Resistivity logging: Difference between revisions
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{{short description|Method of identifying layers of rock in a borehole}} |
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{{More citations needed|date=December 2020}} |
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{{Well logging}} |
{{Well logging}} |
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'''Resistivity logging''' is a method of [[well logging]] that works by characterizing the rock or sediment in a [[borehole]] by measuring its [[resistivity|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- |
'''Resistivity logging''' is a method of [[well logging]] that works by characterizing the rock or sediment in a [[borehole]] by measuring its [[resistivity|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-terminal sensing|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. |
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Indeed, in the borehole fluids the electrical charge carriers are only [[ion]]s ([[cation]]s and [[anion]]s) present in [[aqueous solution]] in the fluid. In the absence of dissolved ions, water is a very poor electrical conductor. Indeed, pure water is very poorly dissociated by its [[Self-ionization of water|self-ionisation]] (at 25 °C, [[Self-ionization of water#Equilibrium constant|pK<sub>w</sub> = 14]], so at [[pH]] = 7, [H<sup>+</sup>] = [OH<sup>–</sup>] = 10<sup>−7</sup> mol/L) and thus water itself does not significantly contribute to conduct electricity in an aqueous solution. The resistivity of pure water at 25 °C is 18 MΩ·cm, or its conductivity (C = 1/R) is 0.055 μS/cm. The electrical charge carriers in aqueous solution are only ions and not [[electron]]s as in [[metal]]s. Most common minerals such as [[quartz]] ({{chem|Si|O|2}}) or [[calcite]] ({{chem|Ca|C|O|3}}) found respectively in siliceous and in carbonaceous formations are [[Insulator (electricity)|electrical insulators]]. In mineral exploration, some minerals are [[semi-conductor]]s, e.g., [[hematite]] ({{chem|Fe|2|O|3}}), [[magnetite]] ({{chem|Fe|3|O|4}}), and [[chalcopyrite]] ({{chem|Cu|Fe|S|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 [[hydrocarbon]]s like [[methane]] and light [[alkane]]s, the conductivity also drops and resistivity increases. |
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| ⚫ | Resistivity logging is |
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| ⚫ | Resistivity logging is used in [[mineral exploration]] (for example for exploration for [[iron]] and [[copper]] ore bodies), geological exploration ([[Deep geological repository|deep geological disposal]], [[geothermal energy|geothermal well]]s), and water-well drilling. It is an indispensable tool for [[formation evaluation]] in oil- and gas-well drilling. As mentioned here above, most rock materials are essentially [[Insulator (electricity)|electrical insulators]], while their enclosed fluids are [[Electrical conduction|electrical conductors]]. In contrast to aqueous solutions containing conducting [[ion]]s, [[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 bond|covalent]] nature of their [[chemical bond]]s. 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. |
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| ⚫ | Usually while drilling, [[drilling fluid invasion|drilling fluids invade]] the formation, changes in the resistivity are measured by the tool in the invaded zone. For this reason, several resistivity tools with different investigation lengths are used to measure the formation resistivity. |
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In geological exploration and water-[[well]] drilling, resistivity measurements also allows to distinguish the contrast between [[clay]] [[aquitard]] and [[sand]]y [[aquifer]] because of their difference in porosity, pore water conductivity and of the [[cation]]s ({{chem|Na|+}}, {{chem|K|+}}, {{chem|Ca|2+}} and {{chem|Mg|2+}}) present in the interlayer space of [[clay mineral]]s whose external [[electrical double layer]] is also much more developed than that of [[quartz]]. |
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| ⚫ | Usually while drilling, [[drilling fluid invasion|drilling fluids invade]] the formation, changes in the resistivity are measured by the tool in the invaded zone. For this reason, several resistivity tools with different investigation lengths are used to measure the formation resistivity. If water based [[drilling mud|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. |
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==See also== |
==See also== |
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* |
* {{annotated link|Archie's law}} |
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* {{annotated link|Drilling fluid|Drilling mud}} |
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* {{annotated link|Formation evaluation}} |
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* [[Formation evaluation#Electric logs|Electric logs (in: Formation evaluation)]] |
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* {{annotated link|Well logging}} |
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==References== |
==References== |
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{{Reflist |
