析,而CO2 吸附—脱附曲线应用DFT模型进行分析。
为了研究可抽提沥青对海陆过渡相煤岩和泥页岩微观孔隙结构的影响,本文选取2组低成熟煤和页岩样品,采用索式抽提法(二氯甲烷∶甲醇=25∶2)分离可抽提沥青,然后同原始样品相同条件下开展低压吸脱附实验。

2 实验结果

2.1 煤和页岩基本地球化学特征

9个煤样总有机碳(TOC)含量为23.13%~87.94%,9个页岩的TOC含量介于5.21%~13.49%之间(表1)。
六盘水煤样镜质组反射率(R O)相对较低,介于1.05%~2.00%之间(表1)。毕节、遵义地区煤样成熟度达到无烟煤阶段, R O 值高达2.20%~4.42%。页岩样品与其相邻的煤具有相似的成熟度变化规律,但整体要比煤的R O值低。岩石热解的峰温(T max)也反映出同样的成熟度变化特征,六盘水地区T max值处于450~520 ℃之间,而毕节、遵义地区样品T max值则高达520~600 ℃。根据I HT max相关性图判定(图1),煤和页岩样品有机质类型以Ⅲ型干酪根为主。
图1 有机质类型T maxI H相关关系

Fig.1 T max vs. I H correlation diagram for organic matter types

煤和页岩的物质组成差异显著(表1),煤样主要以有机镜质组组分为主,含量为50%~80%,其他有机组分含量约为15%,矿物质以黏土矿物为主,其他矿物成分如石英等含量较少。页岩样黏土矿物含量高达54%~96.8%,石英和其他矿物含量相对较低。

2.2 微观孔隙特征SEM观察

与海相页岩普遍发育纳米级有机质孔隙不同18,本文研究观察的龙潭组煤和页岩有机质中均少见明显的有机纳米孔隙[图2(a),图2(b)],更小的微纳米孔(<3 nm)在SEM下不可分辨。页岩样品的黏土矿物较发育,主要为矿物内溶蚀孔或矿物间孔隙[图2(c),图2(d)];煤样矿物含量较低,对孔隙的贡献有限。除此之外,样品中在有机质与矿物交界处以脆性矿物内发育微裂缝为主[图2(e),图2(f)]。
图2 煤和页岩储层SEM镜下微观孔隙特征

(a)煤镜质组孔隙发育情况,少见明显的纳米孔隙(样品 GXM-7,深度为856 m);(b)页岩有机质孔隙发育情况(样品 GXM-8,深度为857 m);(c)页岩发育大的黏土矿物溶蚀纳米孔(样品GXM-8,深度为 857 m);(d)页岩普遍发育黏土矿物粒间或粒内纳米孔(样品GXM-16,深度为641 m);(e)页岩有机质与脆性矿物之间微裂缝(样品 GXM-8,深度为 857 m);(f)煤样脆性矿物内微裂缝以及有机质—矿物间微裂缝(样品 GXM-11,深度为610 m)

Fig.2 Microscopic pore characteristics of coal and shale reservoirs under SEM

2.3 低压吸脱附孔径表征

根据BJH模型计算样品的N2吸附—脱附参数,结果如图3表2所示。煤的平均孔径为7.0~50 nm,总孔容为0.002~0.02 cm3/g,比表面积为0.1~11.9 m2/g。页岩的平均孔径相对小,分布在7.4~10.8 nm之间, 总孔容和比表面积相对大,分别介于0.02~0.04 cm3/g和14.2~27.6 m2/g之间。
图3 典型煤和页岩氮气吸附—脱附曲线

Fig.3 Typical nitrogen adsorption-desorption curves of coal and shale

表2 煤和页岩样品低压氮气和二氧化碳吸附—脱附孔隙分布数据

Table 2 Pore distribution of coal and shale samples by low-pressure nitrogen and carbon dioxide adsorption-desorption

