Electrical Borehole geophysics

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There are many different geophysical investigation techniques, However when there is a need in exploring detailed information about the subsurface, we have to drill the hole and make a detailed collection of subsurface data, this is termed Borehole geophysics. This procedure involves the depth wise sequential record of physical parameters such as resistivity, density, temperature. It is sometimes known as Geophysical Logging or Geophysical well logging.

There are many different geophysical logging however this post will let you know about electrical logging which is commonly includes the resistivity log and self-potential (SP) log.

geologic log vs geophysical log

Geological Log:

This displays variation of subsurface geology (lithology/rock types) with depth. Such as from dry sand (top) to sand (middle) to clay (bottom).

Geophysical Log:

This displays variation of subsurface physical parameters with depth.

The term well log is more general that includes both geological and geophysical logs. It displays variation of physical parameters with depth such as resistivity, fluid conductivity, temperature, rock units.

Types of electrical logging

1. resistivity logging:

This measures the changes in electrical resistance between the lead electrode in the borehole and the fixed electrode at the surface. It deploys different electrodes spacing and configuration. It uses a simple tool device known as Single Point Resistance Device.

The formation resistivity is influenced by salinity and temperature of pore water, drilling mud

For granular formation

Rt = Rw/Qm

But Rt /Rw = F

F = 1/Qm

Where, Rt - Formation resistivity, Rw - water resistivity, Q - porosity function, m - cementation factor, F - Formation resistivity factor, F >1

Note: Cementation factor (m), varies from 1.3 - 1.6 for Non cemented sands and 1.7 - 2.0 for sandstone, 2.2 - 2.6 for densier formation (Guyod, 1952)

Applications of Resistivity Logging

1. It provides information for distinguishing different types of lithologies example sand versus clay (shale)

Sand: high resistivities, it produces a log deflection to the right

Fine grained deposits (clay /shale): They have much less resistivities, it produces a log deflection to the left. See the figure below.

Drawbacks for Resistivity Logging

1. It does not provide quantitative measurements of resistivity, hence it can only be suitable for the petroleum industry.

2. self- potential (sp) logging:

This records depth-wise natural electric potential (mV) of a point in a borehole with respect to a fixed point on the surface.

Self-potential is caused by electrochemical effect between dissimilar layers, electro-kinetic effects produced by movement of borehole fluid through permeable beds.

Usefulness of Self - Potential Logging

1. It used to measure potential difference (p.d) develops in boreholes at the contacts between clay (also shale) beds and sands (also sandstones)

Potential at shale-Line (contact) = 0

Deflection at right from shale-Line = Positive (+ve)

Deflection at left from shale-line = Negative (- ve)

Extreme, negative (-ve) deflection = shale-line

Negative (-ve), means pore (formation) water is more saline than the drilling muds.

Positive (+ve), means drilling mud is more saline than pore (formation) water.

SP = - Klog Rmf/Rw

Where, SP - Is static SP , if it is obtained from SP curve when bed is clean, thick and porous permeable, K - coefficient expressed  by (+64.3 + 0.239T), Rmf - measured resistivity of mud filtrate and Rw - resistivity of formation water

Applications of Self-Potential (SP) Logging

1. In identifying geologic units

2. In correlating units between boreholes

3. It can estimate the resistivity of formation water when the resistivity and Temperature of drilling mud are known, this is mainly used in water quality investigation.

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