Resistivity logging is a method of

well logging Well logging, also known as borehole logging is the practice of making a detailed record (a ''well log'') of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (' ...

that works by characterizing the rock or sediment in a

borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petro ...

by measuring its

electrical resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

. 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

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s (

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s and

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s) present in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

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, pK_w = 14, so at pH = 7, ⁺= H^–= 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

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s as in

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

s. Most common minerals such as

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

() or

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

() found respectively in siliceous and in carbonaceous formations are electrical insulators. In mineral exploration, some minerals are

semi-conductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

s, e.g.,

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

(),

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...

(), and

chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...

() 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 The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...

or gaseous

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

s like

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

and light

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...

s, the conductivity also drops and resistivity increases. Resistivity logging is used in

mineral exploration Mining in the engineering discipline is the extraction of minerals from underneath, open pit, above or on the ground. Mining engineering is associated with many other disciplines, such as mineral processing, exploration, excavation, geology, and ...

(for example for exploration for

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

and

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

ore bodies), geological exploration ( deep geological disposal, geothermal wells), and water-well drilling. It is an indispensable tool for

formation evaluation In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one". Modern rotary drilling usua ...

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 physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge ( electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gen ...

. In contrast to aqueous solutions containing conducting

fluids are almost infinitely resistive because they do not contain electrical charge carriers. Indeed, hydrocarbons does not dissociate in ions because of the

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

nature of their

chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...

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. In geological exploration and water-

well A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...

drilling, resistivity measurements also allows to distinguish the contrast between

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

aquitard An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

and

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

because of their difference in porosity, pore water conductivity and of the

s (, , and ) present in the interlayer space of clay minerals whose external electrical double layer is also much more developed than that of

. 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 A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...

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.

References

{{Reflist, {{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 {{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 {{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 {{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 {{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 {{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

See also

References

Further reading