There are many different geophysical investigation techniques, However when it comes to exploring detailed information about the subsurface, we need to drill the hole and make a detailed collection of subsurface data, this is termed Borehole geophysics. It involves the depth wise sequential record of physical parameters such as resistivity, density, temperature. It may sometimes be known as Geophysical Logging. There are many different geophysical logging however today I want to let you know about electrical logging which includes resistivity Log and Self-Potential (SP) Log.
geologic log vs geophysical log
Geological Log: It displays variation of subsurface geology (lithology/rock types) with depth. Such as from dry sand (top) to sand (middle) to clay (bottom).
Geophysical Log: It displays variation of subsurface physical parameters with depth.
Well Log: It is a more general term that includes both geological and geophysical logs.It displays how various parameters such as resistivity, fluid conductivity, temperature, rock units vary with depth.
resistivity logging: it measures changes in 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.
Formation Resistivity: It 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)
Useful
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
It does not provide quantitative measurements of resistivity, hence it can be suitable for the petroleum industry.
self- potential (sp) logging: It records depthwise 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.
Useful
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 SP-Logging
- In identifying geologic units
- In correlating units between boreholes
- 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.
Thanks for reading! In the next posts I will describe to you other types of borehole geophysical techniques. Stay tuned!
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