BAKER HUGHES - Drilling Fluids Reference Manual

GLOSSARY OF TERMS
Chapter 15
GLOSSARY OF TERMS
The following is a glossary of terms associated with the drilling and development of wellbores.
Absorption - The penetration or apparent disappearance of molecules or ions of one or more
substances into the interior of a solid or liquid. For example, in hydrated bentonite, the planar water
that is held between the mica-like layers is the result of absorption.
Abnormal Pressure - A subsurface condition in which the pore pressure of a geologic formation
exceeds or is less than the expected, or normal, formation pressure. When impermeable rocks
such as shales are compacted rapidly, their pore fluids cannot always escape and must then support
the total overlying rock column, leading to abnormally high formation pressures. Excess pressure,
called overpressure or geopressure, can cause a well to blow out or become uncontrollable during
drilling. Severe under-pressure can cause the drillpipe to stick to the under-pressured formation.
Acid - Any chemical compound containing hydrogen capable of being replaced by positive elements
or radicals to form salts. In terms of the dissociation theory, it is a compound which, on dissociation
in solution, yields excess hydrogen ions. Acids lower the pH. Examples of acids or acidic substances
are hydrochloric acid, tannic acid, and sodium acid pyrophosphate.
Acidity - The relative acid strength of liquids as measured by pH. A pH value below 7. See pH.
Accretion - The mechanism by which partially hydrated cuttings stick to parts of the bottomhole
assembly and accumulate as a compacted, layered deposit.
Acid Gas - A gas that can form acidic solutions when mixed with water. The most common acid
gases are hydrogen sulfide [H 2 S] and carbon dioxide [CO 2 ] gases. Both gases cause corrosion;
hydrogen sulfide is extremely poisonous. Hydrogen sulfide and carbon dioxide gases are obtained
after a sweetening process applied to a sour gas.
Adhesion - The force which holds together unlike molecules.
Adsorption - A surface phenomenon exhibited by a solid (adsorbent) to hold or concentrate gases,
liquids, or dissolved substances (adsorptive) upon its surface, a property due to adhesion. For
example, water held to the outside surface of hydrated bentonite is Silicate or water.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 15-1