Page 90 - Well Logging and Formation Evaluation

80 Well Logging and Formation Evaluation

which surfactants in the OBM filtrate have made the invaded zone oil wet.
Even where WBM (water-based mud) has been used, any mixed wettability
in the formation will tend to result in anomalous results.
The tool will measure a decaying magnetic amplitude vs. time, which
depends on the following parameters:

• B 0 (the static field strength)
• T w (or T r ) (the wait time for longitudinal polarization)
• t (the transverse echo time)
• T 1 of the fluids in the pore space
• T 2 of the fluids in the pore space
• D of the fluids in the pore space
• The total porosity

Note that the signal will arise from only the part of the formation for which
the CPMG pulses correspond to the correct Larmor frequency. Because
the fixed magnet is located in the borehole, with the magnetic field
decreasing with distance from the borehole, this will define the zone of
investigation of the tool.
Having measured the transverse signal as a function of time, the next
step is to invert these data into the corresponding distribution of T 2 values
that make up the signal. This would be a straightforward mathematical
operation were it not for the presence of noise in the signal. In fact, without
some additional form of constraint, at the noise levels typically encoun-
tered in the tool it is possible to produce wildly different T 2 distributions
that can all honor the original decay curve. One constraint that is com-
monly applied, called regularization, is that the T 2 distribution must be
smooth. This results in a more stable solution, although there is no par-
ticular reason why the T 2 spectrum should indeed be smooth. Needless to
say, unless the inversion is correct, the results of the tool will be com-
pletely useless. This is worth bearing in mind in situations where the tool
gives results that cannot be explained in terms of known properties of the
lithology based on core data.
In practice the T 2 spectra are not continuous but divided into “bins”
covering different ranges of T 2. The maximum value of T 2 that can be
measured is determined by the time allowed for the signal to be mea-
sured. This in turn is related to the logging speed. In some situations, wait
times of up to 15 seconds might be needed to capture the full spectra,
translating into logging speeds that are very slow (under 100ft/hour).
Output curves common to NMR tools include the following: