Water-bearing characteristics in overmature shales from coal measure strata based on equilibrium water vapor adsorption and its geological significances

  • Guoxin SHAN , 1 ,
  • Peng CHENG 2 ,
  • Xianming XIAO , 1 ,
  • Jian SUN 1 ,
  • Ping GAO 1
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  • 1. School of Energy,China University of Geosciences of China (Beijing),Beijing 100083,China
  • 2. State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 501640,China

Received date: 2021-07-14

  Revised date: 2021-10-12

  Online published: 2022-04-22

Supported by

The National Natural Science Foundation of China(U1810201)

the Science and Technology Department of Shanxi Province, China(20201101003)

Highlights

The water-bearing characteristics of shale reservoirs have an important impact on shale gas exploration and development, but the related research on high and over-mature coal measure shale is rarely reported. Using equilibrium water vapor adsorption experiments on two shale samples taken from the Yangquan block of the Qinshui Basin, this study investigated the influence of adsorption water on shale effective pore structure parameters, and discussed the occurrence and distribution of adsorption water in shale nanopores. The results show that the adsorbed water of the samples has a significant influence on the pore parameters, and the water content (or saturation) shows a nonlinear negative correlation with the effective pore parameters. A very low water content in the samples can greatly reduce their effective pore structure parameters, but even if the samples reach a high water saturation, there is still a portion of the pore spaces and surface areas can be available for gas adsorption. The water in the shale samples can be stored in nanopores with different pore sizes, but mainly stored in mesopores and 50-100 nm macropores. The water saturation extent of the non-micropore specific surface areas of the samples is significantly higher than that of their non-micropore volumes, and the water saturation extent to the micropore volumes and specific surface areas are similar, further indicating that the water is stored mainly in an adsorption state in the non-micropores and in a filling- state in the micropores. These results provide a reference for further understanding the pore water characteristics of high and over-mature coal measure shales under geological conditions.

Cite this article

Guoxin SHAN , Peng CHENG , Xianming XIAO , Jian SUN , Ping GAO . Water-bearing characteristics in overmature shales from coal measure strata based on equilibrium water vapor adsorption and its geological significances[J]. Natural Gas Geoscience, 2022 , 33(4) : 666 -676 . DOI: 10.11764/j.issn.1672-1926.2021.09.012

0 引言

富有机质页岩储层均含有一定数量的孔隙水1。由于页岩地球化学特征的差异、及其所处地质条件不同,页岩中孔隙水含量差别很大。以我国南方下古生界页岩为例,威远区块的威201井与长宁区块宁201井的含水饱和度为30%~45%,昭通区块的昭101井含水饱和度可达60%~95%2, 焦石坝区块的焦页4井含水饱和度为10%~30%3。页岩中的孔隙水不仅占据了页岩的孔隙空间与吸附点位,减少了页岩的原地气量4-5,而且还会堵塞页岩的部分孔隙,阻碍页岩中气体的运移26-8,并影响页岩气开采中压裂水的运移9-10。因此,页岩的含水特征一直是页岩储层评价的重要研究内容11-12
受海相页岩气的大规模勘探开发影响,近年来对海相页岩的含水特性开展了一系列探索性研究,取得了一些基本认识。如:成熟度是影响页岩孔隙水含量的重要因素,成岩压实、生排烃作用均会导致页岩孔隙水含量的降低13-15,但即使演化到高—过成熟度阶段,页岩中仍可含有少量孔隙水216;与页岩有机孔相比,无机孔(尤其是黏土矿物孔隙)具有较强的亲水性17-18,是孔隙水的主要储集空间,但是有机孔也可赋存少量孔隙水719,其机理与有机质纳米孔隙的毛细管束缚力有关20-22;页岩中孔隙水可包括游离水与束缚水, 但页岩气储层的含水饱和度一般都很低,基本上缺乏游离水,仅含束缚水,含气页岩一般呈超低含水饱和度状态21623。由此可见,页岩含水特征与赋存机理已经成为目前研究的热点。
我国具有页岩气勘探潜力的煤系页岩热演化程度较高,基本上处在高—过成熟度阶段,预测的页岩气资源量巨大,与我国海相页岩气相当24。然而,我国煤系页岩储层的勘探开发力度与研究程度远不如我国南方海相页岩储层,试采井均未取得实质性突破25-27,与页岩气评价相关的基础性研究工作尚在推进中,鲜见对煤系页岩含水性特征的相关报道。我国煤系页岩的有机质以Ⅲ型为主28,矿物组成也不同于我国南方海相页岩29-30,这就决定了其含水特征与我国南方海相页岩存在差异。认识我国高—过成熟煤系页岩的含水特征,对于这套层系页岩气的评价与勘探开发具有重要的意义。
目前,获取可保持地质条件下特征的页岩孔隙水样品非常困难,虽然有文献报道采用现场取心页岩样品研究其孔隙水特征1631-32,但钻井与取样过程中钻井液的污染、样品保存过程中孔隙水的变化等成为不可回避的影响因素。目前的相关研究主要是基于干燥后样品的平衡水吸附实验方法来模拟页岩的含水特性33-37。为此,本文研究采用平衡水吸附实验方法,对采集的高—过成熟度煤系页岩样品进行吸附水的模拟实验,研究吸附水在页岩纳米孔隙中的赋存与分布特征,及其对页岩有效储集空间的影响。

