Tidal Current Energy

8
A. R. H. Goodwin
into blocks. A simulation of the reservoir requires of the order of 10 6 calls to a
package that calculates the thermophysical properties of the fluid and so the
methods chosen to estimate these properties must not contribute significantly
to the time required to perform the simulation. This requirement precludes, at
least for routine work, the use of intensive calculation methods that are based
on molecular models. Because of the requirement for simple correlations, for
a particular process, often over a limited temperature and pressure range, the
industry makes frequent utilization of both empirical and semi-empirical methods.
Typically, the seismic and logging measurements are repeated over the
production time of a reservoir and the parameters further adjusted to represent
the measurements obtained as a function of time in a process known within the
industry as history matching [25–27] .
Some of these measurement techniques are also used to monitor natural gas
storage facilities [28] , while others are used to monitor the plums of contaminated
groundwater within the vadose zone beneath Hanford, Washington, USA [29] .
Hanford, which was built on the banks of the Columbia River in the 1940s,
is where the first full-scale nuclear reactor was located for the production of
weapons-grade 239 Pu.
2.2 . Hydrocarbon types
Hydrocarbon reservoirs were formed by the thermogenic and also microbial
breakdown of organic matter known as kerogen that occurred over 10 6 a. When
the temperature of kerogen is increased to about 353 K oil is produced with,
in general, higher density oil obtained from lower temperatures; microbes are
operative for shallow and thus lower temperature oils, thereby also decreasing
the density. Kerogen catagenesis [30] is a reaction producing both hydrocarbon
and a mature kerogen. As the temperature to which the kerogen is exposed
increases as well as the exposure time the density of the hydrocarbon decreases
and at T � 413 K natural gas is produced. In general, kerogen experienced different
temperatures during burial and thus different types of hydrocarbons
were charged into reservoirs. Models to describe the formation of petroleum
reservoirs from kerogen catagenesis have been proposed by Stainforth [31] . Not
surprisingly, the types of hydrocarbon are as diverse in type as the formation in
which they are located.
The hydrocarbon accumulates in porous, permeable rock and migrates
upward in order of decreasing density, owing to faults, fractures and higher
permeable strata, until prevented by an impermeable barrier. The overriding
assumption is that the fluids do not mix and only in reservoirs that contain fluids
near their critical point does mixing occur solely by diffusion [32] . Recently,
Jones et al. [33] have suggested the biodegradation of subsurface crude oil
occurs through methanogenesis.
This chapter will focus on the hydrocarbon resource that can be solid, liquid
or gas rather than the reservoir which can be at temperatures from 270 to 500 K
and pressures up to 250 MPa with lithostatic and hydrostatic pressure gradients