Quantitative Characterization Of Coalbed Methane Reservoirs:A New Method By Low-field Nuclear Magnetic Resonance

Based on the shortcoming of some conventional experimental techiniques in quantitative characterization coalbed methane(CBM)reservoirs petrophysical characteristics,this study has established a quantitative characterization system for investigation CBM reservoirs petrophysical characteristics,such as pore structure,full-scale pore size distribution(PSD),multiphase methane quantitation and dynamic fluid interactions during CO₂-enhanced coalbed methane(CO₂-ECBM)by using the low-field nuclear magnetic resonance technology(LF-NMR).The main conclusions can be drawn as follows:(1)The multifractal analysis theory is introduced into the characterization 100%water-saturated coals.Multifractal parameters of D-q–Dq,D-q?Dq and??show good correlation with T₂ cutoffs.After multiple linear regression calculation,we proposed a convenient method for NMR T₂ cutoffs determination,independent of any help from other experiments such as centrifugal processes or heat treatment.(2)The values of surface relaxivities calculated from low-temperature N₂ adsorption(LTNA)measurements(?2-SVR)are smaller than those calculated from mercury intrusion porosimetry(MIP)measurements(?2-MIP).The surface relaxivities calculated from these two different methods are applicable only for certain ranges of pore radius but not for the full-scale PSD,and neither method can be used to calculate the full-scale PSD individually.Considering the pore radius of?25 nm as the connection point,the full-scale PSD was restructured by the combination of LTNA and MIP.Based on the result of rebuilt full-scale PSDs and the principle of maximum similarity,the least square method is introduced for a standard of surface relaxivity(?2)calculation,which can be well in calculating an absolute full-scale PSDs from the NMR data.The results show the surface relaxivity 2.1?m/s,3.0?m/s and 1.6?m/s correspond to low-,medium-and high-rank coals,respectively.(3)Based on the self-design NMR methane adsorption setup,we performed a series of NMR adsorption measurements on 15 sub-bituminous coals and anthracites.The adsorption isotherms quantitatively evaluated from the NMR show excellent agreement with those from parallel volumetric measurements.The adsorbed methane content has an excellent linear relationship with P1 peak amplitude for all coals,which can be further used for NMR adsorbed methane calibration coefficient determination.The calculated adsorbed methane calibration coefficient shows a good negative linear correlation with surface relaxivity?2.(4)Based on the self-design NMR CO₂-CH₄ displacement measurement,CO₂-CH₄ flooding experiments exhibit two distinct ad/desorption rates:the first represents a rapid decline in methane content that stabilizes following a long desorptive tail.H₂O wettability of coals decreases with an increasing in the CO₂ injection pressure.This change in H₂O wettability decreases with increase in pressure to<5 MPa before stabilizing at an asymptotic maximum magnitude.CO₂ wettability increases with a decrease in temperature and thus sequestered mass will be increased in lower-temperature coal reservoirs,all other factors being equal.The presence of non-adsorbed water in coals significant decreases the rate of change in CO₂ wettability with pressure and reduces the maximum magnitude of the impact at elevated temperature.