Influences Of Coal Reservoir Structure On Adsorption/Desorption Of CH₄ And CO₂ Associated With CO₂-ECBM

The adsorption/desorption of CH₄ and CO₂ is dependent on the pore-fracture structure and chemical structure of coal reservoir.It is also crucial to the CO₂ sequestration in deep coal seams with enhanced coalbed methane?CH₄?recovery?CO₂-ECBM?.CO₂ is supercritical fluid under the optimum coal reservoir conditions for CO₂ sequestration.It is acknowledged that besides physisorption,the complex fluid-solid interactions do exist between supercritical CO₂?SCCO₂?and coal matrix.The aforementioned complex interactions can alter pore-fracture structure and chemical structure of coals,and further influence both CO₂ sequestration capacity and CH₄ recovery efficiency.Considering the difference existed in coal reservoir property,this work investigated the influences of coal reservoir structure mainly including pore-fracture structure and chemical structure on adsorption/desorption of CH₄and CO₂ during CO₂-ECBM project from three aspects including coal-body structure,maceral composition and coal rank by using micro-CT,low-field nuclear magnetic resonance,mercury porosimetry,probe molecule?N₂/CO₂?adsorption,small angle X-ray scattering,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,Solid-state ¹³C nuclear magnetic resonance spectroscopy,and high-pressure CH₄ and CO₂adsorption/desorption test.The main results and conclusions drawn from this work are summarized as follows:1.The pore-fracture structure of tectonically deformed coal?TDC?reservoirs with various maceral compositions and its impact on CH₄ and CO₂adsorption/desorption performance are analyzed.The results show that tectonic deformation and maceral composition show great influence on the variability and heterogeneity in the inner distribution of seepage-porosity and adsorption-porosity,respectively.The differences in microstructure,the development of pore-fracture and pore connectivity due to tectonic deformation are the leading factors which determine the variations in adsorption/desorption performance of bulk TDC samples.However,maceral composition and surface chemistry are the main controls on gas adsorption performance for power samples.For the test TDC,The maximum adsorption capacity of CO₂ is1.22-1.28 times as much as that of CH₄.The preferential adsorption ratio of CO₂to CH₄ for the test coals is in the range of 1.38-2.03.Thus,it has great potential for CO₂-ECBM process application in the deep TDC reservoirs.2.In this work,three lean coals with the iso-rank but different maceral composition,one predominantly vitrinite and the rest predominantly inertinite,were investigated in laboratory to evaluate changes in pore-fracture structure and chemical structure and high-pressure gas adsorption/desorption isotherms after exposure to SCCO₂.All the coal samples were exposed to dynamic SCCO₂at 45 ^oC and 12 MPa.The proximate analysis shows that there is no evident change in the inorganic component of coals after exposure to SCCO₂.The comparison of solid-state ¹³C nuclear magnetic resonance spectroscopy(¹³C-NMR)curves pre-and post-SCCO₂ exposure shows that extraction yields of hydrocarbon in vitrinite-rich coal are higher than that in inertinite-rich coals.For inertinite-rich coals after SCCO₂ exposure,the surface functional groups decline and the microporosity rapidly decreases,while the meso-,macroporosity and opened porosity obviously increase.However,the reverse trend is found for vitrinite-rich coal after SCCO₂ exposure.The aforementioned results indicate that the coal's structure change due to SCCO₂ exposure is irreversible.SCCO₂exposure leads to an increase in adsorption capacities and the degree of sorption hysteresis for both CH₄ and CO₂ of vitrinite-rich coal.The maximum CH₄adsorption capacity grows but the hysteresis degree of CH₄ sorption is weaken,whereas both the maximum CO₂ adsorption capacity and CO₂ adsorption hysteresis decrease for inertinite-rich coals after SCCO₂ exposure.The change in coal reservoir structure is responsible to the different mechanism of the adsorption/desorption process in relation to vitrinite-rich coal and inertinite-rich coal.3.To evaluate the influence of surface chemistry and pore-fracture morphology due to DH₂O-SCCO₂ exposure on high-pressure CH₄ and CO₂adsorption/desorption behaviors of coals,the interaction of deionized water-SCCO₂ mixture fluid?DH₂O-SCCO₂?with three rank coals,i.e.,weak coaking coal?WCC?,gas coal?GC?and anthracite was simulated on a dynamic supercritical fluid extraction system under conditions of 45 ^oC and 12 MPa.The results confirm that the geochemical interaction occurs between the mineral matters and DH₂O-CO₂ as demonstrated by the change in the content of clays,carbonates and sulfates in coal matrix.DH₂O-CO₂ exposure causes a decrease in the content of volatile matter,organic sulfur groups,oxygen-containing functional groups,but an increase in C-C/C-H species.The aforementioned aspects illustrate the reconfiguration of surface geometry.DH₂O-CO₂ interaction degrades the accessibility of micropores of all the coals.DH₂O-CO₂ interaction facilitates the development of macropores and fractures,and thus improves the permeability of coal seams,which can be attributed to the dissolution and mobilization of mineral matters by the acid water and the shrinkage of coal induced by the water loss.The variation of mesopores due to DH₂O-CO₂exposure is strongly related to coal rank and surface chemistry.Specifically,the decreasing mesoporosity is found for WCC,whereas this trend is opposite for both GC and anthracite.The relationship between mesopores and macropores-fractures of WCC decreases,while the opposite trend is recorded in both GC and anthracite after fluid exposure.The maximum CH₄ adsorption capacities of various rank coals after fluid exposure decrease by 0.80%-6.20%.The maximum CO₂ adsorption capacities of WCC and anthracite decrease by5.35%and 0.77%,respectively,while an increasing trend by 4.59%exists in GC.The diffusion-adsorption rate of WCC and anthracite decreases,but that of GC after fluid exposure increases.DH₂O-CO₂ exposure causes an increase in both CH₄ and CO₂ sorption hysteresis.However,decreasing CH₄ sorption hysteresis and increasing CO₂ sorption hysteresis can be found in GC and anthracite.The results obtained from this work further demonstrate that the comprehensive result of the accessibility of porosity and the surface chemistry in various rank coals due to DH₂O-CO₂ exposure contribute to the different variation in both CH₄ and CO₂ adsorption/desorption behavior.