Log Water Saturation Model Validation Using Nmr Log And Core Data

A low invasion core was obtained in the BM-2 well, B-Field, offshore Northwest Java, Indonesia, for the purpose of validating wireline log porosity and water saturation models. The reservoir sands are lower Miocene in age and are generally clay rich and poorly consolidated. The first step in validating the log models was to compare log and low invasion core porosity data. The average core porosity in the reservoir sands was nearly 4 PU higher than the average porosity obtained with the log model. When examined on the basis of bulk density the core data were low compared to the measured log by nearly 0.1 gm/cc, which is 5 to 10 times larger than the accepted accuracy of the density tool. We concluded that the core, which had high water content, had likely been irreversibly changed when preserved by freezing, so we took an alternate approach for model validation using NMR logs in the adjacent BM-3 welL NMR measurements were carried out on core plugs from the BM-2 well. The NMR data were obtained with the core fully saturated with brine and after desaturating on centrifuge with air at approximately 20 psi. The two data sets were then compared to obtain a T2-cutoff of less than 10 msec. This relatively fast T2 value may be due to the presence of iron-bearing clays. The BM-3 well was drilled nearby and logged with the CMR tool as well as a conventional suite of logs. No core was obtained in this well. The NMR logs from the BM-3 well were initially processed using a 33 msec T2 cutoff, Based on the BM-2 NMR core data, the logs were reprocessed using T2 cutoffs of 10 and 20 msec to obtain revised values of irreducible water saturation. The conventional log model porosity and total NMR porosity agreed on average to within 1 porosity unit. Irreducible water saturation from NMR logs and a Waxman-Smits water saturation agreed qualitatively, but important differences were noted when compared on a more detailed basis. These differences could be understood if explained in terms of variation in T2-cutoff with capillary pressure, and when accounting for separate hydrocarbon columns present in the B-Field. Overall the NMR logs supported the accuracy of the conventional log models. Our results suggest the possibility of using NMR data, under favorable conditions, as another means of calibrating conventional water saturation models. This might be a viable alternative when undisturbed low invasion cores are difficult to recover, for example in unconsolidated sands or sands with high water saturation. A complication in the use of NMR data is the appropriate pressure to use in the core desaturation step during determination of the T2 cutoff. Since capillary pressure varies with height in a reservoir, the T2 cutoff should also be determined as a function of height, which is not normally done. For the reservoir rocks in the B-Field, with a wide range in pore types and sizes, variation in fl-cutoff with capillary pressure must be accounted for when quantitatively interpreting NMR logs for hydrocarbon pore volume.