Interpretation of nuclear magnetic resonance (NMR) logs is challenged by the following three factors (1) incpnrect-critical T2 cutoff foF free fluid index (2) incorrect permeability model and (3) effect of unusual.relaxivity properties of the rock on relaxation times. Some of these challenges can be addressed through the use of NMR core spectrometry data for log calibration. The major difficulty lies in the ability or inability to apply the core data correctly in log calibration.
In this work, we demonstrate that the identification and delineation of NMR relaxivity group or NMR facies are critical in the application of laboratory NMR calibration data to log analysis. The method presented also allows direct application of laboratory-based model for log interpretation irrespective of tool-dependent factors like logging speed, borehole conditions, fluid types and saturation.
In developing this technique, the following were taken into consideration (1), the fundamental physics of the behaviour of the magnetic dipole of hydrogen nuclei in the presence of an applied (external) magnetic field, (2), the mathematical relationship between the relaxation rates of magnetisation, the pore size that contain the liquid and other surface properties of the porous media, and (3), the mineralogital attributes of the formation.
We define NMR facies as units in the reservoir that have similar NMR relaxation characteristics. These units are delineated into groups referred to as relaxivity groups. Each relaxivity group has similar T2 cut off and relaxivity product. The combined influence of mineralogicai and textural attributes on relaxation rate is captured in this method.
Delineation of these groups is based on NMR porosity and normalised median value of the relaxation time, T2m. These data can be obtained directly from NMR log data.
The validity and applicability of the methodology is demonstrated by a regional study of NMR transverse relaxation time constant T2. Measurement were carried out on over 400 core plugs, and mineralogical, textural and pore size attributes on selected samples.
The benefits of NMR relaxivity grouping include:
Basis for tool calibration and enhanced interpretation of NMR log output.
Algorithm for permeaoility and residual oil saturation determination.