1 样品与实验

1.1 样品

沁水盆地位于华北地区大型克拉通盆地内,其广泛发育的上石炭统太原组与下二叠统山西组形成于海陆过渡相环境,为一套含煤岩系。本文研究所选的2个黑色页岩样品(编号:样品1与样品2)分别采自该盆地阳泉区块CSJ-01井的太原组与山西组煤系地层,其有机地球化学和矿物组成特征见表1所示。2个页岩样品R O(镜质组随机反射率)值分别为3.35%与3.38%,T max(最高热解峰温)值分别为584 ℃与586 ℃, I H(氢指数)值分别为15 mgHC/gTOC与14 mgHC/gTOCI O(氧指数)值分别为 2 mgHC/gTOC与32 mgHC/gTOC,均表明具有很高的成熟度,达到过成熟度阶段。2个页岩样品TOC(总有机碳)含量差别大(样品1与样品2的TOC值分别为7.04%与2.37%),但矿物组成相似,主要包含石英、长石和黏土矿物,缺乏碳酸盐矿物。根据文献[28-39]报道的沁水盆地煤系页岩相关数据,这2个页岩样品的TOC含量与矿物组成在沁水盆地具有一定代表性, 可分别代表高TOC与中TOC的煤系页岩。
表1 实验样品基础数据

Table 1 Basic data of the experimental samples

样品号 深度/m 岩性 R O/% TOC/% Rock-Eval分析 XRD分析

T max

/℃

I H(mgHC/gTOC I O(mgHC/gTOC

石英

/%

长石

/%

黏土矿物

/%

黄铁矿 /% 锐钛矿/%
样品1 390.4 页岩 3.35 7.04 584 15 2 44.5 1.1 49.2 0.9 4.3
样品2 300.1 页岩 3.38 2.37 586 14 32 39.6 1.3 50.9 5.3 2.9

