BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
• 3 million grains of sand (74 microns) in line would be about the size of the Astrodome (216
meters in diameter)
• A water molecule and most ions increased a million times would be the size of a grain of
sand
• A clay particle enlarged a million times would be like a piece of poster paper six inches to six
feet across
In terms of reactivity, clays can be classified as either swelling or non-swelling in nature. Regardless
of whether a clay will hydrate in water or not, the presence of clays still contribute to the viscosity, gel
strengths, and other properties of drilling fluids.
Structure
Clay minerals are hydrous aluminum silicates of a layer-type lattice structure (honeycomb) with
magnesium, iron, and potassium located either between the layers or substituted within the lattice.
The major exceptions to this are the attapulgite-sepiolite type clays which have a chain-type structure.
Basic components of clays are silica tetrahedrons and alumina octahedrons, arranged in a sheet-like
structure. These sheets are bound together by shared oxygen between the sheets.
Clays are classified into three major categories depending upon the arrangement of these structural
units. For example, if a clay has two silica tetrahedral layers and one alumina octahedral layer, it is
referred to as a 2:1 clay type. The structure is an alumina sheet sandwiched between two silica sheets.
Six or seven major clay types make up most of the clay minerals. The seven clay types can be divided
into two major groups – layered clays and chain-type clays.
The layered clays include smectites, illites, kaolinites, chlorites, vermiculites, and mixed-layer clays.
The chain-type clays are the attapulgite-sepiolite minerals. Since the smectites are the most important
in terms of drilling fluids, most of this discussion will center on these clay minerals. The other clay
minerals will be discussed only briefly.
Reference Manual
Baker Hughes Drilling Fluids
2-2 Revised 2006