Speaker
Description
Liquid nitrogen (LN₂) cryogenic fracturing has emerged as a promising technique for waterless stimulation in coalbed methane (CBM) reservoirs, primarily by increasing the pore space through freeze-thaw induced effects, thus improving methane recovery from deep coal seams. However, the underlying mechanisms of in-situ gas and water-bearing coal reservoir modification, particularly the dynamic behavior of adsorbed methane and its geomechanical effects during freeze-thaw cycles, remain inadequately understood. This study investigates the in-situ freeze-thaw behavior of gas-bearing coal under pressurized and sealed conditions. Methane-saturated, methane-water co-saturated, and a helium control group of coal samples were subjected to liquid nitrogen-induced freeze-thaw cycles. Multi-scale structural characterization was conducted using Nuclear Magnetic Resonance (NMR) and Micro-Computed Tomography (μ-CT) imaging techniques. The results revealed that: (1) Following the freeze-thaw treatment, NMR spectra exhibited substantial changes, with notable increases in the relaxation times of both small and large pores, and the greatest increase in movable fluid porosity was observed in the methane-water co-saturated samples; (2) The combined analysis of NMR T2 relaxation spectra and μ-CT imaging demonstrated that in the methane-saturated group, damage primarily resulted from the "forced desorption" of adsorbed methane, which triggered matrix contraction and microcracking. In contrast, the methane-water co-saturated samples exhibited both microcracking and macrofractures, with the latter induced by the ice-wedge effect; (3) During the rapid cooling process induced by LN₂, thermal stresses were generated, which, in conjunction with matrix contraction due to methane desorption and the volumetric expansion of water from phase transition, created a synergistic coupling effect. This interaction intensified the damage in fluid-bearing coal, significantly increasing the permeability of the reservoir and weakening the coal's mechanical strength.
| Country | CHINA |
|---|---|
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