1.2 实验方法

1.2.1 平衡水吸附实验

本文研究按照美国测试材料学会推荐的煤岩样品水吸附实验的标准方法(ASTM D1412—07),采用饱和的K2SO4溶液(其在实验温度30℃下可保持96%~97%的平衡相对湿度),根据吸附时间的不同,可以获得一系列不同吸附水含量的子样37。对于本文研究页岩样品,具体实验方法如下:
(1)从大块的页岩岩心中钻取的圆柱形小岩心样品(直径15 mm×高20 mm),在真空烘箱中于105 ℃干燥20 h,去除样品中的水分,获得干燥样品质量(M D)。
(2)将干燥后的样品放入一个小型真空干燥器中,在该干燥器内部放置K2SO4饱和溶液,抽空至约30 mmHg的绝对压力后,将干燥器放入30 ℃的水浴中进行吸附实验。
(3)定时称量润湿岩心,获得质量(MT ),直到质量恒定(M E),并认为此时达到饱和水蒸气条件下的吸附水平衡。
采用式(1)式(2)分别计算一定吸附时间的吸附水含量和平衡吸附水含量,采用式(3)计算吸附水饱和度:
C I W = ( M T - M D ) ÷ M D
C E I W = ( M E - M D ) ÷ M D
S W T = C I W / C E I W × 100 %
式中:C IW为吸附时间为T时的吸附水含量,mg/g;C EIW为平衡吸附水含量,mg/g; S W T为 吸附水饱和度,%;M D为干燥样品质量,mg; M E为达到吸附平衡时样品质量,mg; MT 为吸附时间为T时样品的质量,mg。
每个样品分为2组进行平衡水吸附测试。第1组进行连续吸附,直至达到吸附平衡,确定样品的吸附曲线与达到吸附平衡时的吸附水含量(即最大吸附水量)。第2组样品采用阶段吸附,即根据第1组样品的吸附曲线,选择不同的吸附时间,获取不同吸附水饱和度的样品。本文研究设计的样品点是:吸附水饱和度大致为10%~20%、30%~40%、50%~60%、70%~80%[具体饱和度则根据所取样品的吸附水量按照式(3)计算确定]。对获得的不同含水饱和度的样品进行低压CO2和N2吸附实验,分析与比较吸附水在页岩中的赋存以及对孔隙结构参数的影响。

1.2.2 低压CO2与N2吸附实验

根据IUPAC的方案,页岩中的孔隙可划分为微孔(<2 nm)、介孔(2~50 nm)和大孔(>50 nm)。本文研究使用Micromeritics ASAP2020M装置,通过低压CO2和N2吸附分别表征样品(干燥样品和含水样品)的微孔(0.35~1.1 nm)和非微孔(包括介孔和大孔)的纳米孔隙结构37。在本文实验中,CO2 与N2吸附实验表征的实际孔径范围分别为0.35~1.1 nm与1.7~250 nm。
实验方法简述如下:对于干燥的样品,将其置于样品室,在真空度<10 mmHg的条件下,脱气12 h,再进行N2和CO2吸附实验;对含水页岩样品,将其储存于温度为-196.56 ℃(液氮)或温度为0 ℃(冰水混合物)的样品室,直接进行N2和CO2吸附实验。N2和CO2吸附实验的相对压力分别为0.005~0.995和0.000 01~0.032。基于CO2吸附等温线,应用DA(Dubinin Astakhov)方程计算样品的微孔体积和等效比表面积; 基于N2吸附等温线的吸附分支曲线,应用修正的BET ( Brunauer Emmett Teller)方程计算非微孔比表面积,根据最大实验压力(p/p 0=0.998)下N2的吸附量计算页岩的非微孔孔容37

2 结果与讨论

2.1 平衡水吸附特征

图1(a) 为2个样品吸附水含量随吸附时间的变化。根据IUPAC吸附等温线的分类,样品的等温吸附曲线为I型40。其吸附特征为:在初始吸附阶段,吸附水含量随吸附时间迅速增加;当达到吸附拐点后,吸附速率十分缓慢,直至达到吸附平衡。虽然2个样品吸附量存在差别,但将吸附量转化为吸附水饱和度后[式(3)],2个样品的吸附曲线非常相似[图1(b)]。吸附曲线的拐点均对应于5 h左右,此时的吸附量达到80%左右。值得指出的是,本文煤系页岩样品的吸附曲线与CHENG等37报道的海相高—过成熟页岩很相似,但也存在一些差别,如:本文研究样品吸附速率较快,达到吸附拐点的时间要短,而且吸附量较大,这应当与本文研究样品具有较高含量的黏土矿物有关。本文研究样品的黏土矿物含量是这些海相页岩样品的1.5~2倍37表1);页岩样品中的吸附水优先储集于黏土矿物孔隙中,相比于有机质孔,其对水的吸附要快,吸附要量大41
图1 研究样品平衡水吸附曲线

