Electrical resistivity measurements and their uses

in marine soils.

Manju Ravi Sharma

12/09/08

Outline

• Electrical resistivity

• Factors affecting electrical resistivity

• Archie’s Law

• Factors affecting the cementation exponent

• Modified Archie’s equation

• Conclusions

Introduction

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

Factors affecting resistivity

• Soil type

• Frequency of the current

• Degree of saturation & ionic concentration

• Hydrocarbon saturation

• temperature

Uses of resistivity measurements

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

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’

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

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

Modified Archie’s equations

• oil/gas increases resistivity

• Clay presence increases conductivity

n

Tmw

w R

aRS

/1

1

RTSw

n Sw

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

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.

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.