Glossary - Letter D

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The decarbonization of a country’s energy system involves reducing its greenhouse gas emissions (mainly CO2 and methane). It can be achieved by improving energy efficiency (engine efficiency, building insulation, etc.), gradually replacing fossil fuels with low-carbon alternatives (nuclear and renewables), and promoting more responsible individual behaviors.

Deoxyribonucleic Acid (DNA)

DNA, packaged in chromosomes, is a long molecule in the shape of a double helix. It contains all the genetic information required for a living organism to develop and function.


In the oil industry, depletion corresponds to the gradual decline in production from an oil or gas well. Depletion is signaled by lower pressure in the well than before it was brought on stream. It is also indicated by a steady decline in output under unchanged operation conditions.


An accumulation of natural resources, such as oil, natural gas, coal, uranium, metal ore or another commodity, found at various depths below ground. Also called a field.


Operation that removes most of the sulfur from flue gas, smoke, sour natural gas and refined petroleum products. Also referred to as sulfur recovery. For petroleum products, the process can be carried out at high pressure and high temperature using hydrogen feed. The hydrogen atoms replace the sulfur in the hydrocarbon molecules. Sulfur is naturally present in all fossil fuels, in varying concentrations.

Development (Deposit)

The exploitation stage for an oil or gas deposit. Once a deposit has been discovered (exploration stage) and assessed (appraisal stage), a decision is made to develop it if it is shown to be economically viable, or commercial. Developing a deposit is often extremely costly.


Diesel is the name of an internal combustion engine that works by compression-ignition, invented by the German engineer Rudolf Diesel between 1893 and 1897. By extension, the term diesel, also called gas oil, refers to the specific fuel such engines use. In the hydrocarbon classification system, diesel is a light fuel oil.


Biofuel obtained by converting plant or animal oils, such as rapeseed, sunflower or used cooking oil. Also referred to as biodiesel. It can be used in existing diesel engines when blended into conventional diesel in concentrations of 7 to 30%. In France, diester refers specifically to fatty acid methyl esters (FAME), sometimes also known as vegetable oil methyl esters (VOME), biofuel obtained by the reaction of vegetable oils and methanol.


The solid material remaining after methanation. It consists of non-degraded organic matter and minerals such as ammonium and potassium salts and phosphates. Digestate can be used as a fertilizer, sometimes spread directly, more often after being dried or processed into granules.

Dimethyl Ether (DME)

Dimethyl ether (DME), or methoxymethane, can be substituted for diesel fuel and is far less polluting. It can also be used as an LPG additive. Currently being evaluated, it can be synthesized from coal, natural gas and especially biomass (bio-DME), which makes it more environmentally friendly than the conventional diesel produced by oil refineries. DME is a gas under normal pressure and temperature, which means that it has to be liquefied for transportation and storage, like propane or butane. Chemically, DME is the simplest of the ether oxides.

Directional Drilling

The intentional deviation of a wellbore from the vertical. Despite being more expensive than vertical drilling, this practice is used for economic reasons. For example, multiple wells can be drilled in a single cluster or from a single location, such as an offshore rig; this reduces the number of rigs required and saves money.

Distillate Hydrocracker (DHC)

Unit that performs a refining process in which hydrogen is used to convert certain cuts (fractions) obtained through crude oil distillation into lighter hydrocarbons with an ultra-low sulfur content — mainly diesel, naphtha and kerosene.

Diversion Channel

An artificial channel built to reduce excess water flow, particularly during flooding, by diverting some of this flow to a lake or the sea or to a purpose-built waterway. Diversion channels used to carry water to the turbines in hydropower plants are known as penstocks.

Documented Energy Consumption

The amount of energy that is actually used in a home or building, calculated based on bills. This information can be used to establish an Energy Performance Certificate (EPC).

Double Well Installation (Geothermal)

There are several installation designs used for geothermal energy, among them, the geothermal double well installation (sometimes referred to as a "doublet"). The first well extracts hot water from a geothermal reservoir. A heat exchanger then transfers the geothermal heat to a ground loop that circulates water through a building's heating pipes. The cooled geothermal water is injected back into the reservoir via the second well. This system prevents geothermal water from being discharged above ground; it contains sodium chloride, sulfides and other salts that can cause corrosion and be harmful to the environment.

Drill Bit

Drilling tool that crushes or cuts rock to increase the depth of the borehole. Drill bits consist of three rotating cones (roller cones) with teeth made of very hard steel, tungsten carbide, or even synthetic or industrial diamonds for cutting through very hard rock. Drill bits grind, cut, scrape, crush or pound the rock. Drilling fluid is expelled through jets in the drill bit to clean it and carry cuttings to the surface via the annulus between the wellbore and the drill string.

Drill Pipe

Nine-meter-long base element of the drill string, used to drill several kilometers into the ground. As the borehole gets deeper, new pipes are attached to the previous ones.

Drill String

A column of pipes that connects the drill bit to the surface during drilling.


The process of boring a hole into the ground using special equipment. Drilling is used in exploration to collect direct data about the subsurface, locate hydrocarbons and survey aquifers. It is also used to be able to produce oil, gas and water.

Drilling Mud

Drilling mud is the liquid that circulates from top to bottom inside drill pipes and back from bottom to top in the space between the wellbore and the drill string (known as the annulus) when oil wells are drilled. The drilling mud: lifts the cuttings to the surface, ensures borehole stability by containing formation pressures thanks to its density, protects reservoir formations from drilling fluid invasion by building a mud cake, and helps break apart rock as it is expelled from the drill bit at high pressure.