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can we determine water saturation with the help of porosity logs only & with out the use of resistivity logs?

Dear Muhammad Musa,

Sorry for delayed response due to my business related engagements.
Hope you don't mind it.

This is little long answer and probably you would appreciate it.

Sw from porosity: Yes but you need to know some other information, it's all about saturation height functions.

If you have a function using porosity/permeability and a FWL from pressure data, then you can calcualte Sw easily.

If you think the zone is at Swi and you have a relationship between Swi and porosity/permeability (Pc core data), then you can calculate Sw

If you don't have any of the above.....then no Sw

Water saturation (Sw)can be determined by two ways 1) Direct measurement (from Core analysis)Retort method and Dean Stark method.

Indirect estimation of Sw can also be estimated by Logs and cores. Resistivity logs (Induction and laterologs) that need to be calibrated by core data, NML logs (reconciled by core), Pulsed Neutron Tools (reconciled by core. From core by means of capillary pressure.

NML logs directly measure the amount of hydrogen in formation and we know tht oil and water contain large hydrogen volume. the NMR signal responds only to the liquid filled porosity.
The decay rate of the NMR signal, called the relaxation, is related to the permeability in sandstone formations. By comparing the fast relaxation time (> 33 msecs) with the slow (< 33 msecs) relaxation time, the amount of moveable fluids in the pore space can be detected. This is known as the Free Fluid Index (FFI) and provides an estimate of the irreducible water saturation. The Bulk Volume Index (BVI) represents the volume of capillary bound water and is expressed as: BVI = (φNMR-FFI)

BVI is therefore equivalent to the volume of capillary bound water (Swirr). Since the tool only responds to effective porosity, the BVI provides a direct estimate of Swirr in shaly sands without the need to correct conventional density/resistivity tool data (e.g. Waxman-Smits, Dual Water Model) for shaly sand effects. (reference Chapter 9, Hydrocarbon saturation, Archie and waxman-Miths parameters in Senergy)




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Syed Tariq Hasany


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