Fig.1 The equilibrium water adsorption curves of the studied samples

2.2 吸附水对孔隙结构参数的影响

本文研究对2个样品在不同吸附时间(即不同吸附水含量或饱和度)获得的含水样品,进行了低压N2与CO2的吸附实验。为方便对比2个样品,采用吸附水饱和度进行表示。根据实验结果(图2),随着吸附水含量的增加,2个样品对N2的吸附量均有所降低。与干燥样品具有明显迟滞环有所不同的是,随着吸附水饱和度的增加,迟滞环逐渐变小,当吸附水饱和度达到大约70%,迟滞环消失,吸附和解吸曲线基本重合。本文研究干燥样品N2解吸回线特征类似于ROUQUEROL等42分类的 II(b)型,这类迟滞环的形成是由于广泛发育狭缝状孔与楔形孔隙,含吸附水样品的迟滞环变小、直至消失是由于这类孔隙被水所占据或者堵塞导致31
图2 实验样品在不同吸附水饱和度条件下低压N2吸附—解吸附曲线

Fig.2 Low pressure N2 adsorption-desorption curves of the studied samples under different adsorption water saturation conditions

研究样品低压CO2吸附曲线表明,随着吸附水含量的增加,页岩微孔的吸附能力逐渐降低(图3),与N2吸附有所不同的是,其降低幅度相对较小。
图3 不同吸附水饱和度的页岩样品低压CO2吸附曲线

Fig. 3 Low pressure CO2 adsorption curves of the shale samples with different adsorption water saturation

2个页岩样品在干燥与不同吸附水饱和度条件下的孔隙结构参数见表2表3。在干燥条件下,样品1的微孔与非微孔比表面积分别为28.35 m2/g与10.41 m2/g, 微孔与非微孔孔容分别为0.012 cm3/g与0.017 cm3/g, 微孔比表面积与孔容分别占总比表面积与总孔容的73%与41%;样品2的微孔与非微孔比表面积分别为16.04 m2/g与9.90 m2/g, 微孔与非微孔孔容分别为0.006 7 cm3/g与0.016 0 cm3/g, 微孔比表面积与孔容分别占总比表面积与总孔容的62%与29%。由此可见,2个页岩样品的比表面积主要由微孔贡献,孔容主要由非微孔提供。与样品2相比,样品1具有大得多的总比表面积,尤其是微孔比表面积,同时具有较大的微孔体积,但其非微孔体积略小。结合2个样品的TOC含量与矿物组成,可以认为,二者孔隙结构参数的差别主要与各自TOC含量相关(样品1的TOC含量比样品2要大得多,见表1),也说明页岩样品中有机质主要发育微孔。这与前人43-44对高—过成熟煤系页岩的研究结果相一致。
表2 样品1在不同吸附水饱和度条件下的孔隙结构参数

Table 2 Pore structural parameters of sample 1 under different adsorption water saturation conditions

吸附水饱和度

/%

N2吸附参数 CO2吸附参数
非微孔比表面积/(m2/g) 非微孔孔容/(cm3/g) 微孔比表面积/(m2/g) 微孔孔容/(cm3/g)
0 10.41 0.016 65 28.35 0.011 75
12.81 3.76 0.015 78 20.13 0.008 54
33.48 3.53 0.013 58 15.12 0.005 74
69.84 1.30 0.006 05 13.55 0.005 02
82.86 1.19 0.005 77 12.36 0.004 48
表3 样品2在不同束缚水饱和度条件下的孔隙结构参数

Table 3 Pore structural parameters of sample 2 under different adsorption water saturation conditions

