CN112986089A - 一种构造煤多尺度孔隙结构表征及分形维数计算方法 - Google Patents
一种构造煤多尺度孔隙结构表征及分形维数计算方法 Download PDFInfo
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Abstract
本发明为一种构造煤多尺度孔隙结构表征及分形维数计算方法,属于瓦斯地质领域范畴,其特征是采用压汞实验、低温N2吸附实验和低温CO2吸附实验对构造煤中不同尺度的孔隙结构进行表征;提出了基于孔隙结构全孔径段多尺度联合表征的分形维数计算方法,目的是为描述构造煤全孔径段孔隙分形结构特征,该方法通过对不同阶段分形维数的加权平均计算,实现了不同方法测试结果的归一化,使基于不同测试方法的不同尺度孔隙分形维数按照一定的规律统一起来,为研究构造煤孔隙结构特征和瓦斯扩散规律奠定基础。
Description
技术领域
本发明属于瓦斯地质领域,特别提供一种构造煤多尺度孔隙结构表征及分形维数计算方法,通过以分形维数为主的特征参数对煤体不规则细微孔隙结构特性进行定量表征。
背景技术
煤炭是我国能源的主体,预计在相当长的一段时间里煤炭仍将在我国一次性能源消费结构中居主体地位,但煤矿安全问题依然严峻,而瓦斯仍是我国煤矿安全生产的“第一杀手”。通过对我国的煤与瓦斯突出事故进行分析可以得出:一定厚度构造煤是导致煤与瓦斯突出的重要原因。煤层的形成过程伴随着地质构造的挤压、剪切和拉张的破坏,煤纳米级孔隙结构在构造变形作用下发生重要变化,原有的清晰层理和节理结构丧失、煤体的透气性变差、瓦斯抽采效率低,这一变化不仅影响着煤层瓦斯的储集和赋存状态,同时也对煤与瓦斯突出预测和煤层气开采具有重要影响。
由于我国煤系地层地质构造复杂,受不同期次的构造演化影响,构造煤分布广泛、孔隙结构异常复杂,具有强烈的非均质性和不规则性,难以简单描述,而煤作为一种复杂的天然多孔性固体物质,物性特征是决定煤储层孔隙性、吸附性和渗透性的首要因素,且煤体结构特征是甲烷吸附、渗流以及运移的决定性因素,故其纳米级孔隙对煤层气的储集和运移有着重大影响。构造煤孔隙的多尺度分布 (大、介、微孔均有分布)特性和分形结构特征是研究瓦斯扩散的重要依据,且分形维数是定量表征多孔介质分形的重要参数,能够反映煤体结构的非均质性和表面的复杂程度,因此研究一种构造煤多尺度孔隙结构表征及分形维数计算方法不仅可以合理预测构造煤的发育特征,指导煤矿安全生产,同时对煤层气勘探和开发亦具有十分重要的意义。
发明内容
本发明为一种构造煤多尺度孔隙结构表征及分形维数计算方法,其目的在于采用压汞—吸附联合法对构造煤的孔隙结构特征进行多尺度综合分析,以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,提出多尺度综合分形维数计算方法,为研究构造煤瓦斯扩散规律奠定基础。
本发明技术方案如下:
Ⅰ、煤样采集与制备;
Ⅱ、采用低温CO2吸附法对煤样2nm以下微孔进行测试;
Ⅲ、采用低温N2吸附法对煤样2~50nm介孔进行测试;
Ⅵ、采用压汞实验法对煤样50nm以上大孔进行测试;
Ⅴ、通过压汞—吸附联合法的构造煤孔隙结构多尺度综合表征方法,形成了基于低温CO2吸附法测试数据的微孔分形维数计算方法、低温N2吸附法测试数据的介孔分形维数计算方法和压汞法测试数据的大孔分形维数计算方法,对构造煤不同尺度孔隙的分形维数进行了计算。
Ⅵ、以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,提出的多尺度综合分形维数计算方法如式(1)所示。
式中:dfz为构造煤多尺度综合分形维数;
Bu为微孔孔容占总孔容的比例;
dui为微孔阶段中第i个孔径段的分形维数;
bui为微孔阶段中第i个孔径段孔容占微孔总孔容的比例;
