Research Progress on the Microfracture of Shale: Experimental Methods, Microfracture Propagation, Simulations, and Perspectives

Hydraulic fracturing not only enhances the flow conductivity of coal seams but also transforms the CH 4 adsorbed in coal seams into free CH 4 and then outputs it via the desorption−diffusion process, thus improving the output quantity and efficiency of CH 4 . In this paper, taking the coal from Changping mine in Shanxi as the research object, the 3D macromolecule model of coal was established by using analytical test experiments, and the molecular dynamics simulation method of desorption and diffusion was applied to study the characteristics of methane desorption rate and diffusion behaviors under different driving water pressures through the parameters of gas−water concentration distribution, desorption and diffusion rates, and the interaction energies between the coal and the gas−liquid. The results show that the water drive can increase the desorption rate of methane. With the increase of methane adsorption pressure, the desorption effect of water drive becomes stronger and then weaker. The desorption effect of water drive was best when the methane adsorption pressure was 5 MPa and the pressure difference was 2 MPa. Generally, the water drive promoted the methane diffusion, but if the water drive time was too long, it formed a water plug, and the water drive methane inhibited the diffusion. In shallow reservoirs, water drive can obviously promote the desorption−diffusion of methane. The binding energy between CH 4 molecules is mainly composed of the repulsive force, while that between water molecules is mainly the adsorption force. In the binding energy between CH 4 molecules and coal structures, the van der Waals (VDW) force accounts for 99% of the effect; in the binding energy between water molecules and coal structures, the electrostatic force contributes 84% to ∼88% thereto. The VDW force is significantly influenced by the pressure, while the electrostatic force is slightly affected. As the water displacement differential pressure is increased, the binding energy between CH 4 molecules and coal structures decreases.

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Research on fracture propagation process of coal seam hydraulic fracturing based on simulation analysis and microseismic monitoring

Jing,

Tao,

Liu

et al. 2024

Preprint

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Coal structures significantly influence the efficiency of hydraulic fracturing, which is vital for enhancing coalbed methane production. Experiments, models, and microseismic monitoring show how coal structures affect fracture propagation: (1) Different structural combinations affect the initiation pressure and time, with variations in the way cracks expand depending on the structure. (2) An increase in the thickness of clastic coal leads to fracture extension towards it, while the native coal structure facilitates fracture expansion. Cracks propagate along the largest horizontal principal stress direction due to the increased bonding and flexibility of clump coal when exposed to fracturing fluid. (3) In the "two hards sandwiching one soft" structural combination, where hard coal is thick and soft coal is thin, cracks propagate through the soft coal. The hard coal provides effective crack generation, yet the presence of soft coal notably increases fracturing fluid loss.

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Research Progress on the Microfracture of Shale: Experimental Methods, Microfracture Propagation, Simulations, and Perspectives

Cited by 4 publications

References 111 publications

Grain-based DEM modelling of mechanical and coupled hydro-mechanical behaviour of crystalline rocks

Grain-based DEM modelling of mechanical and coupled hydro-mechanical behaviour of crystalline rocks

Methane Desorption–Diffusion Behaviors in Micropores of Coal under Different Water Displacement Pressures

Research on fracture propagation process of coal seam hydraulic fracturing based on simulation analysis and microseismic monitoring

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