平衡水饱和度

/%

N2吸附参数 CO2吸附参数
非微孔比表面积/(m2/g) 非微孔孔容/(cm3/g) 微孔比表面积/(m2/g) 微孔孔容/(cm3/g)
0 9.90 0.015 98 16.04 0.006 67
9.58 3.65 0.014 65 11.51 0.004 71
14.45 3.34 0.012 74 11.47 0.004 66
52.04 2.37 0.011 88 11.46 0.004 60
79.51 1.27 0.005 58 11.29 0.004 57
随着页岩样品吸附水含量的增加,其比表面积与孔容明显减少,尤其以吸附水饱和度从零增加到10%~15%的减少幅度最大(图4)。此外,随着吸附水饱和度的增加,微孔与非微孔的变化规律也存在一定差别。吸附水饱和度从零增至10%左右,微孔的孔容与比表面积快速减少,之后仅略有降低[图4(a),图4(b)]。随着吸附水饱和度增加,非微孔的比表面积减少趋势与微孔相似,但非微孔的孔容呈现持续减少的趋势[图4(c), 图4(d)]。因此,吸附水对页岩样品的有效纳米孔隙结构参数具有明显的影响,尤其是对非微孔(主要是介孔)的比表面积影响最大。本文研究结果与CHENG等37报道的海相页岩呈现类似规律。
图4 页岩样品吸附水饱和度与孔隙结构参数的关系

Fig.4 Plots showing relationships between adsorption water saturation and pore structure parameters of the shale samples

2.3 吸附水在纳米孔隙中的赋存与分布

根据表2表3的数据,可进一步计算吸附水占据页岩比表面积和孔容的百分比。计算方法为:(干燥样品孔隙结构参数—含水样品孔隙结构参数)÷干燥样品孔隙结构参数×100%。根据计算结果(图5),吸附水对页岩非微孔比表面积和孔容的影响程度不同,但是对微孔孔容和比表面积的影响基本均等,这表明吸附水在页岩非微孔和微孔中的赋存状态不同。
图5 页岩样品吸附水饱和度与其对孔隙空间占据程度的相关性

Fig. 5 Plots showing relationships between the adsorption water saturation and the water-occupied extent of pore spaces for the shale samples

以样品1为例[图5(a)],页岩中大部分非微孔的比表面积在吸附水低饱和度的条件下就被水所占据(吸附水饱和度为12.81%,其占据了非微孔比表面积的76.95%),当吸附水饱和度增加至82.86%时,其占据了比表面积的88.57%,相对只增加了11.62%[图5(a)]。与非微孔的比表面积相比,水占据非微孔孔容的变化幅度相对较小,其与吸附水饱和度呈现近线性正相关[图5(a)]。如:当吸附水饱和度为12.81%时,其占据的孔容仅为28.99%;当吸附水饱和度增加至82.86%时,其占据的孔容为65.34%,增加了34.35%。页岩吸附水对其非微孔比表面积的影响程度显著大于对非微孔孔容的影响程度,这表明水主要以吸附态赋存于页岩非微孔的表面。随着页岩样品吸附水饱和度的增加,微孔比表面积被水占据的比例与孔容被水占据的比例呈近乎等量增加的趋势。如:当吸附水饱和度为12.81%时,水分别占据了微孔比表面积和孔容的30%和25.17%;当吸附水饱和度达到82.86%,水分别占据了56.64 %和61.90%。这表明,与非微孔所不同的是,吸附水主要以填充态赋存于页岩微孔中。DUBININ 等45对微孔中气体吸附实验的研究结果表明,气体是以填充态赋存于微孔中,与吸附水在微孔中的赋存状态具有相似性。由此可见,页岩吸附水可赋存于微孔和非微孔中,在一定的吸附水饱和度条件下,页岩非微孔比表面积被水占据的程度明显高于微孔。
根据图5可知,在相同吸附水饱和度的条件下,吸附水对非微孔的占据程度,2个样品相近,但对微孔的占据程度,2个样品存在一些差异。随着吸附水饱和度的增加,样品1中的微孔持续为吸附水所占据[图5(a)],但样品2中的微孔被水填充到一定程度后不再明显增加,说明样品2中增加的吸附水基本上储集于非微孔中[图5(b)]。2个样品的这种差异可能与其TOC不同所导致的微孔结构特征差异有关。
干燥样品与含吸附水样品孔径分布图的差异可进一步反映水在纳米孔隙中的分布特征(图6图7)。在2~100 nm孔径范围内,干燥样品的孔容增量总体呈现随孔径增大而增加的趋势,但含水样品的这种趋势不如干燥样品明显;与干燥样品相比,含吸附水样品的比表面积增量主峰明显移向低孔径范围(图6)。在孔径2 nm处,干燥样品的孔容与比表面积均有较大的增量,说明样品中含丰富的微孔,但含吸附水样品几乎没有了增量(图6)。在更小的微孔区间(0.42~1.1 nm),含吸附水样品的孔容增量小于干燥样品, 特别是在0.42~0.5 nm之间与1.0~1.1 nm之间,含吸附水样品的孔容增量几乎为零(图7)。因此,吸附水在各种孔径的纳米孔隙中均有分布,尤其是介孔与50~100 nm的宏孔,并基本堵塞了1.1~2 nm的微孔。上述研究结果基本与马继胜等32对现场取心页岩样品研究得出的孔隙水分布特征相一致。
图6 干燥样品与含吸附水样品N2吸附的孔径分布

