Abstract
This paper discusses the calibration of nuclear magnetic resonance (NMR) logs to determine permeability distribution in carbonate reservoirs. The examples presented indicate the need for rigorous and integrated calibration to succeed in permeability estimation.
The promise of the NMR logging tool to deliver, among others, mineralogy independent total porosity and permeability makes it a tool of choice in formation evaluation. Significant savings in coring costs, particularly in horizontal wells, are possible with the NMR tool.
In sandstone reservoirs, NMR has worked remarkably well in the estimation of porosity, irreducible water saturation, producible fluids, and permeability. But the success rate is low in carbonate reservoirs, with respect to permeability. This can be attributed to the complexity of carbonate reservoirs and the lack of proper calibration of the logs with laboratory data or some measured reference.
The paper discusses calibration of NMR logs for permeability estimation, using laboratory NMR measurements, routine core permeability data, and formation test data. The calibrated models are then used to re-compute permeabilities in the logged intervals. The examples illustrate that reliable field-specific permeability modeling is possible when all relevant data are integrated into the calibration effort. The calibration effort also shows that it is not sufficient to adjust only the pre-factors in the SDR and Timur-Coates’ permeability models in order to make the NMR log data match the core data. The porosity exponent (m) and T2LM (or FFI/BVI) exponent (n) also require adjustments. The default values of m=4 and n=2 did not apply in most of the cases examined in this paper.