m为微孔阶段中不同孔径段的个数,为正整数;
Bu为介孔孔容占总孔容的比例;
duj为介孔阶段中第j个孔径段的分形维数;
buj为介孔阶段中第j个孔径段孔容占介孔总孔容的比例;
n为介孔阶段中不同孔径段的个数,为正整数;
Bw为大孔孔容占总孔容的比例;
dwk为大孔阶段中第k个孔径段的分形维数;
bwk为大孔阶段中第k个孔径段孔容占大孔总孔容的比例;
o为大孔阶段中不同孔径段的个数,为正整数。
本发明的有益效果为:
1、基于压汞—吸附联合法的构造煤孔隙结构多尺度综合表征方法,对构造煤的孔隙结构特征进行分析,克服了单一测试方法无法准确测试不同尺度孔隙结构的缺点;
2、定义了描述构造煤全孔径段孔隙分形结构特征的多尺度综合分形维数,并从分形结构的特征与分形维数计算模型出发,提出了相应的分形维数计算方法,即以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,使得分布杂乱无章的多尺度孔隙分形维数按照一定的规律统一起来;
3、为研究构造煤分形孔隙结构中的扩散规律奠定基础。
附图说明
图1微孔孔容分布图;
图2介孔孔容分布图;
图3大孔孔容分布图;
图4全孔径段孔容分布特征;
具体实施方式
实施例1
Ⅰ、实验采自潞安矿区二叠系下统山西组3#煤层,依据《煤样的制备方法》 (GB/T474-2008)的规定并结合各实验的需要,在实验室将煤样通过缩分、逐级破碎等步骤分别制成不同实验所需的煤样;
Ⅱ、使用Quadrasorb SI型孔分析仪,在液态CO2饱和温度(273K)下进行测试,实验样品质量约为1g左右,煤样粒径为60~80目(直径为0.17~0.25mm), 采用NLDFT方法对低温CO2吸附数据进行分析,微孔孔容分布图如图1所示;
Ⅲ、使用Quadrasorb SI型孔分析仪,在液氮饱和温度(77K)下进行测试,实验过程中相对压力范围为0.001~0.995MPa。实验样品质量约为2.5g左右,煤样粒径为60~80目(直径为0.17~0.25mm),本次所测煤样的有效孔径范围为 1.50~27.00nm,介孔孔容分布图如图2所示;
Ⅵ、采用PoreMaster-60型压汞法孔分析仪,测试范围为3.5~106nm,汞和煤表面间的接触角取130°,汞的表面张力取0.485N/m。实验样品质量约1g,测试前将样品在100℃左右的环境恒温干燥12h。基于Washburn公式测定不同压力下注入煤孔隙中的汞体积,得到汞压力与注入汞体积之间的关系曲线,根据曲线分析获得相关孔隙数据,大孔孔容分布图,如图3所示;
Ⅴ、上述孔隙分布数据按照IUPAC的分类方案进行统计分析,归纳出实验样品的微孔、介孔和大孔的孔隙分布图,如图4所示;利用CO2吸附数据计算煤样的孔隙分布密度函数J(r)值,并和对应的孔径r数据按照公式(2)进行线性回归,依据所得直线斜率计算各煤样的微孔分形维数df,同理采用FHH模型计算介孔分形维数,采用热力学模型计算大孔分形维数,结果如下表;
ln J(r)=(2-df)lnr+C (2)
式中:J(r)为孔隙的分布密度函数;
df为构造煤孔隙分形维数;
r为孔隙的孔径,nm。
Ⅵ、以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,提出的多尺度综合分形维数计算方法,得出潞安矿区构造煤多尺度综合维数。
式中:dfz为构造煤多尺度综合分形维数;
Bu为微孔孔容占总孔容的比例;
dui为微孔阶段中第i个孔径段的分形维数;
bui为微孔阶段中第i个孔径段孔容占微孔总孔容的比例;
m为微孔阶段中不同孔径段的个数,为正整数;
Bυ为介孔孔容占总孔容的比例;
dvj为介孔阶段中第j个孔径段的分形维数;
bυj为介孔阶段中第j个孔径段孔容占介孔总孔容的比例;
n为微孔阶段中不同孔径段的个数,为正整数;
Bw为大孔孔容占总孔容的比例;
