In the presence of shaly-sands and low salinity reservoirs the O&G industry commonly makes use of two equations (Indonesia and Waxman-Smits) for the determination of fluid saturation. These equations convert electrical conductivity of rock measured by logs into a corresponding water saturation value using a couple of parameters ("M" & "N") that must be determined in the laboratory. Inaccuracies in the determination of such parameters may result in significant errors when estimating water saturation. Unlike the Waxman-Smits equation, the "M" & "N" parameters of the Indonesia equation are never reported in SCAL reports, because their derivation is complex and not a standard process. In shaly sands, in absence of the Indonesia parameters, petrophysicists are often tempted to replace them with the Archie parameters, but in doing so there is a certain risk of introducing errors in the log interpretation process. In order to remedy this lack of information in SCAL reports, we have developed a procedure to obtain the Indonesia parameters from conventional laboratory measurements. The procedure is easy to apply and the benefit in applying it may be considerable. In addition to the standard electrical measurements needed for the Archie parameters derivation (water conductivity, formation factor and resistivity index curve), the procedure makes use of basic mineralogical information (from X-Ray Diffraction) and cation exchange capacity (CEC) data. We will discuss how the procedure works, we will show the main physics behind it and present the first application on a real case (an indonesian field) in which the comparison between the water saturation logs, determined using the Indonesia and the Archie parameters, shows a more optimistic result for the water saturation coming from the Indonesia "M" & "N".
The Archie equation (Archie, 1942) provides a good description of the electrical behaviour of hydrocarbon bearing and water-saturated clean sands. In shaly sands containing dispersed clay minerals, the Archie equation, under certain conditions, predicts erroneously high values of in situ water saturations. This failure of the Archie equation stems from neglecting "clay conductance" effects, whereby clay minerals provide an additional mechanism for the conduction of electrical currents.