样品

编号

N2吸附BET比表面/(m2/g) N2吸附孔容/(cm3/g) 微孔体积占比(<2 nm)/% 介孔体积占比(2~50 nm)/%

宏孔体积占比(>

50 nm)/%

微孔(<2 nm)BET占比/% 介孔(2~50 nm)BET占比/% 宏孔(>50 nm)BET占比/% 平均孔径(N2吸附)/nm CO2吸附比表面积/(m2/g) CO2吸附孔容/(cm3/g) 平均孔径(CO2吸附)/nm
GXM-1 8.772 3 0.021 406 41.70 49.01 9.29 80.21 19.40 0.38 10.85 35.562 1 0.015 015 0.64
GXM-7 0.552 3 0.002 192 90.90 4.00 5.10 98.90 1.00 0.10 22 49.733 1 0.020 697 0.71
GXM-10 0.896 4 0.005 497 82.40 4.50 13.10 98.64 1.25 0.12 29.1 64.774 2 0.025 808 0.68
GXM-11 11.910 3 0.017 122 49.80 10.00 40.20 77.46 22.26 0.28 7.0 40.922 9 0.016 972 0.66
GXM-14 5.954 3 0.012 339 54.40 35.40 10.20 85.92 13.41 0.67 11.1 36.336 3 0.014 773 0.65
GXM-17 3.997 2 0.005 055 92.30 4.60 3.10 97.64 2.24 0.12 8.6 165.134 7 0.061 022 0.71
GXM-21 6.532 1 0.009 244 83.70 13.00 3.30 95.29 4.55 0.16 8.5 132.040 0.047 806 0.7
GXM-26 0.096 2 0.001 838 96.90 0.70 2.40 99.94 0.06 0.01 50.0 149.112 0.056 745 0.68
GXM-28 0.201 9 0.000 764 98.20 0.60 1.20 99.82 0.14 0.04 41.9 111.113 5 0.041 619 0.73
GXM-2 26.616 5 0.039 962 27.50 66.56 5.94 55.86 43.54 0.60 7.4 33.684 0.015 176 0.63
GXM-8 27.012 8 0.044 688 25.50 68.55 5.95 56.85 42.52 0.62 7.8 35.593 4 0.015 325 0.64
GXM-9 19.304 1 0.034 683 27.10 62.98 9.92 61.06 38.09 0.86 9.2 30.264 6 0.012 902 0.63
GXM-12 27.608 7 0.042 501 27.80 66.23 5.97 58.60 40.89 0.51 7.7 39.086 1 0.016 394 0.63
GXM-15 16.505 3 0.027 042 29.90 61.87 8.23 61.80 37.46 0.74 8.4 26.705 4 0.011 547 0.63
GXM-16 15.039 9 0.029 803 30.70 59.52 9.78 68.63 30.61 0.76 10.8 32.901 8 0.013 263 0.64
GXM-20 24.075 8 0.037 147 25.80 67.36 6.84 56.80 42.48 0.72 8.1 31.651 2 0.012 945 0.65
GXM-27 14.159 7 0.021 32 34.70 58.95 6.35 66.10 33.44 0.47 7.8 27.605 0.011 368 0.63
GXM-29 20.387 6 0.024 195 34.90 52.02 13.08 60.74 37.65 1.61 6.6 31.541 2 0.013 018 0.63
根据国际理论和应用化学学会(IUPAC)分类标准19,N2吸脱附曲线划分为6种类型(Ⅰ—Ⅵ型),滞后环类型分为4种类型(H1—H4型)。页岩N2吸附—脱附曲线相比煤具有更大的滞留环,兼具H2型和H3型,反映页岩中存在更多的墨水瓶形态孔隙或小尺寸喉道连通大孔径,而煤在相应的孔隙空间内多以一端封闭型楔形孔和狭缝孔为主。所有页岩样具有阶段孔容—孔径变化的双峰分布特征,峰值分别在3 nm和30 nm左右;而煤的孔径分布变化较大,部分煤样没有明显峰值特征(图4)。
图4 煤和页岩氮气吸附孔径分布曲线

(a)煤氮气吸附孔径分布曲线;(b)页岩氮气吸附孔径分布曲线

Fig.4 Nitrogen adsorption pore size distribution curves of coal and shale

样品CO2吸附曲线见图5所示,煤总孔容为0.015~0.061 cm3/g,比表面积为35.6~165.1 m2/g,CO2吸附量远高于页岩样品。从CO2 吸附反演的孔径分布来看(图6),煤的0.5~0.6 nm和1 nm左右的微孔较发育,其孔体积要远远大于页岩的微孔。
图5 页岩(a)和煤(b)二氧化碳吸附曲线

Fig.5 Carbon dioxide adsorption curve of shale (a) and coal (b)

图6 煤(a)和页岩(b)二氧化碳吸附反演的孔径分布

Fig.6 Pore size distribution diagram inverted by carbon dioxide adsorption of coal (a) and shale (b)

