BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
The Shrinkage of oil is related to its changes in density, therefore, it is also related to the
composition, pressure and temperature of the fluid. Shrinkage is used to measure the change in
liquid volume in the reservoir from one pressure to another; for example, as the oil moves from the
reservoir to the surface tanks. Normally liquid volumes in a reservoir shrink to a lesser volume as
the reservoir pressure declines, except for the retrograde conditions in reservoirs.
Shrinkage is expressed in B O . B O is the formation volume factor. The B O factor is related to the
amount of oil in the reservoir and the amount in the tanks on surface.
Volume in reservoir
B O
=
Volume at surface
Classification of Oil
Crude oil chemistry is relatively complex with a typical crude oil containing several thousand
different compounds belonging to eighteen different hydrocarbon series. A complete chemical
analysis of crude oils, in terms of compounds present, is a difficult, if not impossible task. Less
complete types of analysis (e.g. by the amounts of lumped elements present) are often useful for
determining its physical characteristics.
Difficulty in classifying oils by the chemical composition of their constituents has led to
widespread use of simpler, less technical classifications. These classifications focus on
differentiating between paraffin base and asphalt base oils. The distinction between paraffin base
and asphalt base oils serves only as a broad classification. Most asphaltic oils contain traces of
solid paraffins, and most paraffin oils contain some asphaltic residue. Some oils, said to be of
“mixed base”, will respond to the tests for both paraffin and asphalt in equal degrees. The tests for
classifying oil are described below.
Paraffin base oils are oils in which paraffins predominate. When cooled to low temperatures, these
oils yield an appreciable amount of light colored wax that is not readily attacked by acids or
dissolved by ether chloroform, or carbon bisulphide.
Asphalt base oils are oils in which asphalt predominates. After slow distillation, these oils yield a
lustrous, solid residue, usually jet black in color, which exhibits a conchoidal fracture and which
dissolves in the previously mentioned solvents.
Often the only classification made on crude oils is made based upon its API° gravity.
Petroleum Reservoirs
Petroleum is produced from the pores between or within the grains of a rock. In order for a rock to
be a reservoir, it must contain sufficient porosity and permeability to permit commercial production
of hydrocarbons. Porosity is a measure of the percent of pores or voids which a rock contains. The
figure below illustrates the position of pores in a typical sandstone reservoir. Oil and gas occurs
within the pores or voids between the individual sand grains. Typical sandstone reservoirs contain
porosity values of 10 to 35%. Permeability is a measure of the ability of the pore fluid to move
from one void to another. Permeability can be envisioned as a “pipeline” between the pores.
The pore in a typical reservoir may contain gas and/or oil. However, all reservoirs contain varying
amounts of water. The water may occur as a thin film around the grains or it may be present as
droplets. Water free oil has been produced from reservoirs which contain up to 50% water.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-10