Rules Of Methane Driven By Hydraulic Fracturing In The Gassy Coal Seam

In the process of hydraulic fracturing in gassy coal seams,there is a phenomenon,which is termed the methane-driven effect of hydraulic fracturing.There are advantages and disadvantages of the effect of methane driven by hydraulic fracturing.In the current,knowledge on the rules of the methane-driven effect by hydraulic fracturing in gassy coal seams is still in a blank state.In this paper,the effect of the methane driven by hydraulic fracturing is considered as the research object.With the methods of physical experiment,theorical analysis,numerical simulation and field test used,the rules of the methane-driven effect by hydraulic fracturing is studied systematically,which has important theoretical significance and extensive prospect of engineering application.With the self-developed pseudo three-axial experimental system of methane displacement by hydraulic fracturing utilized,mechanism of the methane driven by hydraulic fracturing is studied.It is shown by the experimental results that:(1)Competitive adsorption of water and methane can promote adsorption methane into free methane.However,after pressure water enters into coal body,increase of pore(methane)pressure can change the free methane into adsorption methane.Therefore,the net free methane of the both actions is one of methane sources for the water displacing methane effect.(2)The displacement methane volume gradually increases with the increase of the injected water volume,while the displacement methane rate has the trend of increasing first and decreasing last.At the same time,water-displacing-methane process is characterized with time effect.The process of displacement methane lags behind water injection process.(3)Starting pore pressure gradient and ultimate pore pressure exist in the process of methane displacement by water.When the pore pressure gradient is less than the starting pore pressure gradient,there are free methane in the coal rock,but it cannot be displaced.When the pore pressure gradient is greater than the starting pore pressure gradient and the pore pressure is lower than the limit pore pressure,the free methane can be displaced.When the pore pressure is greater than the limit pore pressure,the methane in the coal rock is almost completely adsorption methane,and the water cannot be used to displace the free methane.With the help of COMSOL Multiphysics software,numerical simulation experiment is conducted to study the change rules of the two-phase interface shape,position and water driving efficiency over time in the meso pore channels of the coal rock during the process of water-methane two-phase displacement.It is shown by the simulation results that:(1)Under the combined effects of the boundary layer effect,interfacial tension and pore diameter changes in water-methane displacement,the shape of the water-methane interface changes from a tongue-shape to a U-shape,during which a series of interface shapes,including a piston shape,finger shape,W-shape and ?-shape,are observed.In the pore channel,the area of the water phase below the pore channel axial is larger than that above the pore channel axial.(2)The pore-throat ratio has a considerable impact on the shape and location of the water-methane interface as well as the displacement efficiency.When the water-methane displacement is from a small pore-channel to a large one,the displacement efficiency increases as the porethroat ratio decreases.On the contrary,if the water-methane displacement is from a large pore channel to a small one,the displacement efficiency increases as the porethroat ratio increases.Numerical simulation can be used to study the shape and location of the water-methane interface as well as the displacement efficiency in pore channels with different scales of diameters.The fluid-solid coupling mathematical model of methane driven by water in gassy porous coal is established and verified.It is shown by the numerical simulation results that:(1)water saturation presents a trend of decrease along the driving direction by water,and water saturation in the porous coal will increase over time.Rules of methane saturation are contrary to that of water saturation.The change(increase or decrease)rate of water and methane saturation both decreases.(2)pore pressure gradually decreases along the water-driving direction.In the coal body,there is an area of increasing pore pressure and decreasing pore pressure.The area of increasing pore pressure is decreasing and the area of decreasing pore pressure is increasing.(3)the pore pressure gradient decreases first and then increase along the water-driving direction.The pore pressure gradient reduction zone is located in the pore pressure increase zone,and the pore pressure gradient increase zone is located in the pore pressure reduction zone.With the experimental system of the methane driven by hydraulic fracture and high-resolution 3D X-Ray microscopic imaging system used,the experiment of methane driven by hydraulic fracture is conducted.It is concluded that(1)During the process of methane driven by hydraulic fracturing,the hydraulic fractures will become into dominant paths,which provide methane transport channels for the effect of methane drive by hydraulic fracture.The wider hydraulic fractures are,the easier they will become into dominant path.The more the hydraulic fractures are,the more dominant paths there are in the coal body.However,the dominant paths will gradually weaken or disappear over time.(2)Two approaches are involved during the process of the methane driven by hydraulic fracture.Firstly,hydraulic fractures,which are generated under the coupling action of pressure water and stress field,can provide transport channels for the methane.Secondly,while pressure water flows in the seepage channel,it will permeate along the hydraulic fracture into its two sides of the coal.At the front of the seepage water,pore pressure gradient from high to low is formed along the migration direction of the pore water,and the pore pressure gradient provides power for the effect of methane driven by hydraulic fracturing.Based on the dual-porosity and dual-permeability methane seepage theory in the double-porosity coal seam,the mathematical model of the methane driven by hydraulic fracture is established,and the numerical simulation is conducted.It is concluded that:(1)Along the driving direction of hydraulic fracturing,the pore pressure in the cleats presents a sudden change of first decrase and then increase,and pore pressure gradient presents some certain features of regional change.Along the driving direction of hydraulic fracturing,pore pressure gradient near the fracturing borehole presents a trend of decrease,but pore pressure gradient at the boundary of the coal seam presents a trend of increase.In addition,pore pressure gradient at the space point of cleats is also lower than pore pressure gradient in the matrix blocks on its either side.Pore pressure gradient at the same space point near the fracturing borehole and the coal seam boundary decreases with the extension of time.(2)Along the driving direction of hydraulic fracturing,water saturation shows a trend of decrease on the whole.However,a trend of first increasing and then decreasing is shown near the cleat or hydraulic fracture.Water saturation in the central region of the coal matrix block is smaller than that of its surrounding region.However,water saturation of the whole coal matrix block is larger than water saturation of the cleats and fractures surrounding the coal matrix block.Water saturation at the spatial point of the cleat or hydraulic fracture will gradually increase with the extension of time.Space-time distribution rules of methane saturation are contrary to that of water saturation.(3)Free methane driven by hydraulic fracturing includes original free methane and the free methane desorbed from the desorbed methane.The reduction rate of adsorption methane content is larger than the reduction rate of free methane content.Field test of methane driven by hydraulic fracturing is conducted in the gassy coal seam.It can be concluded that the methane emission rate during the process of hydraulic fracturing is larger than the methane emission rate before and after hydraulic fracturing.In a certain range of the fracturing borehole,methane content reduction zone,methane content increase zone and original methane content zone are generated after hydraulic fracturing.