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 as Borehole or downhole 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 techniques however this post is going to describe briefly about electrical borehole logging.
Electrical Borehole Logging
What is electrical borehole Logging?
Electrical borehole logging measures the flow of electric current in and adjacent to a well, using the same principles as various surface electrical methods.
Types of Electrical Borehole Logging
1. resistivity logging
1.1 Single-Point Resistance Logging
This measures the resistance in ohms between an electrode in a well and an electrode at the land surface, or between two electrodes in a well. 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.
Single point Resistance Logging is excellent for information about changes in lithology; not influenced by bed thickness effects. However it cannot be used for quantitative interpretation of porosity and salinity.
1.2 Normal Resistivity Logging
This measures the resistance by using four electrodes at various spacings on a single probe that is lowered down the hole. If the electrode spacing is short, then it is termed as Short Normal Logging while if the electrode spacing is wide, it is termed as Long Normal Logging.
1.3 Focused Resistivity Logging (Guard Log / Laterolog)
This uses the guard electrodes above and below the current electrode to force the current to flow out into the rocks surrounding the borehole.
Focused Resistivity Logging is designed to measure the resistivity of thin beds or resistive rocks in wells containing conductive fluids. However it is not generally available to water well loggers.
1.4 Lateral Resistivity Logging
This is similar to normal-resistivity electrodes, but electrodes are more widely spaced on the probe.
Lateral Resistivity Logging is designed to measure resistivity of rock farther out from the borehole. It is suitable only for thick beds (> 40 feet) and marginal for highly resistive rocks.
1.5 Microresistivity Logging (Microlog)
This uses numerous variations that all have short electrode spacing and pads or some kind of contact electrode to the effect of borehole fluid.
Microresistivity Logging is designed mainly to determine the presence or absence of mudcake. It is used primarily by the petroleum industry to determine the resistivity of the 3 to 5 inch zone affected by drilling muds.
Effective Resistivity in sedimentary rocks
Archie's Law
It used predominantly in borehole geophysical logging
Resistivity ρ = aϕ-mS-nρw,
Where ρ and ρw are effective rock and pore water resistivity respectively
ϕ - Porosity
Constants: 0.5 ≤ a ⩽ 2.5 and 1.3 ≤ m ⩽ 2.5 while n = 2
S - volume fraction of pores with water.
Formation factor , F = ρ/ρw
For saline ground water - ρ < 0.05 Ω⋅m
For glacial melt water and some groundwater - ρ > 1000 Ω⋅m
The formation Resistivity is influenced by salinity and temperature of pore water, drilling mud
Note: Cementation factor (m), can vary 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
Lithology (Rock types) characterization
Resistivity Logging provides information for distinguishing different types of lithologies example sand versus clay
Coarse grained material such as Sand displays high resistivity so it produces a log deflection to the right.
Fine grained deposits such as clay provide much less resistivity, so it produces a log deflection to the left, as shown in the figure 1.
Figure 1: A simple Resistivity Log trace
2. self- potential (sp) logging
This 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 effects between dissimilar layers, electro-kinetic effects produced by movement of borehole fluid through permeable beds. The formation and origin of self- potential is the same as that described on previous Self-potential post.
Since Self-potential is used to measure potential difference 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
- Identifying geologic units
- Widely used in the petroleum industry for determining lithology, bed thickness, and salinity of formation water. However it is generally not applicable for freshwater aquifers.
- Correlating units between boreholes
- Estimation of Resistivity of formation water when resistivity and temperature of drilling mud are known, this is mainly used in water quality investigation in hydrogeology.
3. Fluid conductivity
This uses a probe that records only the electrical conductivity of the borehole fluids by placing electrodes inside a protective housing.
Fluid conductivity Logging provides data related to the salinity (concentration of dissolved solids in the borehole fluid). It is used to locate sources of saltwater leaking into artesian wells, It can aid in interpretation of electric logs.
It is widely used in ground-water hydrology, primarily to determine water quality. However quantitative interpretations require corrections for bed thickness, borehole diameter, and other factors.
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References
Collier, H.A. (1989a). A Guide to Selecting the Proper Borehole Resistivity Logging Suite. In: Proc. (2nd) Symp. on the Application of Geophysics to Engineering and Environmental Problems, Society of Engineering and Mineral Exploration Geophysicists, Golden, CO, pp. 310-325.
Emilsson, G.R. and R.A. Arnott. (1991). In-Situ Specific Conductivity Monitoring on an Observation Well During a Long Term Pumping Test. In Ground Water Management 5: 533 - 547 (5th NOAC). [conductivity - Temperature probe]
Moran, J.H. and R. Chemali. (1985). Focused Resistivity Logs. Developments in Geophysical Exploration Methods 6: 225 - 260.
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