Abstract
During hydraulic fracturing in a high-methane coal seam, there is a water-displacing-methane effect. A pseudo triaxle experimental system, which is opposite to the name of true triaxial system, for the water-displacing-methane effect was created. First, cylindrical coal samples in a methane adsorption equilibrium state, spontaneously desorbed. And then water was injected into the coal samples. The following was shown: (1) The displacement methane volume gradually rises with an increase of injected water, while the displacement methane rate tends to rise at first before declining later. Simultaneously, the water-displacing-methane process is characterised by a time effect. The methane displacement lags behind water injection. (2) Competitive adsorption and displacement desorption between the water and methane will promote adsorption methane into free methane, while the pore pressure increase caused by water injection will turn free methane into adsorption methane. The net free methane of the combination action provides a methane source for the water-displacing-methane effect. (3) A pore pressure gradient, which provides a power source for the water-displacing-methane effect, is formed and reduces gradually at the front of the water seepage along the seepage direction. The increase in water pressure can rapidly improve the pore pressure gradient and boost the displacement methane volume as well as improve displacement methane efficiency. (4) A starting porosity pressure gradient and limit pore pressure exist in the process of water-displacing-methane. When the pore pressure gradient is less than the starting pore pressure gradient, there is free methane in the coal rock, but it cannot be displaced. When the pore pressure is between the starting pore pressure and the limit pore pressure, the free methane can be displaced. When the pore pressure is greater than the limit pore pressure, the methane is almost completely adsorption methane, and water cannot be used to displace the free methane.
Highlights
Hydraulic fracturing has a role in the exploitation of coalbed methane and has been more widely utilized to improve permeability and eliminate coal and methane outbursts in gassy coal seams (Lu et al, 2015; Ma et al, 2016; Wang et al, 2017; Xu et al, 2017; Zhai et al, 2015; Zhang et al, 2017)
While the parameters of water injection or hydraulic fracturing can be optimised so as to avoid negative effects such as the over-limit of return airway methane concentration induced by the W-D-M effect, the W-D-M effect can be reached the level of methane displacement by hydraulic fracturing
The self-developed pseudo triaxle experimental for methane displacement by water is characterised by good airtightness, and it can be utilized to investigate the laws of methane desorption under different stress load, different water injection volume and different water injection pressure
Summary
Hydraulic fracturing has a role in the exploitation of coalbed methane and has been more widely utilized to improve permeability and eliminate coal and methane outbursts in gassy coal seams (Lu et al, 2015; Ma et al, 2016; Wang et al, 2017; Xu et al, 2017; Zhai et al, 2015; Zhang et al, 2017). When high pressure water is injected into a coal rock mass, an increase in the pore pressure will turn free methane into adsorption methane, resulting in consumption of the free methane (Lu et al, 2020). This shows that the free methane of the coal sample increased on the whole, that is to say, the content of the free methane generated by displacement desorption was greater than the free methane consumed by the pore pressure increment.
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