Page 19 - Drilling Technology in Nontechnical Language

10 Drilling Technology in Nontechnical Language Second Edition

Within a sandstone structure, clay minerals might also be found. Clays
cause serious problems in a reservoir if they react with water (perhaps

from the drilling fluid) because they can block the flow of hydrocarbons
to the well.
Strongly cemented sandstones can be quite abrasive, leading to high
rates of wear on the drill bit and other downhole components.
Sandstones make up about 11% of all sedimentary rocks.

For a sandstone (or other rock) to be capable of acting as a reservoir,
there are two vital physical properties that it must have: porosity
and permeability.
Porosity is a measure of the percentage of the volume of the rock that

is occupied by fluids. In figure 1–7, the white spaces are marked as “Pore

spaces in between grains.” This sandstone therefore has porosity.

Permeability is a measure of the ability of fluids to flow through the
rock. For this to happen, the pore spaces must be connected in such a way

that fluids can flow along the pore spaces. It is possible for a rock to have

porosity but be impermeable (shales are an example). It is not possible for
a rock to be permeable but have no porosity.

Carbonates

Carbonates are composed of fossilized skeletons and mineral grains
of calcite (crystals of calcium carbonate; chemical formula CaCO ).
3
Crystalline limestone is very common, and its texture can be seen by
rotating the rock and observing the light reflected off the numerous crystal

faces. The crystals are soft enough to be scratched by a knife.
Fossiliferous limestone is similar to crystalline limestone except that
it contains fossil fragments, usually composed of calcium carbonate. Most
limestones are fossiliferous.
Carbonates are often fractured due to their brittle nature. Fractured

carbonates make prolific reservoir rocks, as the oil and gas can collect in
the fractures. Since the permeability of fractures is very high, if a well
intersects many fractures, it can produce oil at very high rates. However,
drilling through fractured carbonates can cause large volumes of drilling
fluid to be lost into the formation. Sometimes these formations have to

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