Investigating the Fractal Characteristics of Pore-Fractures in Bituminous Coals and Anthracites through Fluid Flow Behavior

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• 作者：Zhongqiu Ouyang ; Dameng Liu ; Yidong Cai ; Yanbin Yao
• 刊名：Energy & Fuels
• 出版年：2016
• 出版时间：December 15, 2016
• 年：2016
• 卷：30
• 期：12
• 页码：10348-10357
• 全文大小：539K
• ISSN：1520-5029

文摘

The characteristics of pore-fractures are the key petrophysical properties used to assess and evaluate coalbed methane (CBM) reservoirs and include pore types, structure types, porosity/percentage, and pore-fracture space properties. To study the storage and seepage capability of a CBM reservoir, based on 18 coal samples with the maximum vitrinite reflectance (R_o,max) in the range of 1.06–3.04%, we used the nuclear magnetic resonance (NMR) method and fractal analysis to investigate the effect of coalification on the characteristics of pore-fractures. First, the pore-fracture space in a coal reservoir includes irreducible fluid space and moveable fluid space. We built up the moveable fluid NMR fractal based on the saturated/irreducible fluid NMR fractals. Saturated fluid fractal (D_w), irreducible fluid fractal (D_ir), and moveable fluid fractal (D_M) have the following relationship: D_M > D_ir > D_W. Additionally, D_ir has a huge fluctuating value area with the maximum vitrinite reflectance (R_o,max) varying between 1.93% and 2.06%. The curve lg(T₂) and lg(V) for a moveable fluid fractal has four typical types. The shapes “C”, “Z”, “S”, and “T” correspond to four structure types (A, B, C, and D), which provide significant guidance for CBM exploitation. Moreover, the saturated-irreducible T₂ distribution has a special shape, known as the pendular ring, which affects the seepage ability and permeability. Second, we classify the pore-fracture types based on the saturated fluid NMR fractal and the T₂ distribution as follows: adsorbed pore (T₂ < 0.4 ms, < 10 nm), transition pore (T₂ = 0.4–2.5 ms, 10–100 nm), seepage pore (T₂ > 2.5 ms, > 100 nm). At R_o,max = 1.06–3.04%, the percentages of pore-fracture types show that coalification have three coalification jumps at R_o,max = 1.3%, 2.0%, and 2.8%. The porosities of pore-fracture types show that the thermal volatilization of fillers in pore-fractures begins at R_o,max = 1.5% and disappears at R_o,max = 2.5%. Different fluid space fractals demonstrate that when R_o,max < 2.0%, the coal reservoir tends to be compressed and consolidated, whereas when R_o,max > 2.0% the coal reservoir tends to be orderly and uniform. BVM/BVI is the ratio of the irreducible fluid space (BVI) and the movable fluid space (BVM). Finally, the permeability indicates a positive exponential function with BVM/BVI for seepage pores, and it exhibits a negative exponential function with BVM/BVI for adsorbed pores and transition pores. The NMR method combining fractal analysis with coalification enables us to characterize pore-fractures and the effects of coalification on pore-fractures quantitatively, which contributes to guiding CBM exploitation. Moreover, the relationship between permeability and BVM/BVI is beneficial for predicting the favorable areas for CBM exploitation.