3 讨论

3.1 煤和页岩孔隙结构差异性对比

本文中同一口钻井的煤和页岩样品采自临近层位,有机质类型和热演化程度较为接近。但不同钻井的页岩和煤样品在TOC含量和成熟度演化方面存在较大差异。煤主要由有机质所构成,孔隙绝大多数为有机质孔;页岩中由于黏土、石英等矿物大量存在,使得其孔隙结构相对于煤储层孔隙更为复杂20。本文采用IUPIC孔隙分类标准,即微孔直径<2 nm、介孔直径2~50 nm、宏孔直径>50 nm21-22
煤的微孔孔容和比表面积的贡献均占有绝对优势,其占比分别可达50%和90%以上[图7(a),图7(b)],介孔或宏孔体积和比表面积占比相对低,有的甚至小于5%。不同煤样的介孔和宏孔对总孔隙的贡献变化较大,推测可能是煤不同显微有机组分屑间孔、胞腔体及外生孔隙23分布差异所致,或者矿物内或矿物间孔隙所贡献。
图7 煤(a,b)和页岩(c,d)不同孔径孔隙的孔体积占比以及比表面积占比

Fig.7 Proportion of pore volume and specific surface area of pores with different pore diameters in coal(a,b) and shale(c,d)

与之相比,页岩样孔容以介孔为主,微孔孔容和比表面积与煤相比都明显减少,各样品孔径结构基本一致,其中微孔和介孔孔容两者的占比大于90%[图7(c),图7(d)],且孔径分布曲线呈双峰分布(图4)。尽管煤和页岩孔隙分布存在差异,但微孔均是比表面积的主要贡献者。
煤样氮气吸附(>2 nm)的总孔容和比表面积要显著低于相邻的页岩,但CO2吸附(<2 nm)的总孔容和比表面积却是页岩的3~5倍(表2),这与两者之间的物质组成密切相关。煤的有机质含量是相邻页岩的5~10倍,为煤样提供了大量的微孔。页岩中除了一定量的有机质外,还具有较高的黏土矿物含量,除了发育一定量的微孔,还发育了较多的中孔。整体上看,煤主要发育<2 nm的有机质微孔,CH4分子主要以微孔填充的形式存在于这些微孔之中24。而页岩2 nm 到数百纳米的孔径范围内储集空间多样,多尺度的孔隙结构为游离态CH4 分子提供了更多的赋存空间。

3.2 可抽提沥青对煤和页岩孔隙影响

前人对低成熟海相页岩的研究表明,残留沥青会阻塞页岩的孔隙,减少储集空间,从而导致孔隙体积降低或储气空间减少25。对于生气为主的III型有机质,残余沥青的含量相对较低,但对孔隙发育的抑制作用不容忽视26。选取六盘水地区成熟—高成熟的煤和页岩样品(R O值介于1%~2%之间),对比抽提前后的孔径变化。研究表明,煤抽提沥青后,N2吸附量明显增加,N2 吸脱附曲线的滞留环有所扩大,反映出抽提前后煤的孔隙空间明显改善,开始出现一端封闭型孔隙连通性改善[图8(a),图8(b)]。而抽提前后的页岩滞留环并没有发生太大改变[图8(c),图8(d)]。从图9孔径分布曲线来看,抽提后煤的1~10 nm以及大于10 nm孔径的孔隙体积均有一定程度明显增多,并且开始出现孔径峰值。与之不同的是,抽提前后页岩在特定孔径范围内孔径峰值并没有明显变化。
图8 煤(a, c)和页岩(b, d)抽提前后氮气吸附—脱附曲线对比

Fig.8 Comparison chart of nitrogen adsorption-desorption curves of coal (a, c) and shale (b, d) before and after extraction

图9 煤(a, c)和页岩(b, d)抽提前后孔径分布曲线对比

Fig.9 Comparison chart of pore size distribution curves of coal (a, c) and shale (b, d) before and after extraction

由此可见,可抽提沥青对煤的孔隙空间影响较大,这是由于煤中有机质为聚集有机质,生成的液态烃多滞留在有机质内部孔隙空间,而页岩中有机质以分散态赋存在页岩基质中,生成的液态烃更容易排出页岩层系,对页岩孔隙结构影响相对较小。可抽提沥青可能是中低变质煤低孔低渗的重要因素,在编制开发方案时要考虑煤的原始组成、可溶有机质含量、成熟度对含气性的影响。在研究煤系气共采选层时,需要充分考虑可抽提沥青的存在对煤和页岩孔隙结构的影响,以便更好地评估煤系气的存储和运移能力,为资源的高效开发提供科学依据。

