What it is
NMR logging — Nuclear Magnetic Resonance logging, sometimes called MRIL (Magnetic Resonance Imaging Log) or CMR (Combinable Magnetic Resonance) depending on the service provider — is a wireline or LWD (logging-while-drilling) measurement that uses NMR physics to characterize the fluids in the formation immediately adjacent to the tool.
The tool applies a strong static magnetic field to the formation, excites hydrogen nuclei in that field with an RF pulse, and measures the characteristic relaxation signal (T2 distribution) that returns as the nuclei relax back to equilibrium. The distribution of relaxation times can be interpreted to estimate:
- Total porosity (independent of mineralogy, unlike density/neutron logs)
- Free fluid volume vs bound water volume
- Permeability (via empirical correlations)
- Hydrocarbon presence and, in some cases, fluid type
Why it matters
NMR logging became a near-standard tool in unconventional reservoirs because the traditional porosity-from-density and saturation-from-resistivity methods break down in organic-rich shales, tight formations, and complex clay-bearing rocks. NMR logs sidestep many of those failure modes by responding directly to fluids rather than to bulk rock properties.
How it differs from remote NMR mapping
NMR logging is a downhole measurement made inside a borehole that's already been drilled. Remote NMR subsurface mapping (see Inside Earth's technology page) uses NMR as a signal-classification technique applied to surface-derived data — without drilling. The underlying physics is the same; the deployment is fundamentally different.