NMR Imaging And Changes In Pore Structure Of Coal Reservoirs Of CO₂-ECBM Process Simulation

CO₂'s deep coal seam storage technology?CO₂-ECBM?can not only reduce the atmospheric emission of carbon dioxide,mitigate the greenhouse effect,but also improve the recovery of coalbed methane.The NMR technology was introduced into the CO₂-ECBM process simulation study.The real-time imaging of the CO₂-ECBM process simulation and the non-destructive measurement of coal pore structure were completed.The main conclusions are as follows:?1?The transverse relaxation spectra of different coal rank coal samples have different shapes.The Binchang long flame coal is a continuous double peak.In coal samples,micropores,transitional pores,and mesopores are developed at the same time,and the connectivity is good.Datun gas coal and Huaibei fat coal are discontinuous double peaks,with micropores and mesopores mainly developed,and the connectivity between pores is poor.The Yangquan lean coal and Sihe anthracite are discontinuous three peaks.The Sihe coal sample mainly develops transitional holes and develops a small number of micropores and macroporous fissures.The Yangquan coal samples mainly develop micropores,while a small amount of mesopores and macroporosity are developed.Coal samples have poor pore connectivity.?2?It is more accurate to use the peak point method to test the nuclear magnetic porosity of coal samples than the area method.The saturation time has a great influence on the measurement of the nuclear magnetic porosity of coal samples.Under saturated water pressure of 20 MPa,the transition pores,macropores and fractures in coal samples can basically complete the saturation within 24 h,and the micro pores It is at least 72h to fully reach a saturated state.?3?ScCO₂-coal-H₂O reaction increases the porosity of coal samples and improves the connectivity.The transformation effect on micro-holes and transitional holes is more obvious,and the transformation effect on medium and large pores and fissures is relatively weak.The role of ScCO₂-coal-H₂O is beneficial to the storage of CO₂ and the production of CBM.At the same time,coal seams with high mineral content should be avoided as coal seams to avoid subsidence.?4?The pores in coal samples are not uniformly developed,and the storage and distribution of methane in coal are also uneven.The micropores in coal samples are the main sites for the occurrence of methane.Under normal circumstances,carbon dioxide can effectively displace the methane gas in the coal sample.Increasing the displacement pressure difference can increase the displacement rate and the displacement effect.?5?There is an advantage path for gas migration in coal samples.Supercritical carbon dioxide has the same liquid fluid properties as water.During the displacement of methane,supercritical carbon dioxide tends to flow out from the dominant path such as cracks in coal samples.The displacement has no effect.?6?In the displacement of methane by carbon dioxide,on the one hand,the pressure of carbon dioxide at the inlet of the coal sample is greater,and a higher pressure gradient and concentration gradient can be formed,which facilitates the entry of carbon dioxide into the pores of the coal and thus displaces the methane therein.On the other hand,after the methane gas near the inlet end is displaced,it will migrate to the outlet end,causing hysteresis in the methane flooding process near the outlet end of the coal sample.This causes the displacement effect of the inlet end to be better than that of the outlet end.?7?Medium-rank coal-grade coal samples?gas coal,fat coal,etc.?and high coal-grade coal samples with pores or small pores or transitional pores are mainly not able to complete the NMR imaging using the Fourier transform method under saturated water conditions.High coal-grade coal samples with transitional holes can be imaged by projection reconstruction.Coal samples with excessive porosity?more than 25%porosity?cannot be imaged in the saturated methane case.The coal samples that can be imaged are mostly low-rank coal?long flame coal,lignite?and moderate porosity coal briquettes.