Study On Characteristics And Mechanism Of Tectonically-deformed Coal Reservoirs Response With Stress Release

Tectonically-deformed coals(TDC)are especially prevalent in China.Conventional stimulation methods,such as fracturing,drainage and decompression,are not suitable for coalbed methane(CBM)extraction in TDC reservoir.Many practices show that the rapid increased permeability of TDC reservoir after stress release has been successfully applied to CBM development.However,the theory and technology of in-situ stress release development of CBM development in tectonic coal needs to be explored urgently,and the response characteristics of TDC reservoir and the fluid changes after stress release still need to be deeply studied.The typical TDC samples from Huainan coal mine were selected in this study.Based on the fully understanding of the control effect of structural conditions on coal physical properties,and the experimental research,numerical simulation methods and theoretical analysis were comprehensively used to systematically analyze the response characteristics and control mechanism of volumetric strain,pores-fissures,and permeability of coal samples after the stress released.And then establish a constitutive model about the influence of stress release on coal structure,porosity and permeability changes and gas migration.Finally,the numerical simulation method is used to solve and verify this model.The research conclusions were draw as follows:(1)Tectonic stress causes the TDC reservoirs to undergo different degrees of deformation and destruction,resulting in difference evolution in coal structure and mechanical properties.The brittle deformation of coal increases the volume of mesopores and macropores,which is conducive to rapid gas migration;the ductile deformation contributes to the formation of micropores in coal and enhances the adsorption capacity of coal,but the pore connectivity of mylonite coal is relatively poor.Compared with the PUC,the strain and permeability of TDC are more sensitive to stress changes.The strong compression of pores-fissures in broken-soft coal by in-situ stress is bound to change the gas migration behaviors.(2)The stress changes affect the pore-fissure structure of coal and hence changes the gas adsorption/desorption behaviors in coal.The increase in effective stress reduces the coal reservoir connectivity,decreasing its adsorption capacity within a set time.The adsorption-induced swelling of coal matrix grows inward to narrow fissures under the limitation of the high confining pressure condition,leads to the densification and low permeability of TDC with strong adsorption capacity.The stress release affects the CBM extraction by improving the pore-fissure connectivity and desorption rate.When the effective stress decreases,due to the rapid elastic recovery of coal particles and the expansion of fissures,the initial desorption gas volume of the TDC increases significantly,but the whole desorption volume increase does not change obvious.The reduction in effective stress can continue to increase the desorption rate of PUC.(3)The difference pore structures and mechanical properties of PUC and TDC samples affect their permeability response to stress changes.Under the condition of stress change,the permeability of primary structure coal is controlled by the response of pore-fracture structures in coal matrix,while the permeability of structural coal mainly depends on the changes of intergranular pores.In this study,the fracture volume strain is used to uniformly analyze the permeability changes of different coal samples with stress,and hence establish a corresponding empirical model to quantify the control of stress on the permeability.(4)Based on the transformation mechanism of in-situ stress release on coal deformation,desorption-migration state,porosity and permeability,a constitutive model of coal reservoir structure response under stress release conditions was established.According to the results of physical simulation experiments,the empirical parameters describing the desorption response and permeability response during stress release of different coal structure samples were determined,and numerical simulation methods were used to calculate the permeability change trend of CBM extraction under the stress release,which provides a reference for subsequent engineering practice.This dissertation contains 62 figures,26 tables,and 313 references.