Dimensions in NMR Logging
WT (n) WT (3) WT (2) WT (1) Full CPMG (1) Full CPMG (2) Full CPMG (3) Full CPMG (n)
Time
> T1 logging. Traditional NMR CPMG sequences measure T2 distributions and begin after a sufficient WT for polarization of hydrogen nuclei. For T1 acquisition, a succession of short CPMG cycles with WTs selected over a range of values is used. In a departure from earlier T1 acquisition methods, the MR Scanner tool acquires full T2 echo trains for each chosen WT value, and thus the resulting data can be subjected to a multidimensional inversion and provide both T1 and T2 distributions. T1 logging is especially useful in low signal-to-noise environments and for fluids with long polarization times, such as are found with light hydrocarbons and in large pores. In addition, T1 distributions, unlike T2 distributions, are free of diffusion effects and provide more- accurate results in highly diffusive fluids.
Humans generally visualize in three dimensions, and geometric relationships are understood as adding levels of complexity with each dimension. For instance, a 1D image may have length, 2D adds width, 3D adds depth, and 4D adds the element of time.1
Analogous to
spatial relation ships, NMR measurements can be described using dimensionality, with each dimension adding a degree of complexity. The 1D NMR distribution refers to T2 transverse relaxation time measurements. T2 distributions are obtained by inverting raw NMR echo-decay signals. The distributions contain information about both fluid properties and pore geometry. However, signals from different fluids often overlap, and it is not always possible to distinguish water from oil, or water from gas purely on the basis of the T2 distribution.
The T1 relaxation measurement, from the buildup of polarization, also provides a 1D distribution. A single echo (or a small number of echoes) is acquired for a series of different wait times, WTs.2
The observed increase in
echo amplitude with increasing WT is the polariza tion buildup, which is governed by the distribution of T1 relaxation times (above). With a mathematical inversion similar to the one employed to derive T2 distributions from echo-decay signals, the T1 distribution is extracted from the polarization buildup. During the T1 acquisition, a full T2 echo- decay signal can be acquired for each WT, rather than a single echo or a short series of echoes, and thus a 2D dataset can be generated with T2 and T1 data. The individual echo-decay signals are inverted to obtain a separate T2 distribution for every WT. Each T2 component follows its own characteristic buildup with increasing WT, governed by the T1 distribution (buildup) associ ated with that
T2 component. In practice, a 2D inversion directly transforms the original echo dataset to a 2D T1-T2 distribution, some times referred to as a T1-T2 map (next page, top left).3 For many fluids, T1 and T2 distributions are very similar since they are governed by the same physical properties. Under typical measurement conditions the T1/T2 ratio for water and oil ranges between 1 and 3. However, an important difference between the two relaxation times is that T2 times are affected by molecular diffusion whereas T1 times are free of diffusion effects. In NMR measurements, diffusion causes a reduc tion in echo amplitude and therefore shortens T2 times. The magnitude of the diffusion effect is a function of the molecular-diffusion constant of the fluid, D, and the echo spacing, TE. TE is an adjustable measurement parameter defining the time between consecutive radio frequency (RF) pulses in the measurement sequence. Diffusivity is an intrinsic fluid property, depending only on the fluid composition, temperature and pressure. Once quantified, it identifies the fluid type.4
For water, D is
prima rily related to temperature, and for natural gas, it is determined by both tempera - ture and pressure. Crude oils exhibit a distribution of diffusion rates governed by the molecular composition, temperature and pressure. Diffusion, therefore, is the key to identifying fluid type with NMR. For example, T2 relaxation times for gas are much shorter than T1 times because of diffusion. By identifying the differ ence between T1 and T2 measurements in a gas reservoir, the hydrocarbon type can be inferred. Diffusion distributions are determined by measuring echo-amplitude decays for echo trains acquired with different echo spacings, TEs. However, increasing the TE to allow diffusion to take place comes at a price. The increased time between echoes means there
8
Oilfield Review
Polarization
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