NMR - Back to Basics

Nuclear Magnetic Resonance (NMR) logging has come a long way since its inception as a permeability tool, when researchers in the 1960s discovered the link between NMR T2, pore size and hence permeability. Today, it stands as one of the most versatile and insightful technologies in the formation evaluation toolkit. Yet, despite its capabilities, NMR is often misunderstood or underutilized - frequently only called upon when conventional logs fall short.  

Let’s take a step back and revisit the fundamentals of NMR and explore why it deserves a more central role in reservoir evaluation. 

More Than Just Permeability 

Originally developed to provide a continuous log of permeability, NMR has evolved into a multi-dimensional diagnostic powerhouse. Advances in tool technology now allow for: 

• Multiple wait times and echo spacings 

• Multi-frequency acquisition and simultaneous inversion 

• Pre-polarizing magnets, improved Signal-to-Noise and higher vertical resolution 

• T1 logging and 2D NMR maps 

These enhancements have enabled NMR to tackle a wide range of reservoir challenges - often independently of other logging tools. 

A Fluid-Focused Perspective  

Unlike conventional logs, NMR measures fluids only. This makes it inherently independent of lithology, unaffected by variations in mineralogy or clay content. This is a game-changer as all other porosity tools are significantly affected by these, and have other limitations: 

• Density logs, measure electron density and require environment corrections 

• Neutron logs, require even more tool-specific environmental corrections including excavation correction 

• Sonic logs, are inherently non-linear and don’t ‘see’ secondary porosity 

These logs are all affected by fluid properties as well, as is NMR, but with all modern NMR tools having pre-polarizing magnets, the effect of T1 is negligible and applying the correction for Hydrogen Index (HI) is straightforward.  

With NMR, total porosity (PHIT) is directly measured, and, because it can distinguish clay-bound water from capillary-bound water, effective porosity (PHIE) is also clearly defined.  

Solving Real-World Problems 

NMR’s capabilities extend far beyond porosity and permeability: 

• NMR can discriminate between free and bound fluids, helping to identify productive zones in what we like to “Spot the Reservoir” 

• Identify low resistivity or low contrast pay and accurately compute hydrocarbon pore volumes, without having to resort to ill-posed thin-bed techniques  

• Fluid Identification: Accurately quantify water, gas, and oil volumes independent of resistivity 

• Hydrocarbon Properties: Determine gas density and oil viscosity 

• Heavy Oil and Wettability: Assess wettability and capillary pressure (Pc), even in complex reservoirs 

Interpretation Doesn’t Have to Be Complicated

If NMR seems complex, it’s often because it’s only used in the most challenging scenarios. But with a solid understanding of its principles and a few key interpretation techniques - like combining NMR with density logs (LHC) - the process becomes intuitive. 

Conclusion

NMR is not just a niche tool for complex problems - it’s a robust, standalone solution for a wide range of reservoir evaluation tasks. By embracing its full potential, we can unlock more accurate, reliable, and insightful interpretations across the board.