Saturation Profile from Multi-frequency NMR Measurements
- State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing, China
Invasion often causes the variation of fluid saturation varying along radial depth of borehole, which would always contribute to the uncertainty of the information of uninvaded formation obtained by well logging. Studying the variation of radial saturation can help correct the measured data to acquire more accurate information of uninvaded formation. With multi-frequency NMR well logging, the information of various saturation along radical depth can be obtained from T2 spectrum inversed from echo trains acquired by different frequency [1] .
However, with deeper investigation, the ratio of signal to noise(SNR) of acquired data from the instrument decreases. Influenced by SNR, inversion at different DOIs(Depth of Investigation) would cause jittery or oscillatory of the fluid saturation and T2 distribution. B spline function can be used as basic function to smooth T2 distribution and can be used as a smooth constraint in radial dimension [2] . The transformed data by B cubic function were inversed by the method of jointing inversion. A rock core model is developed to simulate NMR echo trains at different DOIs with different SNR. The simulation and inversion results showed that the combination of interpolation of B spline function and the jointing inversion method can effectively reduce the jittery and oscillatory. Besides, the simulation results showed that multi-frequency NMR method can reflect the variation of the radial fluid saturation.
- [1] Coates G R, Xiao L, Prammer M G., (1999), NMR logging: principles and applications, Gulf Professional Publishing
- [2] Sun B, Dunn K J, Clinch S., (2010), Inversion along the depth dimension to enhance the continuity of multi-dimensional NMR logs, SPWLA 51st Annual Logging Symposium. Society of Petrophysicists and Well-Log Analysts