NMR logs taken over hydrocarbon intervals show significant differences with routine core data. Ideally, one should do laboratory NMR at full reservoir conditions, which is very expensive. As an alternative, we propose that the same can be modeled, starting from routine NMR on core, and estimated values of oil properties and saturation. The advantage of this approach is that changes in these estimates can easily be made. The modeled NMR response is then used to calibrate interpretations such as bound water, permeability and capillarity.
Another new concept is a variable, rather than a fixed, T2cutoff for the prediction of bound water from the NMR log. In-situ modeling was also used to calibrate this cutoff against features of the actual log.
We show that the actual log response is faithfully predicted by our modeling for two wells in the Valdemar area. Well A was drilled with water-based mud (WBM) and logged with a wireline NMR tool, and Well B was drilled with oil-based mud (OBM) and logged with an LWD NM tool.
The results compare generally well with the resistivity derived saturation, but local deviations are found. Similarly, permeability correlations calibrated to in-situ conditions produced values that compare favorably with core data where available. Pseudo-capillary-pressure curves computed from the NMR log, using calibration against available core data, were used to locate the free-water level (FWL). Wettability was assessed from the shift of the oil-relaxation time away from its predicted bulk value under reservoir conditions. The reservoir appears to be strongly water-wet.
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