Applicability Analysis of Fractal Model with Poricidal Fracture in Coal Bed Based on Low-field Nuclear Magnetic Resonance Technology

doi:10.3969/j.issn.1006-6535.2024.05.010

Abstract

Abstract: 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 (D_T) 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.

Key words: coal bed, pore fracture, heterogeneity, fractal model, nuclear magnetic resonance, western Guizhou-eastern Yunnan

CLC Number:

TE122

Ren Haiying, Wen Shupeng, Hou Jianjun, Kong Lingfei, Zhou Zeni, Guo Zhijun. Applicability Analysis of Fractal Model with Poricidal Fracture in Coal Bed Based on Low-field Nuclear Magnetic Resonance Technology[J]. Special Oil & Gas Reservoirs, 2024, 31(5): 85-94.

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