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

2. Outline Electrical resistivity
Factors affecting electrical resistivity
Archie’s Law
Factors affecting the cementation exponent
Modified Archie’s equation
Conclusions

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

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

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

6. Archie’s 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’

7. 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

8. Limitations of Archie’s 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

9. Modified Archie’s equations
oil/gas increases resistivity
Clay presence increases conductivity

10. 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

12. 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.

13. 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.