Abstract
Coal fines produced during drainage of coalbed methane reservoirs can affect the permeability of the coal reservoir and damage production facilities such as downhole pumps, shafts, and valves. Thus, to clarify the mechanism of coal fines output is very important to high production of coalbed methane. The characteristics of coal fines can be used to identify the sources of the coal fines and to develop reasonable means of controlling coal fines output. In this study, different coal fines characteristics were investigated to reveal the output mechanism of coal fines. The coal fines samples were collected from 16 coalbed methane wells, which located in Eastern Ordos Basin of China. And the wells are in different drainage stages. The coal fines samples were analyzed by using transmission light microscopy, reflection polarized optical microscopy, laser particle size analysis, X-ray diffraction, and scanning electron microscopy with energy dispersive X. The results show that the concentration of coal fines is in the range of 3–8% (volume percent). The sizes of the coal fine particles tended to be below 200 µm. The main components of pulverized coal are vitrinite and inorganic minerals and the average content of inorganic minerals account for 50.56% and the standard deviation is 0.0685. The morphology analysis results show that the shape of coal fines is different in different parts of the coalbed methane wells. The coal fines concentration increases with the increase in the thickness of the deformed coal, and decreases with the increase in the burial depth. The concentration of coal fines becomes higher with the increase of casing pressure and coal fines concentration increases with the increase of the variation of bottom hole pressure.
Highlights
The recovery of coalbed methane can be classified into three stages of dewatering: single water flow, gas-water two-phase flow, and single gas flow
According to the quantitative analysis of the micro-components of raw coal (Table 1), the vitrinite content of the main recoverable coal seam in Linfen coalbed methane block is more than 70% and the inorganic mineral content is below 15%
The vitrinite and inorganic minerals are the main components of the coal fines, and the average content of inorganic minerals accounted for 50.56%
Summary
The recovery of coalbed methane can be classified into three stages of dewatering: single water flow, gas-water two-phase flow, and single gas flow. Liquid erosion and pressure fluctuations, coal fines will be inevitably produced in all dewatering stages (Singh et al, 2015a; Zhang et al, 2011, 2015; Zou et al, 2014; Wei et al, 2016). Factors that affect generation of coal fines include: (1) coal properties and the geological conditions; (2) the mechanical grinding during drilling, fracturing stimulation, and completing process; (3) the erosion of liquids flow, pressure fluctuations, and shrinkage of coal during production (Chen et al, 2009; Civan, 2007; Gentzis et al, 2009; Magill et al, 2010; Marcinew and Hinkel, 1990; Yuan and Shapiro, 2011; Zeinijahromi et al, 2011). A lot of research works are focusing on the generation mechanisms, influencing factors and control measures of coal fines from theoretic analysis, laboratory tests and production practices. Palmer et al (2005) summarized different coal failures during drilling, fracturing, and CBM production and concluded that failures in weak coal could cause fines generation. Chen et al (2009) believed that the main factors that affect the output of coal fines are the properties of coal. Cao et al (2012) proposed a system to classify coal fines as original coal fines, which generated from tectonic damage during coal forming period and secondary coal fines, which generated from mechanical destruction and pressure changes during coalbed methane exploitation period. Wei et al (2013) showed that the formation of tectonically deformed coal is the key factor of coal fines generation. Guo et al (2015) conducted a laboratory investigation on fines production and suggested that the main compositions of produced fines are clay minerals. Zhang et al (2016) made gray relational analysis and concluded that the development of tectonic coal is the most critical factor. Guo et al (2015) conducted a laboratory investigation on fines production and suggested that the main compositions of produced fines are clay minerals
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