Insights into fractal characteristics of pores in different rank coals by nuclear magnetic resonance (NMR)
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ORIGINAL PAPER
Insights into fractal characteristics of pores in different rank coals by nuclear magnetic resonance (NMR) Yang Chen 1,2,3 & Dameng Liu 1 & Yidong Cai 1,2 & Quan Gan 4 Received: 16 June 2018 / Accepted: 24 September 2018 # Saudi Society for Geosciences 2018
Abstract Low-field nuclear magnetic resonance (NMR), as a nondestructive and time saving method, has been widely used to evaluate the petrophysical properties of coalbed methane (CBM) reservoirs. To better understand the impacts of pore structures on permeability, NMR T2 spectrums of different rank coals and their meaning to porosity and permeability were investigated. Fractal dimensions of coal pores with NMR T2 spectrum and their correlations with porosity, Ro,m, and permeability were synthetically analyzed. The results show that T2 spectrums can be divided into three types named as unimodal, bimodal, and trimodal based on the spectrum peaks. The percentage of spectrum peak P1 increases with increasing Ro, m and the connectivity of different peaks can effectively reflect the permeability. The irreducible fluid and movable fluid porosity have an opposite relationship with increasing percentage of centrifugal fluid and increasing Ro,m. Fractal dimensions of adsorption pores (D1, 0.82–1.94) and seepage pores (D2, 2.50–2.99) demonstrate that coal pores have multiple fractals. Due to the coalification jump, both D1 and D2 show binomial relationship with Ro,m at the inflection section of 1.3–1.5% Ro,m. Generally, movable fluid porosity, irreducible fluid porosity, and total porosity show exponentially decreased or binomial relationship with the increasing D1 and D2. The development of microfractures could be the main cause of abnormal tendency between D2 and total porosity in the high-rank coals. The movable fluid porosity that reflects the proportion of seepage pores decreases with the increasing D2, which have a significant impact on low permeability coals. Most seepage pores that contributed permeability is lower than 0.1 mD, which contributes less than 20% to total permeability. However, seepage pores that contributed permeability may have an important role on CBM production, especially at the middle to late stage. Keywords Coals . Pores . Fractal . Permeability . Nuclear magnetic resonance (NMR)
Introduction Coal, as a complex porous medium, has strong heterogeneity in pore structure with pore size from nanometers to over a
* Yidong Cai [email protected] 1
Coal Reservoir Laboratory of National Engineering Research Center of CBM Development and Utilization, China University of Geosciences, Beijing 100083, China
2
Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, Shandong, China
3
National Geological Library of China, Beijing 100083, China
4
School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK
micrometer (Close 1993). Pores are the space for coalbed methane (CBM) storage and migration, which have complicated stru
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