gas hydrate - CCOP

Q-values computed from the zero-offset Mallik data by the spectral ratio method are shown in
Figure 7. The depths of reflections marked ‘A’, ‘B’, and ‘C’ in Figure 6 is indicated. As shown
in Figure 7, the gas hydrate zone has low Q-factor and the variation in Q-factors matches very
well with the boundaries of the gas hydrate zone. Moreover, at the depth of reflection ‘B’, the
Q-factor becomes large compared to those of surrounding formations.
Q-factor
0 100 200 300 400 500
500
600
Depth (m)
700
800
900
1000
Top of gas hydrate zone
Coal seam in gas hydrate zone
1100
Bottom of gas hydrate zone
1200
Figure 7. Q factors extracted from the zero-offset VSP data at Mallik 3L-38 well
by using the spectral ratio method.
Frequency-dependent Q-factors were computed by the modified spectral ratio method for the
Mallik zero-offset data although the results are not shown here. The Q-factors for all layers
showed lower values compared to those obtained by the spectral ratio method. Further
preprocessing and analysis are needed to get more precise Q-factors by the modified spectral
ratio method.
CONCLUSIONS
The algorithm of the spectral ratio method and the modified spectral method were developed
and applied to synthetic data sets and the zero-offset data at Mallik 3L-38 gas hydrate
production research well. The frequency-independent Q-factors from the synthetic data
obtained by using the spectral ratio method provide the information on the pattern of the
change in Q-factors along the depth. While the computed Q-factors approach the true values
for the layers of low Q-factors, they deviate from the true values for the layers of high Q-
factors. In the layers with large Q values, the differences of the frequency spectrums of traces
are very small and it makes the computation of Q-factors very unstable. The variation of the
Q-factors extracted from the zero-offset VSP data at Mallik 3L-38 well by the spectral ratio
method agrees well with the boundaries of the layers, including the gas hydrate zone,
described in the reflection image. The results of applying the modified spectral ratio method,
which provides frequency-dependent Q-factors, show very unstable Q-factors for some layers
in the synthetic data and the Mallik data. Further analysis and research are required to obtain
more precise Q-factors by the modified spectral ratio method.
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 41