Resistivity logging
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| Well logging methods |
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Resistivity logging is a method of
electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current. In these logs, resistivity is measured using four electrical probes
to eliminate the resistance of the contact leads. The log must run in holes containing electrically conductive mud or water, i.e., with enough ions present in the drilling fluid.
Indeed, in the borehole fluids the electrical charge carriers are only
semi-conductors, e.g., hematite (Fe
2O
3), magnetite (Fe
3O
4), and chalcopyrite (CuFeS
2) and when present in sufficiently large quantities in the ore body can affect the resistivity of the host formation. However, in most common cases (oil and gas drilling, water-well drilling), the solid mineral phases do not contribute to the electrical conductivity: electricity is carried by ions in solution in the pore water or in the water filling the cracks of hard rocks. If the pores of the rock are not saturated by water but also contains gases such as air above the water table or gaseous hydrocarbons like methane and light alkanes
, the conductivity also drops and resistivity increases.
2O
3), magnetite (Fe
3O
4), and chalcopyrite (CuFeS
2) and when present in sufficiently large quantities in the ore body can affect the resistivity of the host formation. However, in most common cases (oil and gas drilling, water-well drilling), the solid mineral phases do not contribute to the electrical conductivity: electricity is carried by ions in solution in the pore water or in the water filling the cracks of hard rocks. If the pores of the rock are not saturated by water but also contains gases such as air above the water table or gaseous hydrocarbons like methane and light alkanes
Resistivity logging is used in
electrical conductors. In contrast to aqueous solutions containing conducting ions, hydrocarbon fluids are almost infinitely resistive because they do not contain electrical charge carriers. Indeed, hydrocarbons does not dissociate in ions because of the covalent nature of their chemical bonds
. When a formation is porous and contains salty water, the overall resistivity will be low. When the formation contains hydrocarbon, or has a very low porosity, its resistivity will be high. High resistivity values may indicate a hydrocarbon bearing formation.
In geological exploration and water-
electrical double layer is also much more developed than that of quartz
.
Usually while drilling,
mud
is used and oil is displaced, "deeper" resistivity logs (or those of the "intact zone" sufficiently away from the borehole disturbed zone) will show lower conductivity than the invaded zone. If oil based mud is used and water is displaced, deeper logs will show higher conductivity than the invaded zone. This provides not only an indication of the fluids present, but also, at least qualitatively, whether the formation is permeable or not.
See also
- Archie's law – Relationship between the electrical conductivity of a rock to its porosity
- Drilling mud – Aid for drilling boreholes into the ground
- Formation evaluation – Assessing if boreholes drilled for oil or gas are able to deliver a profitable production
- Electric logs (in: Formation evaluation)
- Well logging – Detailed record of borehole contents
References
- OSTI 6982729. Retrieved 13 December 2020.
- ^ "AAPG Datapages/Archives: AAPG Methods in Exploration, No. 16, Chapter 1: Basic relationships of well log interpretation". archives.datapages.com. Retrieved 13 December 2020.
- ^ OnePetro. "Tutorial: Introduction to resistivity principles for formation evaluation: A tutorial primer – OnePetro". onepetro.org. Retrieved 13 December 2020.
- ^ OnePetro. "In situ measurements of electrical resistivity, formation anisotropy, and tectonic context – OnePetro". onepetro.org. Retrieved 13 December 2020.
- ISSN 2366-1585.
- ISBN 978-3-662-54976-6.
Further reading
- Apparao, A. (1997). Developments in geoelectrical methods. Taylor & Francis.