3.2 Reservoir Fluids
Keywords: organic compounds, alkanes, isomers, olefins, aromatics, refining, standard
conditions, API gravity, gas:oil ratio, dry gas, wet gas, condensate, volatile oil, black oil,
phase behaviour, bubble point, dew point, isothermal depletion, compressibility, gas
recycling, density, viscosity, Wobbe index, hydrate, oil formation volume factor, PVT
analysis, overburden pressure, abnormal pressures, surface tension, wettability, free
water level, transition zone.
Introduction and Commercial Application: This section introduces the various types of
hydrocarbons which are commonly exploited in oil and gas field developments. The
initial distribution of the fluids in the reservoir must be described to be able to estimate
the hydrocarbons initially in place (HCIIP) in the reservoir. The relationship between
the subsurface volume of HCIIP and the equivalent surface volume is important in
estimating the stock tank oil initially in place (STOIIP) and the gas initially in place
(GIIP). The basic chemistry and physical properties of the fluid types are used to
differentiate the behaviour of the fluids under producing conditions. For the petroleum
and process engineers, a representative description of the reservoir fluid type is important
to predict how the fluid properties will change with pressure and temperature and is
essential for the correct design of the surface processing facilities. Looking further
downstream, the chemical engineer would be concerned about the composition of the
hydrocarbon fluids to determine the yields of various fractions which may be achieved.
3.2.1 Hydrocarbon chemistry
The fluids contained within petroleum accumulations are mixtures of organic compounds,
which are mostly hydrocarbons (molecules composed of hydrogen and carbon atoms),
but may also include sulphur, nitrogen, oxygen and metal compounds. This section will
concentrate on the hydrocarbons, but will explain the significance of the other compounds
in the processing of the fluids.
Petroleum fluids vary significantly in appearance, from gases, through clear liquids with
the appearance of lighter fuel, to thick black, almost solid liquids. In terms of weight
percent of crude oil, for example, the carbon element represents 84-87%, the hydrogen
element 11-14%, and the other elements typically less than 1%. Despite this fairly
narrow range of weight percent of the carbon and hydrogen elements, crude oil can
vary from a light brown liquid with a viscosity close to that of water, to a very high
viscosity tar-like fluid.
The diversity of the appearance is due to the many ways in which the carbon atoms are
able to bond to each other, from single carbon atoms to molecules containing hundreds
of carbon atoms linked together in linear chains, to cyclic arrangements of carbon atoms.
It is the ability of carbon molecules to combine together in long chains (catenate) which
makes organic (i.e. carbon containing) compounds far more numerous than those of
other elements, and the basis of living matter.
The various arrangements of carbon atoms can be categorised into "series', which
describe a common molecular structure. The series are based on four main categories
which refer to
the arrangement of the carbon molecules
- open chain (which may be straight chain or branched)
- cyclic (or ring)
the bonds between the carbon molecules
- saturated (or single) bond
- unsaturated (or multiple) bond
The alkanes
The largest series is that of the alkanes or paraffins, which are open chain molecules
with saturated bonds, and have the general formula CnH2n+2.
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