Oil and Gas Terms Beginning with “O” — Page 3

The optical index is a quantitative measurement of the amount of light reflected by a fluid from the tip of an optical probe, and it corresponds directly to the relative refractive index of light between the probe… Read more →

What Is an Optical Probe in Formation Testing? An optical probe in wireline formation testing is a near-infrared absorption spectrometry module in the formation tester tool string (MDT, RCI, RDT) that analyses the… Read more →

An organic acid in petroleum engineering is a carbon-containing acid compound — most commonly acetic acid (CH₃COOH), formic acid (HCOOH), or citric acid — used as an alternative to or in combination with inorganic… Read more →

An organic deposit in oilfield production operations is a specific type of formation damage in which heavy hydrocarbons (paraffins, asphaltenes, and other heavy organic components of crude oil) precipitate from the… Read more →

Organophilic (from Greek "organo" meaning organic compound and "philia" meaning affinity or attraction) describes a material that preferentially associates with organic compounds, oils, and non-polar hydrophobic… Read more →

What Is Organophilic Clay? Organophilic clay is a modified smectite or hectorite clay mineral in which the naturally hydrophilic surface has been chemically altered by replacing the exchangeable inorganic sodium or… Read more →

Organophilic lignite is a chemically modified form of lignite, a soft brown coal rich in humic acids, that has been treated so it disperses in oil and synthetic base fluids rather than water, where it serves as a… Read more →

What Is an Orifice Meter? Orifice meter (also called an orifice plate meter or differential pressure meter) is a flow measurement device consisting of a thin plate with a precisely machined circular opening installed… Read more →

An orifice valve is a valve, usually installed on small-diameter pipework, that incorporates a fixed or selectable orifice or other flow-restriction device so that it controls the rate of fluid flow rather than simply… Read more →

Orogeny is a major episode of mountain building driven by plate tectonic processes, in which lithospheric plates collide, subduct, or slide past one another to deform, thicken, and uplift the continental crust into… Read more →

Osmosis is the spontaneous movement of water from one aqueous system to another through a semipermeable membrane that allows water molecules to pass while restricting larger ions and dissolved species — driven by… Read more →

Osmotic pressure in oil and gas operations is the pressure differential generated across a semipermeable membrane when two solutions of different chemical activity (or water activity) are separated by that membrane,… Read more →

An outcrop is a body of rock exposed at the surface of the Earth, where bedrock is visible directly rather than buried beneath soil, vegetation, water, or younger sediment. Outcrops occur naturally where erosion has… Read more →

Outside or outer diameter. Casing and tubing are commonly described in terms of inside diameter (ID) and outside diameter. Read more →

The ovality limit is the maximum permitted deviation of a coiled tubing string's cross-sectional profile from a perfect circle — specified as the maximum difference between the outer diameter measured at the widest… Read more →

What Is Overbalance? Overbalance (also called overbalanced pressure or positive pressure differential) is the positive difference between the hydrostatic pressure exerted by the drilling fluid column in the wellbore and… Read more →

(noun) The total thickness of rock and sediment overlying a formation of interest, from the surface (or mudline in offshore environments) to the top of that formation. Overburden stress, generated by the cumulative… Read more →

What Is an Overflush? An overflush is the final volume of fluid pumped during a scale inhibitor squeeze treatment or a chemical wellbore treatment that displaces the preceding chemical slug further into the formation… Read more →

Pertaining to a hydrocarbon source rock that has generated as much hydrocarbon as possible and is becoming thermally altered. Read more →

What Is Overpressure? Overpressure (also called abnormal pressure or geopressure) is a subsurface pore fluid pressure that exceeds the hydrostatic pressure expected for the depth of burial — the pressure a continuous… Read more →

What Is an Overriding Royalty Interest? An overriding royalty interest (ORRI) is a non-cost-bearing fractional interest in oil and gas production carved out of the working interest, entitling the holder to a percentage… Read more →

What Is an Overriding Royalty Interest? Overriding royalty interest (ORRI) (also called an override or production override) is a fractional share of gross production from an oil and gas lease that is carved out of the… Read more →

An overshot is a fishing tool run on a drill string or wireline into the wellbore and lowered over the outside of a fish (a stuck or lost downhole assembly) to grip it on its outer diameter, allowing the fish to be… Read more →

A thrust fault having a relatively large lateral displacement. Read more →

Overtravel is the distance a drilling jar mandrel continues to travel upward after the jar fires — the additional stroke that occurs as the released energy drives the mandrel past the impact anvil position and against… Read more →

What Is the Oxide Closure Model? The oxide closure model is a geochemical log interpretation technique that converts the elemental weight fractions measured by a pulsed neutron spectroscopy tool into mineral weight… Read more →

What Is Oxygen Activation? Oxygen activation is a nuclear logging technique that detects and quantifies water flow in or around a wellbore by using the electronic neutron generator of a pulsed neutron tool to activate… Read more →

An oxygen scavenger is a chemical additive injected into oilfield water systems, drilling fluids, completion brines, and injection water to remove dissolved oxygen before it can cause corrosion of steel pipelines,… Read more →