Analyses of Influencing Factors of Hydrocarbon Identification Using NMR Time Domain Analysis
核磁共振测井TDA识别油气的影响因素

Time-domain analysis (TDA) is one of the most important NMR methods used in fluid identification and porosity calculation. It shows great advantages in reservoir evaluation, but there are also certain limitations. In order to investigate the adaptability of TDA in hydrocarbon identification under different reservoir conditions, numerical simulation was used in this study to analyze the factors influencing the outcomes of TDA, including longitudinal relaxation time T1, polarization time TW, porosity, hydrocarbon saturation, hydrogen index HI and polarization function. It is concluded that when acquisition mode D9TW is chosen, the viscosity of oil that can be measured by TDA is less than 9 mPa.s. After changing the short waiting time up to 2 s while keeping the long waiting time constant, the minimal viscosity reduces to less than 4 mPa.s. We also found the intensity of differential signals correlated positively with all the influencing factors mentioned above. With real low SNR NMR data, TDA can effectively identify hydrocarbon and calculate porosity only under the conditions that the differential porosities of hydrocarbon are greater than 1.5 p.u.