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It ensues from previous chapters that electrical resistivity
of a rock depends mainly on quantity of water in a unit volume of rock,
and on the quality of this water.These factors are taken into account
in an extremely important experimental law established for saturated media:
It is the "Archie law" (Archie 1942)
which links the resistivity of the rock, the porosity, the nature of distribution
and the resistivity of the electrolyte.
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Resitivity of the imbibition water =
porosity- a = factor which depends of the lithology
and that varies between 0.6 and 2 (a < 1 for rocks with intergranular
porosity and a > 1 for rocks with fractured porosity)
- m = cementation factor (It depends of the
pores shape, of the compaction and varies between 1,3 for unconsolidated
sands to 2,2 for cimented limestone.
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| Generally the parameters are known
as Formation Factor |
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Pratically the following relationships are generally
accepted:
| For unconsolidated formations
(sandstones, sands) : Humble's formula |
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| For well cemented and consolidated
formations |
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Download graph 3 |
| Archie's expression for saturated
rock then becomes : |
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