HEAVY OIL VISCOSITY DETERMINATION USING NMR LOGS

摘要

The apparent Hydrogen Indices (HI_(app)) of heavy (high density) oils as seen by NMR logging tools are ＜1, and dependent on the echo spacing (TE) value used in the measurement. The actual HI values of the samples we tested, however, were determined by geochemical means to be within 5% of 1.rnTransforms for estimating heavy oil viscosity for the MRIL and CMR logging tools were determined using laboratory NMR measurements (T_2) made on oil samples with viscosity values ranging from ＜1000 cp to ＞100,000 cp at the probe temperature of 28℃, and API gravity values ranging from 8.7 to 14.5. For the echo spacings associated with each tool, the viscosity transforms were based on either the loss of signal (HI_(app)) or from the log mean average T_2 value of the oil. HI_(app) was related to viscosity (η) as η=a~*exp(b~*HI_(app)) where a is inversely proportional to the square of TE and b is approximately constant.rnThe T_2 of the oil samples (η ＞1000 cp) can be related to the viscosity through an adaptation of the Vinegar equation (Kleinberg and Vinegar, 1996). A larger prefactor is used (1.8 vs 1.2), and T_2 is replaced by T_2 -10~(-3)(TE-0.5) where T_2 is in seconds and TE in ms. The larger prefactor may occur because the effects of any dissolved oxygen (paramagnetic) in the heavy oils would have negligible effect on the short T_2 values measured. The TE terms are needed to account for under-sampling of the rapidly relaxing T_2 signals from the heavy oils.rnWe applied these transforms to NMR logs from a heavy oil reservoir where the oil viscosity was found to be 1,000 cp to 4,000 cp in one zone and 5,000 to 9000 cp in a deeper zone. High noisernlevels in the NMR logs, resulted in the large uncertainty in the viscosity estimates.