为了研究煤储层孔裂隙分布非均质性,揭示煤层气藏赋存状态和传输特性,以黔西多煤层发育区9块二叠统龙潭组中高阶煤样为例,利用饱和-离心测试方法进行煤样核磁共振(NMR)测试,明确样品可动水、束缚水分布及孔裂隙结构特征;通过单重和多重分形理论进行煤样孔裂隙分布非均质性的定量表征,探讨不同分维值与孔隙结构参数的相关性。研究表明:单重分形模型能够更好地表征不同流体状态或孔隙分布的非均质性,而多重分形模型则更适用于表征孔裂隙分布的非均质性;单重分形参数中,渗流孔分维值(D2)和总分维值(DT)表现为弱负相关性;多重分形参数中,随着吸附孔孔隙体积增加,谱宽D-10—D10呈线性增大,谱宽D-10—D0变化较小;样品可划分为吸附孔发育型和渗流孔发育型2类,前者普遍发育小孔,孔径分布呈“双峰态”,后者普遍发育大孔,孔径分布呈“单峰或双峰态”;随着渗流孔孔隙体积减小,渗流孔和孔隙总体分布非均质性增加;吸附孔孔隙是影响孔裂隙分布的主要因素,对煤储层孔裂隙结构的多重分形特征具有明显的控制作用。单重分形模型和多重分形模型在物理意义上存在一定的差异,但都对研究煤储层的孔裂隙结构提供了重要的理论支持。
To study the heterogeneity of pore and fracture distribution in coal bed and reveal the occurrence state and transmission characteristics of coalbed methane reservoirs, taking the high-order coal samples of the Permian Longtan Formation at 9 Blocks in the multi-coal seam development zone of western Guizhou as an example, the nuclear magnetic resonance (NMR) test of coal samples is performed using saturation-centrifugation test method to define the distribution of movable water and irreducible water, and the characteristics of pore fracture structure. Quantitative characterization of the heterogeneity of pore fracture distribution in coal samples is conducted through single tractal theroy and multi-fractal theory, and the correlation between different fractal dimension values and pore structure parameters is discussed. The results show that the single-fractal model can better characterize the heterogeneity of different fluid states or pore distribution range, while the multi-fractal model is more suitable for characterizing the heterogeneity of pore fracture distribution. In the single-fractal parameters, the fractal dimension value (D2) of the seepage pore and the total fractal dimension value (DT) show a weak negative correlation; in the multi-fractal parameters, with the increase of the pore volume of the adsorption pore, the spectral width (D-10-D10) increases linearly and has a slight variance. The samples can be divided into two categories, that is, adsorption pore development and seepage pore development. The type of adsorption pore generally develops fine pores,with a "bimodal pore size" distribution, while the type of seepage pore generally develops large pores with a "uni-modal or bimodal pore size" distribution. As the volume of the seepage pore decreases, the heterogeneity of the seepage pores and the overall distribution of pores increases. The adsorption porosity is the main factor affecting the distribution of pore and fracture, and it plays a significant controlling role in the multi-fractal characteristics of pore and fracture structure in coal reservoirs. There are certain differences in physical significance between the single-fractal model and the multi-fractal model, but both provide important theoretical support for the study of the pore fracture structure of coal reservoirs.
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