Fig 8:Formation and mud resistivity
The invasion diameter calculated for different layers of the same lithology are very consistent.
Furthermore, it is also reasonable to assume that the calculated invasion diameter is reliable when the formation resistivity and the invaded zone resistivity are reliable because Russian interpretation results are interdependent.
Tab. 1: Invasion diameter values Mud resistivity = 0.03 ?m Invasion diameter (m) 2.4 3.4 2.3 2.7 2.4 2.6 12 8 1 2 5 1.5
Layer thickness (m) The calculated invasion diameters are generally much greater than the typical values found for western wells. This is one of the reasons way interpretation based on laterolog measurements, like the western methodology, can not provide consistent results. In fact, the investigation depth of laterolog tools is smaller than the calculated invasion diameter and, therefore, the simulated resistivity profiles are not representative of the true formation resistivity.
The water saturation values are calculated on the basis of the true formation resistivity and porosity measurements. In the Russian countries the neutron-gamma tool, NGK, and the sonic tool, AK, provide the most reliable porosity measurements. In the case of the examined wells the porosity profile from the sonic tool were consistent with the porosity values measured on cores whereas the neutron-gamma tool overestimated the actual formation porosity (fig 9). This can be explained by considering that the neutron-gamma measurements are related to total formation porosity, while sonic measurements are related to the rock primary porosity only.
Fig 9:Porosity profiles from logs and porosity values from cores
Water saturation values were calculated using Archie’s law which is suitable for carbonate rocks in the absence of shales. Due to the lack of special core analyses standard values for the Archie’s law parameters in carbonate lithologies were assumed (a=1, m=2, n=2).
Comparison between water saturation values calculated as a function of porosity from neutron-gamma (fig. 10) and sonic measurements (fig. 11), respectively, show that consistent values are obtained on the basis of resistivity evaluated from modeling and Russian interpretation. Conversely, water saturation profiles obtained on the basis of resistivity evaluated by western interpretation did not prove reliable. Water saturation values calculated as a function of sonic porosity measurements seemed more
consistent with well testing with respect to saturation values calculated as a function of neutron-gamma porosity. In any case the application of Archie’s law requires reliable porosity measurements because it was verified that water saturation is affected more by uncertainties in porosity measurements than by formation resistivity. Furthermore, true formation resistivity squared profiles are not coherent with the adopted porosity curves. In fact, vertical resolution of resistivity and porosity measurements are not the same, and the calculated water saturation is often influenced by porosity variation even if the true formation resistivity does not change.
Finally, it was observed that the combination of the true formation resistivity evaluated with the western interpretation methodology and the porosity measurements from the neutron gamma ray tools lead to a water saturation profile which appeared locally consistent with well testing results. However, it must be emphasized that such agreement was purely casual, and due to a combination of errors (fig. 12).
Fig 10: Water saturation values as a function of porosity measured by neutron gamma tool.
Fig 11: Water saturation values as a function of porosity measured by sonic tool.
Fig 12: Comparison between different water saturation profiles.
Fig 12: Comparison between different water saturation profiles.
CONCLUSIONS
Results clearly showed that interpretation of the Russian resistivity logs according to the western methodology can not provide reliable true formation resistivity profiles, both due to the differences
existing between Russian and western tool configurations and to an unexpectedly deep mud invasion in the formation. Since the western interpretation approach is based on laterolog measurements the simulated resistivity profiles are not representative of the true formation resistivity because the instrument investigation depth is generally smaller than the calculated invasion diameters.
The resistivity values obtained by application of the Russian manual methodology and by numerical simulations are sufficiently consistent. The Russian methodology is very ingenious although extremely
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complex and very time consuming. The Rt-MOD software is very simple, only requires few data to run log interpretation, and quickly performs simulations.
The true formation resistivity values obtained with the Russian approach are reliable only for thick layers (more than 2 meter-thick) whereas results obtained from the software application are fairly consistent for any layer thickness. However, calculated mud resistivity values are not always consistent with the reported fluid property and when the simulated value differs significantly from the reported
measurement, the obtained interpretation results might be questionable. Although the layer sequence reproduced by the software may not be representative or the real formation, resistivity results seem to be consistent with the results obtained with the Russian interpretation.
Finally, it was verified that also porosity measurements can significantly affect water saturation and, therefore, the porosity curve to adopt for calculation of water saturation should be accurately selected. In fact, combination of erroneous true formation resistivity and porosity profiles might even lead to a locally consistent water saturation profile.
ACKNOWLEDGEMENTS
The authors are very grateful to Petroleum Software Technologies for providing access to the software
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RtMOD used in this research.
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