Abstract
Coalbed methane (CBM) resources cannot be efficiently explored and exploited without a robust understanding of the permeability of fracture-size heterogeneities in coal. In this study, two sister coal samples were imparted with pre-developed cleat and connected fractures, and the permeability of the coal samples was measured under different conditions of controlled confining and gas pressures. Furthermore, the implications of the results for CBM exploration and exploitation were discussed. The permeability of coal with cleat development ranged from 0.001–0.01 mD, indicating ultra-low permeability coal. The gas migration in this coal changed from a linear flow to a non-linear flow, with the increase in gas pressure (>1 MPa). Thus, the permeability of the coal initially increased and then decreased. However, the Klinkenberg effect does not exist in this ultralow-permeability coal. For the coal sample with connected fracture, permeability ranged from 0.1–10 mD, which is larger by hundred orders of magnitude than that of the sample with cleat. For this coal, with a decrease in gas pressure (<1 MPa), the Klinkenberg effect significantly increased the permeability of the coal. With an increase in the applied confining pressure, both the Klinkenberg coefficient and permeability of the coal presented a decreasing trend. It is suggested that field fracture investigation is a prerequisite and indispensable step for successful CBM production. The coal beds that cleat network is well conductive to the connected fracture can be an improved target area for CBM production. During CBM production, a variety of flow regimes are available owing to the decrease in CBM reservoir pressure. In particular, under the low CBM reservoir pressure and low in situ geo-stress conditions, the gas migration in the CBM reservoir with connected facture development exhibits remarkable free-molecular flow. Thus, the reservoir permeability and predicted CBM production will be enhanced.
Highlights
Coalbed methane (CBM) is a rapidly growing source of clean energy worldwide
Where, k0 is the initial permeability of the coal under the condition of σ = σ0, p = p0, Cf is the fracture compression coefficient of the coal (MPa−1); σ is the applied confining pressure to the coal (MPa), and p is the gas pressure used in the experiment (MPa)
Where k0−p is the initial permeability of the coal for each constant gas pressure, Cf is the fracture compression coefficient to the variable confining pressure, expressed by C fp−σ; and 3 Cf represents the sensitivity of permeability to the variable confining pressure, expressed by C f σ
Summary
Coalbed methane (CBM) is a rapidly growing source of clean energy worldwide. In addition to the USA, Canada, and Russia, the Asia-Pacific countries are presently the fastest-growing CBM markets, with a compound average growth of 14.9% between 2014 and 2020 (Bandyopadhayay et al, 2020). The efficient exploration and exploitation of this huge resource necessitate accurate reserve estimation and production forecasting This cannot be achieved without a robust understanding of the permeability of coal (Kumar et al, 2018). Gensterblum et al (2014) studied the relationship between coal deformation and permeability under the action of cyclic loading and inferred that the volumetric strain of the coal gradually increases with the increase in temperature under applied loading, with a corresponding decrease in its permeability. Wang et al (2017) discussed the relationship between coal permeability and porosity under high-temperature and high-pressure conditions and concluded that an exponential relationship exists between the permeability and porosity of coal. Research on the permeability of coal primarily has focused on the influences of stress, temperature, pore pressure, coal lithotype, bedding, and adsorption effect
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