Abstract
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.
Introduction
NMR has long promised to measure porosity that is independent of mineralogy. It can also provide information on irreducible water saturation.1 Depending on the tool and the mode of operation, the type of fluid in the formation can be indicated, whether it is gas, oil or water.2,3 Permeability and pore size distribution can also be derived4. All these are in real time and on depth. If this potential were fulfilled, there would be a reduced need for coring to identify rock facies and for permeability measurements from core plugs. All of these provide additional strategies to optimize the cost benefits of formation evaluation. The application of NMR logging technology takes on even more significance in horizontal wells, where core is frequently unavailable.
NMR logging in carbonate reservoirs has shown mixed results.5,6,7,8 In some cases, the porosity and permeability estimates compared favorably with core data. But in many carbonate reservoirs porosity has been underestimated. With this underestimation in porosity, the permeability estimation becomes problematic, since permeability is derived from NMR porosity.
Carbonate rocks pose the greatest challenge to NMR logging and interpretation. This can be attributed to heterogeneity, wettability, and surface relaxivity phenomena of the rock. Laboratory experiments are providing insight into these phenomena.4,9,10,11NMR response in carbonates is not very well understood in the oil industry. For this reason, several industry consortia have been formed to develop improved permeability models for carbonates through the integration of NMR logs, core data, and other openhole logs. It is widely accepted that more work needs to be done to develop field-specific models. The models should then be tested and validated.