Oil & Gas Terms in Category I

Igneous rock

A rock mass formed by the solidification of material poured (when molten) into the earth’s crust or onto its surface.

Granite is an igneous rock.

Inland barge rig

A drilling structure consisting of a barge upon which the drilling equipment is constructed.

When moved from one location to another, the barge floats.

When stationed on the drill site, the barge can be anchored in the floating mode or submerged to rest on the bottom.

Typically, inland barge rigs are used to drill wells in marshes, shallow inland bays, and areas where the water covering the drill site in not too deep.

Also called swamp barge.

See floating offshore drilling rig.

Inhibited acid

An acid that has been chemically treated before the acidizing or acid fracturing of a well to lessen its corrosive effect on the tubular goods and yet maintain its effectiveness.

See acid fracture, acidize.


Preventing the passage of fluid.

A formation may be porous yet impermeable if there is an absence of connecting passages between the voids within it.

See permeability.

Initial shut-in period

The comparatively short shut-in period following the initial flow period of a drillstem test.

This period is followed immediately by much longer flow and shut-in periods to allow the pressure to closely approach initial reservoir pressure.

The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour.

When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure.


1.a dial gauge used on the rig to measure the hookload.


Substances in acid-base that, in solution, change color or become colorless as the hydrogen ion concentration reaches a definite value, these values varying with the indicator.

In other titrations, such as chloride, hardness, and other determinations, these substances change color at the end of the reaction.

Common indicators are phenolphthalein, and potassium chromate.

Impression block

A block with lead or another relatively soft material on its bottom.

It is made up of drill pipe or tubing at the surface, run into a well, and set down on the object that has been lost in the well.

The block is retrieved and the impression is examined.

The impression is a mirror image of the top of the fish and indicates the fish’s position in the hole, i.e., whether it is centered or off to one side.

From this information, the correct fishing tool can be selected.

Inside blowout preventer

Any valve installed in the drill stem to prevent a blowout through the stem.

Mud can be pumped in but flow back up the stem is prevented.

Also called an internal blowout preventer.

Inclination (wellbore)

The measurement of a wells deviation from vertical.

When used with fluids, a positive number indicates upflow and a negative number may represent downflow.


An intrusion of formation fluids into the borehole, i.e., a kick.


International association of drilling contractors



Installation / workover control system

find more info about iwocs at hydrafit-subsea.co.uk

In situ combustion

The setting afire of some portion of the reservoir in order that the gases produced by combustion will drive oil ahead of it to the producing wells.

Impression tool

A lead-filled cylindrical device used to ascertain the shape of a fish.

Insulating flange

A flange equipped with plastic pieces to separate its metal parts, thus preventing the flow of electric current.

Insulating flanges are often used in cathodic protection systems to prevent electrolytic corrosion and are sometimes installed when a flow line is being attached to a wellhead.