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Electrical resistivity measurements and their uses in marine soils. Manju Ravi Sharma 12/09/08

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Outline Electrical resistivity Factors affecting electrical resistivity Archies Law Factors affecting the cementation exponent Modified Archies equation Conclusions

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Introduction Electrical resistivity; a geophysical method = R. (A/L)

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Factors affecting resistivity Soil type Frequency of the current Degree of saturation & ionic concentration Hydrocarbon saturation temperature

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Uses of resistivity measurements Resistivity of soil related to –Compressive strength –Elastic modulus –Density –Degree of Saturation –Max past pressure –Hydraulic conductivity

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Archies Law Law relating Formation factor FF and porosity FF = Ro/Rw= a. -m FF describes pore geometry;removes the effect of pore fluid resistivity Values of the reservoir constant a and the cementation exponent m

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Factors affecting m Shape of grain; angularity increases m Type of grain; clay high m, sand low m Tortuosity; higher tortuosity higher m Anisotropy Overburden pressure; higher pressure flattens the grains and angularity increases

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Limitations of Archies eqn F= a/ m Archie assumed a=1 and m empirically found as 2. For = 0, F=Q The equation does not consider water saturation effect or the shale effect

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Modified Archies equations oil/gas increases resistivity Clay presence increases conductivity

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Use of Formation Factor From the FF, porosity can be estimated, which can be used to predict behavior of soil under loading. water saturation Sw is needed for locating the potential oil wells

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Conclusions Though the values of a and m influence the values of the estimated porosity, it has been now agreed that the water saturation Sw exerts more significant influence on the values of porosity and also the formation factor. Research should focus on methods for accurate estimation of Sw.

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Conclusions The clay presence reduces resistivity causing overestimation of Sw. This may result in oil companies missing oil wells. Accuracy of Resistivity measurements is questionable. Inclined penetration of the probe combined with strong porosity gradients result in spurious high resistivity.

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