钻井液侵入致密气储层核磁共振T2响应数值模拟OA

Nuclear magnetic resonance(NMR)technology,detecting the relaxation decay signals of hydrogen nuclei in pore fluids,provides critical information about reservoir pore fluids,which has been widely applied in exploration and development of the reservoirs.However,the distribution pattern of pore fluids can be altered by drilling fluid invasion,resulting in the NMR T2 spectra containing some drilling fluid signals.Consequently,the reliability of fluid identification and reservoir fine evaluation based on T2 spectra is reduced.In order to improve the accuracy of nuclear magnetic resonance logging interpretation,it is of great significance to study the NMR T2 responses for rock with drilling fluid invasion.Taken a tight gas reservoir as an example,three-dimensional digital core models for tight gas reservoir with different drilling fluid types and invasion degrees are constructed in this study.NMR T2 responses of tight gas reservoir under drilling fluid invasion conditions are simulated utilizing the random walk method,and the influences of internal magnetic field gradient,echo spacing,hydrogen index,and signal-to-noise ratio(SNR)on T2 responses of tight gas reservoir also are investigated.The results demonstrate that water peak and gas peak overlap and the overlapped peak gradually moves along the direction of relaxation time enhancement as the degree of water-based drilling fluid invasion increases.As the degree of oil-based drilling fluid invasion increases,water peak and gas peak also overlap but the overlapped peak gradually moves along the direction of relaxation time reduction.As the increase of the internal magnetic field gradient or echo spacing,the transverse relaxation times for water peak,oil peak,and gas peak in T2 spectra shift toward shorter relaxation times,and the movement amplitude is faster for gas peak than for water peak and oil peak.In additional,the signal intensity of the gas peak in T2 spectra gradually strengthens with gas hydrogen index increases.The accuracy of the inverted T2 spectra is enhanced as data SNR increases.It is recommended that the SNR of the acquired NMR data is greater than 40.The research results provide a theoretical foundation for NMR log data interpretation under drilling fluid invasion conditions.