Viscosity Evaluation of Heavy Oils from NMR Well Logging
Hirasaki, George J.
Doctor of Philosophy
Heavy oil is characterized by its high viscosity, which is a major obstacle to both logging and recovery. Due to the loss of T 2 information shorter than the echo spacing ( TE ), estimation of heavy oil properties from NMR T 2 measurements is usually problematic. In this work, a new method has been developed to overcome the echo spacing restriction of NMR spectrometer during the measurement of heavy oil. A FID measurement supplemented the CPMG in an effort to recover the lost T 2 data. Constrained by the initial magnetization ( M 0 ) estimated from the FID and Curie's law and assuming lognormal distribution for bitumen, the corrected T 2 of bitumen can be obtained. This new method successfully overcomes the TE restriction of the NMR spectrometer and is nearly independent on the TE applied in the measurement. This method was applied in the measurement of systems at elevated temperatures (8 ∼ 90 °C) and some important petrophysical properties of Athabasca bitumen, such as hydrogen index ( HI ), fluid content and viscosity were evaluated by using the corrected T 2 . Well log NMR T 2 measurements of bitumen appear to be significantly longer than the laboratory results. This is likely due to the dissolved gas in bitumen. The T 2 distribution depends on oil viscosity and dissolved gas concentration, which can vary throughout the field. In this work, the viscosity and laboratory NMR measurements were made on the recombined live bitumen sample and the synthetic Brookfield oil as a function of dissolved gas concentrations. The effects of CH 4 , CO 2 , and C 2 H 6 on the viscosity and T 2 response of these two heavy oils at different saturation pressures were investigated. The investigations on live oil viscosity show that, regardless of the gas type used for saturation, the live oil T 2 correlates with viscosity/temperature ratio on a log-log scale. More importantly, the changes of T 2 and viscosity/temperature ratio caused by solution gas follows the same trend of those caused by temperature variations on the dead oil. This conclusion holds for both the bitumen and the synthetic Brookfield oil. This finding on the relationship between the oil T 2 and its corresponding viscosity/temperature ratio creates a way for in-situ viscosity evaluation of heavy oil through NMR well logging.