Nuclear magnetic Resonance (NMR) Logs

NMR is a technique that observes the behavior of protons (hydrogen nuclei) in a magnetic field. The NMR tool normally has a permanent magnet in the center of the probe (Fig. 1), which provides an external magnetic field to protons. In the absence of an external magnetic field, the hydrogen nuclear spin axes are randomly aligned. In the presence of an external magnetic field, the spin axes attempt to line up with the field either parallel or anti-parallel to the magnetic field. After this so-called polarisation process, the radio-frequency (RF) antenna surrounding this magnet (Fig. 1) transmits into the formation of precisely timed bursts of radio-frequency energy in the form of an oscillating magnetic field (CPMG pulse sequences), which tips the protons to resonate with the field from the permanent magnet. Between these pulses, the antenna receives the signal from those hydrogen protons that decay from the resonance state to the initial state. There are two kinds of decay detected by the antenna, longitude and transverse decay (Fig. 2).

The received signal of transverse decay is transformed into a diagram of porosity and spin-spin relaxation time t2. In porous rocks t2 is related to the pore environment that the proton is in, therefore the tool can be tuned to adjust to different situations due to the fluid properties. The total porosity is usually divided by t2 cut-offs into porosity occupied by clay-bound water (CBW), irreducible capillary-bound water (BVI), and free fluid index (FFI) (Fig. 3). The tools are normally available in a variety of different frequencies to provide investigate cylindrical regions at different diameters around the tool (Fig. 1).

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Technical Details

“Javelin Wireline slim” is a magnetic resonance logging tool with an optional natural gamma sensor. It provides direct and high-resolution measurements of hydrogeological properties including porosity, bound and mobile water content, pore size distribution, Hydraulic conductivity, and natural gamma. The tool allows two independent DOI shells measured during a single logging run and could provide accurate measurement of clay-bound water content by ultra-short echo spacing of 500 microseconds.

 

Figure 1: A simple diagram of the structure of a NMR probe, with three different frequencies. (G.R. Goates et al. (1999), redrawn)

Figure 2: Longitude and transverse decay (S. Giannini, 2013)