08 - Electrical properties

Question 1

Ohm’s law:

Resistivity

Answer 1

Res = U’/I

how strongly a material opposes the flow of electric current

Question 2

measurement resistivity

slide 2

Answer 2

cylindrical rock sample

2 electrodes for electric current I measurement

voltage U measurement

R_t = A/L * U/I

Question 3

Dielectric permittivity or absolute permittivity epsilon

Answer 3

measure of polarizibility

conductive and dielectric properties are coupled complex and are frequency-dependent

Question 4

only effective conductor among pore fluids

Answer 4

water

Question 5

resistivity of water is controlled by .. (5)

Answer 5

chemical composition
concentration
Temperature of brine/electrolyte

Volume fraction (porosity,saturation)
electrolyte distribution in rock

Question 6

correlation of resisticity to salinity

Answer 6

decrease with higher salinity

Question 7

resistivity in rocks

order from low to hight

sandstone, limestone, shale&clay, gneiss etc.

Answer 7

shale,clay
sandstone
limestone
gneiss etc.

Question 8

Resistivity log SP

Answer 8

electrical resisticity

in ohm m

Question 9

Spontaneus Potential log

Answer 9

natural or spontaneous potential difference between
the borehole and the surface

in mV

detect permeable beds and to estimate clay content and formation water salinity

Question 10

cationic membranes

Answer 10

permeable to positively charged ions (cations) and impermeable to negative ions (anions)

Question 11

mud-filtrate salinity is greater than the formation water salinity

Answer 11

positive SP deflections

Question 12

salinities of the mud filtrate and formation water are similar

Answer 12

no SP is generated

Question 13

Shale-baseline shifts

Answer 13

can occur when formation waters of different salinities are separated by a shale bed that is not a perfect cationic membrane

Question 14

resisitivity depends on lithology:

Evaporates
Carbonates
Shales
Sandstones

Answer 14

Evaporates: high
Carbonates: high
Shales: medium
Sandstones: low

Question 15

effect of geological processes on resistivity

Answer 15

All processes that enhance the amount of water in the rock likely decrease the bulk resistivity of the rock

clay alteration, faulting, salt, shearing, etc

Question 16

hot dry rocks

vs.

water bearing rocks

Answer 16

hot dry rocks:
meinly temperature dependent -> exp. decrease with increasing T

water bearing rocks:
mainly pressure dependent -> increase with increasing p

Question 17

Microresistivity logs

measures..

Answer 17

measure the resistivity of the flushed zone and delineate permeable beds by detecting the presence of mudcake

Question 18

Microresistivity logs

how is it measured..

Answer 18

Microlog pad is pressed against the borehole wall by arms and springs

pad has three small in-line electrodes spaced 1 inch apart

invaded permeable zone, Rmc is usually significantly lower than in the invaded zone Rxo