NMR relaxation in crude oils at elevated temperatures
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Until recently, most nuclear magnetic resonance (NMR) measurements in support of lab petrophysics and well logging were conducted at room temperature. However, the T2 relaxation of bulk crude oils strongly depends on temperature. Measurement of the crude oil relaxation response at reservoir temperatures is needed for optimal interpretation of log data. This work used a Can-Purcell-Meiboom-Gill (CPMG) pulse sequence to measure the NMR T2 relaxation at 2 MHz. Four stock tank crude oils were studied over the temperature range 30-100°C at a constant pressure of 300 psig. For comparison, the T2 temperature response of five viscosity standards was measured over the same temperature range and compared to the crude oil data. A two component exponential decay described all the T2 data. As expected from the temperature dependence of translational diffusion, there is not a universal curve that fits all hydrocarbon mixtures. The standards and the crude oils did not fit a single curve. The observed T2 varied directly with absolute temperature and inversely with the viscosity. For the crude oils, the log mean T2 was related to a viscosity value calculated from the stock tank oil viscosity conelation as developed by Beggs and Robinson and modified by Egbogah. This T2 relation did not agree with a widely accepted T2 conelation developed at room temperature from oil samples of differing viscosity (0.7 to 1000 cp). When the viscosity of one of the crude oils was measured as a function of temperature, that data then fit the room temperature T2 conelation. Viscosity values derived from correlations based only on API gravity are evidently insufficient to make close estimates of crude oil T2 relaxation at elevated temperatures. In the T2-viscosity plot, one of the crudes was a distinct outlier from the others. The composition of this sample suggests further investigations into the T2 relaxation of oil-based mud filtrate and mixtures of crude and oil-based mud filtrate.