BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
Chapter 2
Successful drilling requires knowledge of the formation type into which the borehole
is drilled. The characteristics of that formation, i.e., geological composition, liquid or
gas content, pressure, permeability, porosity, reactivity, hydration, etc. will help
determine the type of fluid to be used in drilling and completing the wellbore.
Clay and Clay Minerals
The importance of clay and clay minerals in the drilling industry is evident from two viewpoints:
• Commercially mined clays are added to drilling fluids to build viscosity, thixotropy, and to
contribute to wall building properties.
• Clay minerals are present in nearly all sedimentary rocks.
• Clay reaction to the drilling process can produce major problems such as,
• Hole enlargement
• Sloughing
• Hydration
• Rheological problems.
The magnitude of the potential problems is apparent when one realizes that more than half of
sedimentary rocks are composed of shales, siltstones, mudstones, etc. These rock types are composed
of more than 50% clay minerals. Even sandstones may have as much as 20% to 30% clay minerals.
The better understanding one has of clays and clay minerals, the greater the control over problems that
may develop in the drilling process.
Definitions / Terminology
Clays can be defined according to physical properties, chemical properties, size, structure, and
reactivity in water. One definition cannot adequately describe all types of clays. A simple definition
used by many people is that clay is a finely ground, earthy material that can be molded into shapes
when wet.
Geologists and mineralogists usually define clays according to the chemical and mineral content that
make up the clay. Elements that commonly make up clays are oxygen, silicon, aluminum, calcium,
magnesium and potassium. Non-clay minerals generally found are quartz, feldspar, pyrite, and calcite.
Clays are also defined according to their size. The American Petroleum Institute (API) defines clays
as being those particles that are two microns or less in size. A micron is equal to one millionth of a
meter. In order to grasp the concept of this size, it is sometimes easier if the sizes are scaled up a
million-fold. For example:
• A million yard sticks end to end would stretch from Houston to El Paso
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 2-1