Temperature Dependence of NMR Relaxation Time in Carbonate Reservoirs

NMR-based carbonate interpretation models are commonly calibrated using laboratory ambient core NMR measurements. For applying the core calibrated models to downhole NMR logging interpretation, the difference between the NMR responses measured at ambient and reservoir conditions needs to be evaluated. The temperature dependence of NMR relaxation time in high-quality carbonate reservoirs was investigated, and NMR temperature dependence models were determined using data analytic methods (Hursan et al, 2019). This paper focuses on temperature dependence of NMR relaxation time in low-quality carbonate formations. For more than 95% of the samples investigated, NMR relaxation time shows a positive correlation with temperature. The correlation is similar to that observed in high-quality carbonate rocks but slightly less significant. Temperature dependent correlations for predicting T2GM from a measured temperature to any other temperature are derived from high- and low-quality carbonate rocks independently first, then a unified T2GM correlation is derived including both the high- and low-quality carbonate reservoirs. Predicting T2 distribution from one temperature to other temperatures is achieved using dimension reduction approach involving principal component analysis (PCA) technique. It is found that the T2 distributions at any given temperature for both the high- and low-quality carbonate reservoirs can be predicted robustly from the T2 distributions at the ambient temperature by representing the T2 distributions with principal components (PCs) at the ambient temperature then using these PCs to predict the PCs at a different temperature. The optimal number of PC components depends on the multimodality of the T2 distribution. This work extends the validity range of a data analytic method that quantifies the temperature dependence of carbonate NMR properties. The new NMR temperature model enables the integration of NMR laboratory studies and dowhole measurements for advanced petrophysical analyses in a wide range of carbonate reservoirs.