NMR Influence Factors Analysis And Log Interpretation Of Igneous Rocks

Nuclear magnetic resonance(NMR) logging has significant advantages in reservoir identification, calculation of porosity and permeability and fluid identification, which is widely used in sedimentary rocks. In recent years, large oil and gas reservoirs in igneous rocks are found in succession. But igneous rocks are such complex and diverse in lithology, lithofacies, reservoir space, pore structure, mineral composition and element composition, causing the low accuracy of NMR logging in interpretation and evaluation applying in such reservoirs, and the application of NMR is limited. Therefore, the study of NMR influence factors analysis and log interpretation of igneous rocks not only has important theoretical significance, but also has important practical application value.Based on the characteristics of igneous lithology, influence factors from two aspects of experiments of NMR and theoretical perspective are analyzed in this thesis, combined NMR fundamental principle, and proposes a correction method of NMR porosity and T2 spectrum. Firstly, based on NMR experiment results, the effect of different lithology on NMR, and investigate the relationship between different elements, magnetic susceptibility and the relative error of NMR porosity are analyzed. Secondly, based on NMR response mechanism, the influence of magnetic susceptibility, fluid type, pore structure and relaxation type to NMR response by using the method of numerical simulation are analyzed. Thirdly, based on element content, the empirical formula of NMR porosity correction are built, using the data from Chepaizi district. Finally, in NMR logging interpretation, the model of permeability calculation and calculate saturability combined with resistivity logs are improved.The researches show that the core NMR porosity, mud logging NMR porosity and wire line logging NMR porosity of the igneous are less than the core buoyancy porosity; NMR porosity relative error decreases with the lithology from basic to acid and the relative error increases with paramagnetic substance increasing; the igneous rock magnetic susceptibility is significantly higher than that of sedimentary rock, resulting in strong internal magnetic field gradient, the T2 time is shorter, the T2 spectrum is forward; The longer echo interval T2 spectrum moves forward more obviously; internal magnetic field gradient is stronger, T2 spectrum move forward more obviously; The influence of the internal magnetic field gradient on the T2 spectra increases according to the oil, water and gas; Under the same conditions, the diffusion relaxation of sandstone is small, and the diffusion relaxation of igneous rocks is high, and the diffusion relaxation of gas bearing rocks is the largest, followed by water and oil. The example analysis shows that the empirical formulas of NMR porosity correction built are consistent with the core experiment; the inversion of T2 spectrum moves backward, and the calculation porosity and permeability is consistent with the core experiment.