NMR Fluid Properties

Accurate interpretation of the formation properties (porosity, permeability and irreducible water saturation) requires reliable estimates of NMR fluid properties or the relationship between diffusivity and relaxation time. Estimation of oil viscosity and solution-gas content require their correlation with NMR measurable fluid properties. These include the hydrogen index, bulk fluid relaxation time and bulk fluid diffusivity [8]. 

Degassed water under ambient conditions has a relaxation time (T and T2) of about 4 s at 30 °C [11,12], However, air-saturated brines may have a relaxation time of about 2-3 s. Light hydrocarbons are even more sensitive to dissolved oxygen [10], For example, the relaxation time of deoxygenated pentane is 14 s while air-saturated pentane is about 3 s. The correlation for degassed alkanes between the relaxation time (Ti), viscosity (q) and temperature (T) is given by Eq. (3.6.1) [13]. 

The relationship between Ti and T2 was examined for a number of liquid alkanes and crude oils [15]. It was concluded that there is no difference for light oils, apparently because light oils satisfy the fast-motion condition (the correlation time is less than the Larmor period). However, viscous oils do not satisfy this condition as the departure between Tx and T2 correlates with an increasing viscosity and Larmor frequency. 

The relaxation of gaseous methane, ethane and propane is by the spin-rotation mechanism and each pure component can be correlated with density and temperature [15]. However, the relaxation rate is also a function of the collision cross section of each component and this must be taken into account for mixtures [16]. This is in contrast to the liquid hydrocarbons and their mixtures that relax by dipole-dipole interactions and thus correlate with the viscosity/temperature ratio. 

Porosity (ej ) determination with NMR is a direct measurement as the response is from the fluid(s) in the pore space of the rock. The initial amplitude (before relaxation) of the NMR response of the fluid(s) saturated rock (corrected for hydrogen index) is compared with the amplitude of the response of bulk water having the same volume as the bulk volume of the rock sample. The 2 MHz NMR 

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