In situ study of intensity of weathering-induced fractures and methane emission to the atmosphere through these fractures
Highlights
► The intensity of weathered-induced fractures was studied in situ. ► The original device for measurement of fracture parameters was proposed. ► The relations between methane seepage and fracture parameters are obtained.
Introduction
Methane seepage from rock mass is a problem in many parts of the world. In order to extract methane from coal and remove it from the mine, different gas drainage systems are widely used in engineering practice (Palchik, 2002a, Booth, 2007, Jenkins and Frazier, 2010, Karacan and Luxbacher, 2010, Sang et al., 2010). A number of researchers such as Karmis et al., 1983, Peng, 1992, Chekan and Listak, 1993, Kendorski, 1993, Palchik, 2003, Karacan, 2008 have defined three zones of overburden movement during longwall coal excavation. In caved (zone 1) and fracture (zone 2) zones, there are mining-induced fractures, and the thickness of the fracture zone can achieve 100-fold coal seam thickness (Palchik, 2005, Palchik, 2010). Zone 3 above the fractured zone is a continuous deformation zone without any mining-induced fractures. The upper boundary of the continuous deformation zone is the ground surface.
Although zone 3 does not contain mining-induced fractures, weathering-induced fractures can be formed in the subsurface part of this zone. The ground surface is the zone where the processes of weathering are very active. In the presence of gas-bearing rock layers (gas sources) located at a shallow depth below the ground surface, weathering-induced fractures can conduct gas from gas sources into the atmosphere. In the southern part of Donetsk city (Ukraine), weathered sandy shale layer (7.6 m thick) overlies the coal seam pillar (at the outcropping of coal seam) located at a depth of 1.5 m below ground surface (Figure 1a). The coal seam was extracted more than hundred years ago, and there are no reliable plans of the mining excavations. Methane seepage from the coal pillar into the atmosphere occurs. As a result, there are cases of methane poisoning of humans. Fire and explosion can also occur when methane concentration on the ground surface reaches a potentially dangerous value (Burrel and Friel, 1996, Sizer et al., 1996, Kral et al., 1998). Methane is a greenhouse gas that remains in the atmosphere about 100 years and is 20 times more effective in trapping heat in the atmosphere than carbon dioxide (Wuebbles and Hayhoe, 2002, CRS report, 2009).
The goal of this paper is to study the gas volumetric flow rate to the atmosphere, dimensions and number of gas-conducting fractures in weathered sandy shale surface layer and to define how fracture characteristics and gasodymanical parameters (pressure and viscosity of methane) influence methane seepage. The ratios between gas emission through weathering-induced fractures and rock matrix are revealed.
Section snippets
Theoretical background
It is well known that the flow of a fluid through a porous medium can be described by Darcy's law. The phenomologically derived constitutive equation formulated by Darcy (1856) is based on the results of experiments on the flow of water through beds of sand. The volumetric flow (q) of gas in porous media can be described by Darcy's equation as:where K is the effective permeability, ∂P/∂L is pressure gradient, A is the cross-sectional area of the rock formation.
Darcy's law is a simple
Description of studied surface rock layer
In this geological setting, the depth of the bottom border of the weathering zone is between 55 and 85 m. (Palchik, 2002b). The studied sandy shale consists of weakly-cemented quartz grains and mica in the form of thin horizontal scales. This rock is very fractured due to near-surface weathering and small amount of cement (calcareous cementation) at quartz grains contacts (Palchik, 2002b). It is well known (e.g. Palchik, 1999, Gupta and Rao, 2001, Borrelli et al., 2007, Gokceoglu et al., 2009,
Stage 1
The area of ground surface (equal to cross-section area (Ab) of future vertical borehole) is cleared from filled-up grounds in order to expose the weathered sandy shale. The value of Ab is 0.00322 m2 for a borehole with the diameter (d) of 64 mm. Gas emission from subsurface coal seam (pillar) to ground surface occurs through fractures in overlying sandy shale. These fractures form a system of through gas-conducting channels, whose number (k) is between 2 and 4 (see Section 5). The outlets of
Test results
Gypsum casts of all fractures formed in the sides of five boreholes were made. The values of qf, qtot, gas pressure difference (ΔP), minimum and maximum values of width (s) and length (l) of fractures at different intervals of depth (H), average horizontal distance between through channels (δ) and numbers of fractures (n) and through channels (k) observed in each of the five boreholes are presented in Table 1. Here, the number of fractures (n) observed in borehole surface in each borehole is
Semi-empirical models for qf, si, Sf and Jf
In each of the five boreholes, the measured values of fracture characteristics and gas pressure difference were linked with measured values of gas volumetric flow rates. As a result, it was established that the number of through channels, width and number of fractures defined in a borehole, horizontal distance between through channels in weathered rock surface layer and gasodymanical characteristics (pressure difference and methane viscosity) are input parameters for calculating gas flow rate
Conclusions
In situ study of fracture characteristics of weathered sandy shale surface layer and methane seepage to the atmosphere was performed. The main conclusions are the following:
- 1)
The sandy shale surface layer in the depth interval of 0–1.5 m is very weathered, and weathering-induced fractures form a system of through interconnected channels which conduct methane into the atmosphere.
- 2)
The value of gas emission (qf) from through channels is between 1.44 × 10− 3 and 4.62 × 10− 3 m3/s, whereas gas emission (qm)
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