Page 53 - The Geological Interpretation of Well Logs

- RESISTIVITY AND CONDUCTIVITY LOGS -

carbons. This is still their principal quantitative use: it measures conductivity. A current is induced in the
Tesistivity logs furnish the basic numbers for petrophysi- formation around the borehole and the capacity to carry
cal calculations. However, a formation’s resistivity is one the current is observed. This carrying capacity is the con-
of its typical geophysical characteristics and as such can ductivity. The resistivity is simply the reciprocal of the
contribute information on lithology, texture, facies, over- conductivity. Thus in oilfield units:
pressure and source rock aspects.
1x 1000
The log is used frequently for correlation (Table 6.1).
resistivity (ohms m? /m) =
conductivity
Limitations
Resistivity tools (Section 6.4) can only function in bore-
As previously stated, rock materials are essentially
holes containing conductive muds, that is muds mixed
insulators (like all generalities this is a half-truth, and will]
with salt water. They cannot be mun in oil-based muds or
be modified later). However, normal rocks consist not
freshwater based muds. Induction logs (Section 6.5), on
just of rock materials, but also voids or pores. The pore
the contrary, are most effective with non-conductive
spaces are principally filled with water, in subsurface
muds, oil-based or fresh water based. However, induction
logs are also run in salt water based muds and are reason-
ably effective, although corrections to the raw readings
mairix formation water
may be necessary for quantitative use (Table 6.8). non-conductive conductive

6.2 Theoretical considerations
current current
Earth resistivity and conductivity
The laws which govern electrical resistance in a wired
circuit apply also to currents flowing in the earth.
electrical electrical
Resistance (in ohms) is the electrical term, while resistivity
is the logger’s term. Resistivity is in units of ohms m?/m,
being the resistance with normalized dimensions.
Two tests can be applied under subsurface conditions
to measure resistivity. The first test is a direct measure-
ment. A current ig passed between two electrodes on a
Figure 6.2 Formation conductivity - schematic. The electrical
logging tool and the potential drop between them pro- current is restricted to the formation fluids (formation water):
vides the resistivity. The second test is indirect in that the matrix is non-conductive.

Table 6.1 The principal uses of the resistivity and conductivity (induction) logs.

Discipline Used for Knowing

Quantitative Petrophysics Fluid saturations: Formation water resistivity (R,)
Formation (5,,) Mudc-filtrate resistivity (R,,)
Invaded zone (S_.) Porosity (#) (and F}
i.e. detect hydrocarbons Temperature (7)

Semu-quantitative Geology Textures Calibration with laboratory samples
and Qualitative
Lithology Mineral resistivities

Correlation —

Sedimentology Facies, Gross lithologies
Bedding characteristics

Reservoir geology Compaction, Normal pressure trends
overpressure and
shale porosity

Geochemistry Source rock identification Sonic and density log values
Source rock maturation Formation temperature

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