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 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 ions (

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 anions) 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 electrons as in metals. Most common minerals such as quartz () 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 An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...

. In mineral exploration, some minerals are semi-conductors, 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 (), and chalcopyrite () 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

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 (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

, 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

s (, , and ) present in the interlayer space of

clay mineral Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...

s whose external

electrical double layer A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...

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 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