改进压汞表征方法及其在褐煤储层中的应用

Application of mercury injection correction in pore characterization of lignite reservoir

压汞试验由于其模型简单且测试范围广、速度快、精度高等优点,被广泛应用于多孔材料孔隙表征。而褐煤样由于变质程度低,压实作用弱,煤质松软,易受麻皮效应、基质压缩效应等影响,导致测试结果存在较大误差。为提高压汞试验测试精度,对研究区(二连盆地)褐煤样压汞数据进行校正。首先基于各样累计进汞曲线及分形特征,确定各样排驱压力及麻皮系数。其次针对传统基质压缩效应校正方法在低压段校正不足,高压段校正过度的缺陷,提出了基于三次样条插值、分段求取煤样基质压缩系数的新方法,提高数据利用率及校正结果的准确性。最后,联合压汞及液氮吸附试验数据分析了该区褐煤样孔隙发育情况。研究结果显示,麻皮效应对块状煤样的主要影响范围为<0.0274 MPa压力段,麻皮系数均值为16.9%。基于新方法计算该区褐煤样平均基质压缩系数介于1.73×10^(-4)~3.48×10^(-4)MPa^(-1),均值为2.28×10^(-4)MPa^(-1),且随煤样孔隙流体压力、有机质含量、含矿物基腐植组含量增加,煤基质压缩系数增大,基质压缩校正前后进汞量下降0.0383~0.0792 mL/g,平均降幅为35.1%。研究区褐煤孔容(>1.7 nm)介于0.03~0.25 cm^(3)/g,均值为0.15 cm^(3)/g,孔隙由微孔至大孔孔容占比均值依次为12.5%、16.8%、16.2%、54.5%,B-E-T比表面积介于2.4~37.19 m^(2)/g,均值为13.85 m^(2)/g。煤样孔径分布曲线自然分为Ⅰ(>5μm)、Ⅱ(0.1~5μm)、Ⅲ段(<0.1μm),其中Ⅰ段孔隙主要为管胞及胞腔孔,Ⅱ段孔隙主要为纹孔及气孔,Ⅲ段孔隙主要为链间孔及分子结构孔,Ⅰ、Ⅱ段孔隙孔容与含矿物基惰质组含量呈正相关,Ⅲ段孔隙孔容受变质程度控制,且与Ro呈正相关。

Mercury intrusion experiment is widely used in porous material characterization because of its simple model,wide testing range,high speed and high precision.However,lignite samples are easy to be affected by Hemp skin effect and matrix compression effect due to low metamorphic degree,weak compaction and soft coal quality,resulting in large error in test results.In order to improve the test accuracy,the mercury injection data of lignite samples in Erlian basin were corrected.Firstly,based on the cumulative mercury injection curve and fractal characteristics,the displacement pressure and hemp skin coefficient are determined.Secondly,aiming at the deficiency of traditional matrix compression correction method in low-pressure section and over correction in high-pressure section,a new method based on cubic spline interpolation and subsection to obtain matrix compression coefficient of coal sample is proposed to improve data utilization and accuracy of correction results.Finally,the pore development of lignite samples in this area was analyzed by combining mercury injection and liquid nitrogen adsorption experimental data.The results show that the hemp skin effect mainly affects the pressure range of<0.0274 MPa,and the average hemp skin coefficient is 16.9%.Based on the new method,the average matrix compressibility of lignite samples in this area ranges from 1.73 to 3.48×10^(-4)MPa^(-1),with an average of 2.28×10^(-4) MPa^(-1).With the increase of pore fluid pressure,organic matter content and mineral based humic group content,the matrix compressibility increases.Before and after correction,the mercury intake decreases by 0.0383-0.0792 mL/g,with an average decrease of 35.1%.The pore volume(>1.7 nm)of lignite in the study area is between 0.03-0.25 cm^(3)/g,with an average of 0.15 cm^(3)/g.The average pore volume from micropore to macropore is 12.5%,16.8%,16.2%and 54.5%,respectively.The specific surface area of B-E-T is between 2.4-37.19 m^(2)/g,with an average of 13.85 m^(2)/g,which is mainly provided by micropores.The pore size distribution curve of coal sample can be divided into I(>5μm),Ⅱ(0.1-5μm),Ⅲ(<0.1μm).Among them,the pores in section I are mainly tracheid and cell cavity pores,the pores in sectionⅡare mainly striated pores and pores,and the pores in sectionⅢare mainly interchain pores and molecular structure pores.The pore volume of the first and second sections is positively correlated with the content of the content of inertinite,and the pore volume of the third section is positively correlated with Ro.