Fig.6 Pore size distributions of the studied dry samples and their moist samples based on N2 adsorption

图7 干燥样品与含吸附水样品CO2吸附的孔径分布

Fig.7 Pore size distributions of the studied dry samples and their moist samples based on CO2 adsorption

2.4 地质意义

页岩中的孔隙水不仅控制游离气含量,而且对吸附气含量也有重要的影响。页岩含气性现场测试数据表明,页岩中孔隙水含量与吸附气含量呈现非线性负相关关系。具体表现为:在低含水饱和度条件下,吸附气含量(或者容量)随着孔隙水含量的增加快速降低,当达到一定含水饱和度后(大约40%)出现拐点,随着含水量的进一步增加,吸附气含量仅略有降低46。本文研究实验结果可以为这一地质现象提供一些理论解释。根据本实验结果,在很低吸附水饱和度的前提下,大部分非微孔比表面积被水所占据,从而大幅度降低页岩的吸附能力(图4图5)。随着吸附水含量的进一步增加,页岩的非微孔比表面积仅略有减少,而吸附水仍然可使微孔的比表面积与孔容持续减少(图4图5),进一步导致吸附性能降低。因此,页岩的吸附气含量与孔隙水含量或饱和度的这种非线性负相关性,主要受控于水在页岩纳米孔隙中的赋存与分布, 但其深层次机理解析尚需进一步研究。
页岩吸附气评估通常是基于对干燥样品的甲烷吸附实验,并假定有机孔隙由于其疏水性基本上不存在孔隙水47。然而,页岩中的水可存在于有机与无机孔隙中16,非常低的含水量可显著减少可供气体吸附的比表面积或者微孔体积。因此,在评估页岩吸附气含量时,应充分考虑孔隙水的影响,这对于低—中成熟度页岩尤其重要,因为这些页岩尚未经历强烈的气携排水作用过程,页岩中可能还存在较多的孔隙水48-50。本文研究结果也表明,对于过成熟度煤系页岩,即使页岩样品含水饱和度较高(如:吸附水饱和度达到60%),也有约20%的非微孔比表面积与50%左右的微孔可作为气体吸附的有效空间,页岩吸附气仍然是煤系页岩气的重要组成部分。

3 结论

本文研究基于平衡水吸附实验,对采自沁水盆地阳泉区块2块过成熟煤系页岩样品开展吸附水含量与孔隙结构参数关系的研究,主要获得了以下认识与结论:
(1)页岩样品吸附水含量与孔隙结构参数呈现非线性负相关关系,即使很低的吸附水含量,也可大幅度减少页岩的有效孔隙结构参数,显著降低页岩的吸附性能。
(2) 页岩样品中吸附水在不同孔径的纳米孔隙均有分布,尤其是介孔与50~100 nm的宏孔,吸附水主要以吸附态储集于非微孔表面,以填充态赋存于微孔中。
(3) 页岩样品的吸附水未完全占据其孔隙空间,仍有部分孔隙空间与表面积可用于气体吸附。对于高—过成熟煤系页岩气评价,不可忽视页岩孔隙水对吸附气与含气性的影响。
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