BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Most of the prolific oil production and indeed most of the
giant oilfields are in sandstones. Sandstones generally exhibit
high primary permeability as well as secondary permeability
characteristics. For example, most of the oil and gas produced
in Russia is from clastic reservoir rocks. Much of the
production from the USA has also been from clastic reservoir
rocks. There are some notable exceptions. The Permian Basin
of the south-western U.S.A. is a carbonate (limestone)
reservoir as are the majority of the huge oilfields in the
Middle East.
Sandstones and limestones are two distinct rock types.
Compositionally, sandstone is formed through inorganic and
clastic processes. Erosion of land surfaces containing all types of existing rocks creates sediments
which are transported into a basin where compaction occurs, creating sandstone rock. Looking
closely at the sediments, one would see that it contains pebbles, sand grains, and other bits and
pieces of rocks. All the sediment of this kind is referred to as clastic rocks, meaning accumulated
particles of broken rock and of skeletal remains. The clastic materials are held together in the rock
by cement, generally silica.
Limestones (carbonates) are primarily made of the mineral calcite. They are the result of sediment
formed by precipitation of minerals from solution in water, either the result of biochemical
reactions or by inorganic chemical processes. Inorganic processes mean that calcite is precipitated
directly from water; small spheroidal grains, about the size of sand grains, called oolites are found
on the floor of oceans. They are composed of calcium carbonate (CaCO 3 ). Oolites found in
limestones mean that they were formed in ancient oceans. Cave deposits are also calcite, but they
are formed in a wet cave on land.
Biochemically formed limestones are created by the action of plant and animal life that extract
calcium carbonate (CaCO 3 ) from the water in which they live. The CaCO 3 may be either
incorporated into the skeleton of the organism or precipitated directly. Regardless of the
mechanism, when the organism dies, it leaves behind a quantity of CaCO 3 and over geologic time
thick deposits of this material build up. Reefs are examples of such accumulations.
Another precipitant is the mineral CaMg(CO 3 ) 2 ; it occurs in large concentrations and it forms a
rock called dolomite. It is not the result of direct precipitation in sea water, but rather is formed by
replacement of pre-existing deposits of calcite.
The manner in which the type of rock creating the reservoir rock was formed is important in
understanding the reservoir behavior. The manner in which they were derived creates essential
differences as it affects their fundamental properties of porosity and permeability. Primary
permeability is dependent upon the manner of formation. Sandstones generally have greater
primary permeability than limestone unless the latter is a reef, or similar, structure. Clastic
sedimentary rocks tend to be more porous because of the uniqueness of their origin. They are the
result of erosion and transportation; these two processes tend to cleanse the material of everything
but silica sand grains and they are often of similar size and shape. Carbonates on the other hand,
tend to have greater secondary permeability because they are more soluble and reactive. Fractures
and faults within carbonates (which are highly soluble) tend to enlarge in size and lateral extent
when water flows through these fractures and faults.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-11