dwk为大孔阶段中第k个孔径段的分形维数;
bwk为大孔阶段中第k个孔径段孔容占大孔总孔容的比例;
为大孔阶段中不同孔径段的个数,为正整数。
由以上数据可以发现通过对不同阶段分形维数的加权平均计算,使得分布杂乱、无明显规律的不同尺度孔隙分形维数按照一定的趋势统一起来,所得到综合分形维数与构造煤结构演化表现出一定的关联性,随着构造煤破坏程度(煤体结构类型)的增加,多尺度综合分形维数也不断增大,表明孔隙结构的不规则性、非均质性也相应增强。
本发明未尽事宜为公知技术。
上述实施例只为说明本发明的技术构思及特点,其目的在于让熟悉此项技术的人士能够了解本发明的内容并据以实施,并不能以此限制本发明的保护范围。凡根据本发明精神实质所作的等效变化或修饰,都应涵盖在本发明的保护范围之内。
Claims (5)
1.一种构造煤多尺度孔隙结构表征及分形维数计算方法,其特征在于:采用压汞—吸附联合法对构造煤的孔隙结构特征进行多尺度综合分析,实现了不同测试方法结果的归一化;从分形理论的基本原理出发,提出了能够表征构造煤全孔径段孔隙结构分形特征的多尺度综合分形维数及其计算方法,使基于不同测试方法的不同尺度孔隙分形维数按照一定的规律统一起来。
2.按照权利要求1所述的压汞—吸附联合法,其特征在于:根据低温CO2吸附法、低温N2吸附法、压汞法对不同尺度孔隙的测量精度,采用三种方法联合测量的方式,从不同测试方法的优势孔径段出发,将上述测试方法的数据分别在2nm、50nm处进行衔接,利用低温CO2吸附法表征2nm以下微孔,用低温N2吸附法表征2~50nm的介孔,用经过压缩性修正的压汞法数据表征50nm以上的大孔。
3.按照权利要求1所述的多尺度综合分形维数,其特征在于:基于低温CO2吸附法测试数据的微孔分形维数计算方法、基于低温N2吸附法测试数据的介孔分形维数计算方法和基于压汞法测试数据的大孔分形维数计算方法,对构造煤不同尺度孔隙的分形维数分别进行计算。
4.按照权利要求1所述的分形维数计算方法,其特征在于:从分形结构的特征与分形维数计算模型出发,提出了相应的分形维数计算方法,即以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,所得的分形维数即为多尺度综合分形维数。
5.按照权利要求1所述的一种构造煤多尺度孔隙结构表征及分形维数计算方法,其具体步骤为:
Ⅰ、煤样采集与制备;
Ⅱ、采用低温CO2吸附法对煤样2nm以下微孔进行测试;
Ⅲ、采用低温N2吸附法对煤样2~50nm介孔进行测试;
Ⅵ、采用压汞实验法对煤样50nm以上大孔进行测试;
Ⅴ、通过基于压汞—吸附联合测试的构造煤孔隙结构多尺度综合表征方法,形成了基于低温CO2吸附法测试数据的微孔分形维数计算方法、基于低温N2吸附法测试数据的介孔分形维数计算方法和基于压汞法测试数据的大孔分形维数计算方法,对构造煤不同尺度孔隙的分形维数进行了计算。
Ⅵ、以各孔径段孔隙的孔容比例为权重对各阶段孔隙的分形维数进行加权平均计算,提出的多尺度综合分形维数计算方法如式(1):
式中:dfz为构造煤多尺度综合分形维数;
Bu为微孔孔容占总孔容的比例;
dui为微孔阶段中第i个孔径段的分形维数;
bui为微孔阶段中第i个孔径段孔容占微孔总孔容的比例;
m为微孔阶段中不同孔径段的个数,为正整数;
Bv为介孔孔容占总孔容的比例;
dvj为介孔阶段中第j个孔径段的分形维数;
bvj为介孔阶段中第j个孔径段孔容占微孔总孔容的比例;
n为介孔阶段中不同孔径段的个数,为正整数;
Bw为大孔孔容占总孔容的比例;
dwk为大孔阶段中第k个孔径段的分形维数;
bwk为大孔阶段中第k个孔径段孔容占总孔容的比例;
o为大孔阶段中不同孔径段的个数,为正整数。
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