Hou Haihai, Qin Qiuhong, Shao Longyi, Liang Guodong, Tang Yue, Zhang Huajie, Li Qiangqiang, Liu Shujun
Liaoning Technical University, Fuxin 123000, China.
China University of Mining and Technology, Beijing 100083, China.
ACS Omega. 2021 Nov 19;6(48):32495-32507. doi: 10.1021/acsomega.1c03904. eCollection 2021 Dec 7.
In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle-high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content ( ), low and medium ash yield ( ), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension ( ) is positively correlated with the Langmuir volume ( ) under the three-peak transverse relaxation time ( ) spectrum. The fractal dimension of all effective points under saturated water ( ) is positively correlated with and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension ( ) is negatively correlated with the porosity of full saturated water (Φ) and the porosity of movable water (Φ). There is a negative correlation between Φ and the seepage space fractal dimension ( ) in the coal samples with one-peak and two-peak T spectra, but a positive correlation can be found with the three-peak T spectrum. Therefore, it is necessary to consider the types of T spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, Φ, and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance ( ) at 2.6-2.8%, which would be attributed to the third coalification jump. Generally, is the fractal dimension representing the coal pore surface, and and are closely related to the pore structure. Furthermore, not only represents the roughness of the pore surface but also the complexity of the pore structure.
为了评估孔隙 - 裂隙结构分形表征在煤储层中的适用性，并确定孔隙度、渗透率、煤变质程度与孔隙 - 裂隙结构之间的内在关系，利用低场核磁共振（NMR）对中国北方安河、焦作和淮北煤田的21个中高阶煤样的孔隙 - 裂隙特征进行了研究。所有煤样的特点是水分含量低、灰分产率低和中等，煤岩显微组分中镜质组含量高。在三峰横向弛豫时间（ ）谱下，吸附空间分形维数（ ）与兰氏体积（ ）呈正相关。饱和水（ ）下所有有效 点的分形维数与 及吸附孔体积呈正相关，但与渗流孔隙和裂隙的体积比呈负相关。自由流动空间分形维数（ ）与全饱和水孔隙度（Φ）和可动水孔隙度（Φ）呈负相关。在单峰和双峰 谱的煤样中，Φ与渗流空间分形维数（ ）呈负相关，但在三峰 谱中呈正相关。因此，在分析孔隙 - 裂隙参数与NMR分形维数之间的相关性时，有必要将 谱峰类型作为前提条件加以考虑。随着煤阶的升高，吸附孔含量、Φ和体积不可动部分（BVI）分数先增加后降低，而渗流孔含量、裂隙发育程度、体积可动部分（BVM）分数和BVM/BVI先降低后增加。这些变化的拐点对应于镜质体最大反射率（）为2.6 - 2.8%，这可归因于第三次煤化跃变。一般来说， 是代表煤孔隙表面的分形维数， 和 与孔隙结构密切相关。此外， 不仅代表孔隙表面的粗糙度，还代表孔隙结构的复杂性。
