Due to a high noise-to-signal ratio, 10 of the 24 acquired magnetic resonance echo data logs from a field in the Niger Delta were excluded from the processing result. The reduction in the total number of echoes led to a high diffusivity, especially in the gas-bearing reservoir sand. High diffusivity caused some of the movable fluids to appear as irreducible bound water. The low hydrogen index of the gas also caused a low-permeability profile.

The effect of high diffusivity was very prominent in the Apparent Transverse Relaxation Time (T2app) spectrum, but the intrinsic transverse relaxation time (T2intrinsic) spectrum was unaffected. Consequently the, T2intrinsic spectrum was used to determine partial porosities and fractional fluid volumes. However, the total and effective porosities were not seriously impacted.

2D NMR (T2intrinsic – Diffusivity) maps were used to determine the partial porosities and fractional fluid volumes. These maps are produced by plotting diffusivity, D, against the Transverse Relaxation Time, T2. 2D NMR maps aid the discrimination of the magnetic resonance fractional volumes – clay-bound water (CBW), irreducible bound water (BVI) and movable volume (BVM). The maps can further be used for fluid-typing analysis.

The common processing result using T2app showed little or no permeability and little or no BVM in the gas interval, while the 2D NMR result showed an average permeability index of 3mD and an average movable fluid volume of 8pu. There was also very little BVI in this gas interval.

This paper effectively demonstrates the ability to use advanced magnetic resonance techniques to address issues related to high a noise-to-signal ratio, as well as minimal sensor failures. As presented, the paper clearly shows that we can get useable data from as high as 50% echo trains failure rate.

Though efforts are made to prevent signal failures and high noise-to-signal situations, there is a fix to such problems when they arise. Rather than re-log, the data can be savaged and the desired result achieved.

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