Self-sealing of coals and CBM accumulation effect constrained by in situ stress

Research on the superimposed coalbed methane system usually pays attention to the sequence stratigraphic framework of the coal-measure stratigraphy.There is insufficient attention to the “elastic self-sealing effect” of the reservoir under the restriction of in-situ in situ stress.The non-monotonic changes in coal reservoir permeability and its control on reservoir pressure,gas content are often overlooked.Here,the vertical variation in in-situ stress and its structural control factors in western Guizhou were systematically analyzed,and the non-exponential variation in permeability with increasing depth in several coal-bearing basins were revealed.Finally,the matching relationship between reservoir pressure,pressure coefficient and permeability in the vertical were further discussed.In western Guizhou,the horizontal stress gradient changes with depth is non-uniform and is overprinted by the effect of syncline,and the stress gradient is remarkably high nearing the axis.These observations within 200-500,500-750,750-1 000 and >1 000 m depths are similar with the variations in horizontal stress difference and lateral pressure coefficient,corresponding to stress extrusion (horizontal stresses dominated),stress release (vertical stress dominated),stress transition (stress concentration in syncline axis begins to appear),and stress concentration zones (the low point of the syncline axis,high stress area),respectively.The stress release zone is favorable for a relatively high permeability reservoir (mean 0.2×10-15 m2).Below (750-1 000 m，mean 0.04×10-15 m2;>1 000 m，mean 0.003×10-15 m2) and above(200-500 m，mean 0.06×10-15 m2)this zone,the reservoirs have a very low permeability.The non-monotonic change law in permeability with depth is universal,the relatively high permeability zones at the middle depths can be found in the Panguan-Tucheng and Bide-Santang synclines,as well as in the southern Qinshui Basin (650-800 m) and the eastern margin of the Ordos Basin (800-950 m).Under in-situ conditions,the “self-sealing” effect of low-permeability reservoirs (<0.1×10-15 m2) makes it possible to form relatively independent fluid units or accumulation units without relying on caprocks and other sealing conditions.For seams at 200-750 m and >750 m in depth,penetration is restricted by reduced permeability,resulting in discontinuous gas-bearing systems with irregular gas content distributions and unpredictable reservoir pressure gradient.The higher permeability reservoirs at 500-750 m depths need tight sealing cap rock to block the fluid connection between layers,and thereby is favorable to form a unified gas-bearing system,where the gas content generally increase with depth to a“peak gas”horizon under hydrostatic pressure.The relatively tight part of the reservoir may also block the high porosity and permeability part,causing multiple section of fluid pressure system in the same gas reservoir.