适用于长弛豫组分的CMR-NG核磁共振测井二维采集模式OA

This paper addresses the problems of signal non-convergence,low fluid identification accuracy and distorted calculation of nuclear magnetic resonance porosity when the original T1—T2 two-dimensional acquisition mode of combinable magnetic resonance next generation(CMR-NG)nuclear magnetic resonance(NMR)logging processes reservoirs with long relaxation components.Multiple technical approaches are adopted for research,including analyzing defects of existing acquisition parameters,optimizing pulse sequence parameters,quantitatively evaluating polarization rate,quantitatively evaluating attenuation rate,quantitatively evaluating porosity sensitivity,carrying out forward numerical simulation of two-dimensional NMR,and comparing and verifying logging data from actual drilled wells.It explores the influence laws of four key acquisition parameters(waiting time,echo spacing,number of echoes and number of repetitions)on long relaxation NMR signals,establishes a new two-dimensional acquisition parameter system applicable to long relaxation components,defines the convergence rate index to quantify the convergence degree of two-dimensional signals,and completes accuracy verification of data.Research results show that:①The original acquisition mode features short waiting time and insufficient echoes.Hydrogen nuclei in fluids with long T1 and T2 fail to achieve full polarization,attenuation signals are truncated,T2 spectra deform and produce false trailing tails.②The new acquisition mode sets the maximum waiting time to 3 193 ms,increases the maximum number of echoes to 3 000,covers a long relaxation T2 range of 10～600 ms,achieves porosity sensitivity approaching 1,and significantly improves the completeness of signal polarization and decay acquisition for long relaxation times.③Numerical simulation of five formation models proves that the convergence rate of long relaxation components under the new mode is generally higher than 0.8.The convergence rate only drops to 0.63～0.68 when both T1 and T2 exceed 1 000 ms simultaneously.④Comparative analysis of two actual wells in Tarim oilfield demonstrates that the new mode eliminates false noise at the long relaxation end of two-dimensional spectra.Its calculated NMR porosity values match core helium-measured porosity better and avoid risks of fluid misinterpretation.It concludes that this optimized two-dimensional acquisition mode effectively improves the convergence of two-dimensional NMR signals in fractured-vuggy and large-pore sandstone reservoirs with long relaxation characteristics and enhances the accuracy of quantitative porosity calculation and fluid discrimination.For field application,acquisition parameters should be customized according to the relaxation characteristics of reservoir blocks to further improve the quality of logging data.