Application NMR Well Log Calibration

NMR logs are performed to get several key petrophysical properties, such as porosity, bound versus mobile fluid, and permeability prediction.  Laboratory measurements make these values more accurate by defining the T2 cut off and permeability model coefficients that are specific for your reservoir

NMR technology can be affected by changing temperatures, polarity anomalies, and radio signals created naturally and by other electronics equipment. Downhole NMR well log tools work in very dirty, volatile and signal rich environments. As a result, it is very difficult to perform accurate and controlled measurements.

NMR well log calibration assists reservoir engineers and petrophysicists by ensuring the downhole data is interpreted accurately. Lab based NMR instruments work in a much cleaner and more controlled environment, so lab-based rock core NMR analysis is used to calibrate the data collected by the downhole NMR tools.

The so-called T2 cut-off in a NMR T2 distribution is the T2 value that divides the small pores that are unlikely to be producible from the larger pores that are likely to contain free fluid.  The integral of the distribution above the T2 cut-off is a measure of the free fluid (mobile fluid) in the rock.  The portion of the curve below the cut-off is bound fluid and is made up of clay bound fluid and the capillary bound fluid.  An accurate determination of the T2 cut-off is essential for an accurate determination of recoverable reserves (mobile fluid).

Permeability is one of the key rock properties affecting production.  NMR can be used to predict permeability through several models utilizing the NMR pore size distribution as input.  These models require coefficients which are more accurate when calibrated for the specific reservoir.

The data plotting and calculation for this measurement are carried out automatically through GIT Systems software supplied as the standard operating system for a GeoSpec NMR core analyzer.

For more information on how NMR well log calibrations are performed, see the this Application Note.