Oil and Gas Glossary

impermeable

Pertaining to a rock that is incapable of transmitting fluids because of low permeability. Shale has a high porosity, but its pores are small and disconnected, so it is relatively impermeable. Impermeable rocks are desirable sealing rocks or cap rocks for reservoirs because hydrocarbons cannot pass through them readily.

immiscible

Not capable of mixing or being permanently mixed (as oil and water).

inclination

Deviation angle. Measured in directional surveys and used in calculating true vertical depths. It is the angle between the axis of the measuring tool (hence, borehole axis) and true vertical. The inclination can also be measured with respect to true horizontal.

induction log

A log recorded in uncased boreholes which involves the use of electromagnetic induction principles for the measurement of formation conductivity or resistivity. The induction logging tool has advantages for use in nonconductive borehole fluids (air. oil gas) where other electrical resistivity logging tools cannot be easily used or should not be used. The induction log is widely used in electrically conductive drilling muds where it works well provided the formations are not too resistive and borehole effects are known and not too great (i.e., mud not too saline and hole diameter not too large).

injection well

A well into which fluids have been pumped, and from which fluids have been injected into an underground stratum to increase or maintain reservoir pressure.

in situ

In place. In its natural location.

invaded zone

The portion of formation surrounding a well bore into which drilling fluid has penetrated. displacing some of the formation fluids. This invasion takes place in porous, permeable zones when the pressure of the mud is greater than that of the formation fluids. A mud filter cake builds on the formation wall, limiting further invasion into the formation by mud filtrate. Directly behind the mud cake is a flushed zone from which almost all of the formation water and most of the hydrocarbons have been displaced by filtrate. The invasion process alters the distribution of saturations and other properties and, consequently, alters the values which are recorded on logs. The depth of invasion is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate. In oil-bearing zones, the filtrate may push a bank of formation water ahead of it to produce what is referred to as an annulus.

irreducible saturation

The irreducible saturation of a fluid is the minimum saturation of that fluid attainable when that fluid is displaced from a porous medium by another fluid immiscible with the first.

irreducible water saturation

The fraction of the pore volume occupied by water in a reservoir at maximum hydrocarbon saturation. In water-wet rock it represents the layer of adsorbed water coating solid surfaces and the pendular water around grain contacts and at pore throats. Irreducible water saturation is an equilibrium situation. It differs from "residual water saturation," measured by core analysis because of filtrate invasion and the gas expansion that occurs when the core is removed from the bottom of the hole and brought to the surface.

isopach

A line joining points of equal bed thickness