Influential factors analysis and porosity correction method of nuclear magnetic resonance measurement in igneous rocks

摘要

Nuclear Magnetic Resonance (NMR) log can provide accurate porosity independent of lithology, and NMR logging is more advantageous over conventional logs. In recent decades, it has been broadly applied in reservoir evaluation and pore structure study of sandstone and carbonate formations. But in igneous rocks, NMR porosity is underestimated compared to the actual porosity, which limits the application of NMR measurement in such reservoirs. To tackle this problem, NMR measurements were firstly introduced from DX, XS, and CPZ regions, China, and the effect of different igneous rocks on NMR porosity was analyzed in detail. NMR transvers relaxation time (T-2) of igneous rock is very short and the T-2 distribution is in fast relaxation region, and the amplitude of NMR signals is so low. From NMR measurement analysis, NMR porosity relative error generally increases from acid, middle to mafic igneous rocks. Then, combined with element measurement, some influential factors were summarized from laboratory experiments and the relationships of NMR porosity relative error between different paramagnetic elements and magnetic susceptibility were investigated. The elemental analysis from plasma emission spectrometer showed that, NMR porosity relative error is related to paramagnetic mineral contents, and it generally increases as the iron and manganese contents increase. Moreover, the magnetic susceptibility of igneous rock is usually significantly greater than the sedimentary rocks, and from acid, middle to mafic igneous rocks, the magnetic susceptibility trends to increase. According the above analysis, some NMR porosity correction correlations are constructed. Finally, in case study from CPZ region, NMR logging porosity was correlated by the constructed correction correlation from iron content of element spectrum capture log, the corrected NMR porosity is in good agreement with core porosity, which proves the empirical correction method from the paramagnetic element contents rel