{{Reflist| |
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<ref name="osti_Basi">{{Cite web |title=Basic exploration geophysics (Book). OSTI.GOV |author= |work=osti.gov |date= |osti= 6982729|access-date=13 December 2020 |url= https://www.osti.gov/biblio/6982729}}</ref> |
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<ref name="arch_AAPG">{{Cite web |title=AAPG Datapages/Archives: AAPG Methods in Exploration, No. 16, Chapter 1: Basic relationships of well log interpretation |author= |work=archives.datapages.com |date= |access-date=13 December 2020 |url= http://archives.datapages.com/data/specpubs/method16/me16ch01/me16ch01.htm}}</ref> |
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<ref name="onep_Tuto">{{Cite web |title=Tutorial: Introduction to resistivity principles for formation evaluation: A tutorial primer – OnePetro |author=OnePetro |work=onepetro.org |date= |access-date=13 December 2020 |url= https://www.onepetro.org/journal-paper/SPWLA-2019-v60n2t2}}</ref> |
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<ref name="onep_InSi">{{Cite web |title=In situ measurements of electrical resistivity, formation anisotropy, and tectonic context – OnePetro |author=OnePetro |work=onepetro.org |date= |access-date=13 December 2020 |url= https://www.onepetro.org/conference-paper/SPWLA-1990-M}}</ref> |
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<ref name="Liu2017a">{{cite book|last1=Liu|first1=Hongqi|title=Principles and Applications of Well Logging |chapter=Integrated interpretation of well logging data|series=Springer Mineralogy |year=2017|pages=289–323|issn=2366-1585|doi=10.1007/978-3-662-53383-3_10|isbn=978-3-662-53381-9 }}</ref> |
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<ref name="Liu2017b">{{cite book|last1=Liu|first1=Hongqi|title=Principles and Applications of Well Logging |chapter=Electrical Logging|year=2017|pages=9–58|doi=10.1007/978-3-662-54977-3_2|isbn=978-3-662-54976-6 }}</ref> |
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}} |
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==Further reading== |
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* Apparao, A. (1997). Developments in geoelectrical methods. Taylor & Francis. |
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[[Category:Well logging]] |
[[Category:Well logging]] |
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Latest revision as of 13:08, 4 July 2022
 | This article needs additional citations for verification. (December 2020) |
| Well logging methods |
|---|
Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its 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 ions (cations and anions) present in aqueous solution in the fluid. In the absence of dissolved ions, water is a very poor electrical conductor. Indeed, pure water is very poorly dissociated by its self-ionisation (at 25 °C, pKw = 14, so at pH = 7, [H+] = [OH–] = 10−7 mol/L) and thus water itself does not significantly contribute to conduct electricity in an aqueous solution. The resistivity of pure water at 25 °C is 18 MΩ·cm, or its conductivity (C = 1/R) is 0.055 μS/cm. The electrical charge carriers in aqueous solution are only ions and not electrons as in metals. Most common minerals such as quartz (SiO
2) or calcite (CaCO
3) found respectively in siliceous and in carbonaceous formations are electrical insulators. In mineral exploration, some minerals are 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.
Resistivity logging is used in mineral exploration (for example for exploration for iron and copper ore bodies), geological exploration (deep geological disposal, geothermal wells), and water-well drilling. It is an indispensable tool for formation evaluation in oil- and gas-well drilling. As mentioned here above, most rock materials are essentially electrical insulators, while their enclosed fluids are 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-well drilling, resistivity measurements also allows to distinguish the contrast between clay aquitard and sandy aquifer because of their difference in porosity, pore water conductivity and of the cations (Na+
, K+
, Ca2+
and Mg2+
) present in the interlayer space of clay minerals whose external electrical double layer is also much more developed than that of quartz.
Usually while drilling, drilling fluids invade the formation, changes in the resistivity are measured by the tool in the invaded zone. For this reason, several resistivity tools with different investigation lengths are used to measure the formation resistivity. If water based 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
[edit]- 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
[edit]- ^ "Basic exploration geophysics (Book). OSTI.GOV". osti.gov. 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.
- ^ Liu, Hongqi (2017). "Integrated interpretation of well logging data". Principles and Applications of Well Logging. Springer Mineralogy. pp. 289–323. doi:10.1007/978-3-662-53383-3_10. ISBN 978-3-662-53381-9. ISSN 2366-1585.
- ^ Liu, Hongqi (2017). "Electrical Logging". Principles and Applications of Well Logging. pp. 9–58. doi:10.1007/978-3-662-54977-3_2. ISBN 978-3-662-54976-6.
Further reading
[edit]- Apparao, A. (1997). Developments in geoelectrical methods. Taylor & Francis.