3.3 煤与页岩孔隙发育的地质影响因素

岩石热解结果(图1)表明,黔西上二叠统的煤和相邻页岩的有机质类型相似,均以Ⅲ型干酪根为主,但煤与页岩储层形成的沉积微相有所不同,使得二者在有机碳含量、矿物组成含量等方面差异很大,这是煤与页岩孔隙发育差异的主要原因。煤中不同显微组分具有不同的孔隙结构,有机组分与矿物之间普遍发育微裂缝27。煤中>2 nm的孔隙具有较强的非均质性,与显微组分之间孔隙差异有关28
成岩作用对煤和页岩储层孔隙均有显著影响。在成岩作用的早期阶段,在机械压实和胶结作用下,煤和页岩储层中大的孔隙大大减少。在进入生油窗或生气窗之后,煤及页岩中有机质由于生烃作用产生一定量的气孔,这些气孔以微孔与小孔为主,增加了储集空间。值得注意的是成熟煤演化过程生成的残余沥青会堵塞一部分孔隙,使得储集空间明显降低[图9(a)],在高成熟度阶段,残余沥青含量降低,但仍有一部分残余沥青充填在中孔和大孔阶段[图9(c)]。在成岩中后期阶段,页岩黏土矿物转化或部分矿物溶解,产生黏土矿物粒间或粒内孔隙、溶蚀孔等29。煤中残余沥青会进一步热解,伴随形成更多有机孔。同时,煤和页岩有机质与矿物之间容易产生成岩微裂缝,改善储集空间。
煤中有机质占有绝对优势,其<2 nm孔隙具有较高的孔体积和比表面积。煤和页岩微孔(<2 nm)孔容和比表面积与TOC含量明显正相关30,反映出微孔主要为有机质贡献[图10(a),图10(b)]。煤的热演化程度越高,其微孔孔容和比表面积越大,而页岩的微孔孔容和比表面积与热演化程度没有明显关系[图9(c),图9(d)]。因此,煤的有机质纳米微孔发育(<2 nm)受煤的TOC含量和热演化成熟度控制,而页岩的孔隙空间多样,除有机质孔外,还发育了大量的黏土矿物孔,孔隙的发育受成岩作用与热演化作用共同控制。
图10 样品有机碳含量、热演化程度与微孔孔容及比表面积关系

(a)样品有机碳含量与微孔比表面积关系;(b)样品有机碳含量与微孔孔容关系;

(c)样品热演化程度与微孔比表面积关系;(d)样品热演化程度与微孔孔容关系

Fig.10 The relationship between organic carbon content and thermal evolution degree of samples and micropore pore volume and specific surface area

4 结论

(1)黔西上二叠统龙潭组煤的物质组成主要以有机镜质组组分为主,其他矿物组分含量较少,孔隙大多数为有机质孔;而页岩的黏土矿物含量占绝对多数,因此页岩孔隙结构相对于煤储层孔隙更为复杂。煤比表面积为44.2~168 m2/g,总孔容为0.024~0.065 cm3/g,页岩比表面积为43.2~66.6 m2/g,总孔容为0.032~0.059 cm3/g。页岩中存在更多的墨水瓶形态孔隙或小尺寸喉道连通大孔径,孔径分布曲线呈双峰分布(3 nm和30 nm);而煤岩在相应的孔隙空间内多以一端封闭型楔形孔和狭缝孔为主,孔径分布变化较大,没有明显峰值特征。煤的微孔孔容和比表面积的贡献均占绝对优势,介孔或宏孔体积和比表面积占比相对低;页岩样品孔容以介孔为主,微孔孔容和比表面积与煤相比都明显减少。
(2)高有机质含量为煤样提供了大量的微孔,而页岩除了一定量的有机质外,还具有较高的黏土矿物含量,除发育微孔外,还发育较多的中孔。煤主要发育<2 nm微孔,CH4 分子主要以微孔填充的形式存在这些微孔之中;页岩发育2 nm至数百纳米储集空间,多尺度的孔隙结构为游离态CH4 分子提供了更多的赋存空间。
(3)抽提沥青前后,煤的空间明显改善,出现一端封闭型孔隙连通性改善,而页岩抽提前后无明显变化。煤的热演化程度及有机质含量是孔隙结构的主要影响因素,而页岩的有机质含量及矿物类型是影响孔隙结构的主要因素,其热演化程度影响相对较小。
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