dc.description.abstract | The basic equations which govern the flow of fluids in a geothermal
reservoir (often a two-phase mixture of steam. and water) are outlined. The
governing nonlinear system of partial differential equations that arises is
solved by numerical methods which involve replacement of both the spatial
and time derivatives by their finite difference equivalents. The equations
that result are nonlinear and are linearized using the Newton-Raphson
procedure. These equations with their solutions are applied to the Olkaria
geot hernial reservoir. Two significant variables, namely pressure and
enthalpy change are modelled. Both variables are investigated for the
present producing reservoir of Olkaria East field and the anticipated extension
to the Olkaria North East field which is to have an installed capacity
of 64MvVe. In order to examine these variables by modelling, two
models are set up i.e the natural state model of the field before exploitation
and a production model which gives the response of the reservoir to
exploitation for electricity generation. The solutions to the models are
generated by the MULKOM computer program. The main aim of carrying
out this study is to model exploitation of Olkaria East field with a
capacity of 45MWe extension of exploitation to the Olkaria North East
field with a capacity of 64MvVe, and to assess if Olkaria North East field
can sustain the said capacity for a period of 20 years. The natural state
model which occupied a surface area of '~l2..km. x 22km. had heat inputs
at the base of some of the reservoir blocks. When this model is run over a
long period of time, the simulated pressure and temperature, to a reasonable
degree of accuracy, matches the observed values at any given location
in the field. The actual process of simulation includes inserting heat of a
certain magnitude at specific locations at the bottom of the reservoir to
represent magmatic bodies. The system is then heated over a long period
of time until the temperature and pressure resemble those we observe in
the field by measurements. The production model uses these parameters
together with reservoir enthalpy as starting information. Each reservoir
block was assigned specific values of pressure, temperature, enthalpy etc.
As withdrawal of fluids from the reservoir for electricity generation commences.
these parameters change with time. These changes a.re calculated
by the !\IULEOi\I program which enables us to find the values of these
parameters at. any location and time in the reservoir. Production using a
45MWe power station from Olkaria East field was simulated for 14 years of
production from 1981 to 1995. The production was then run for another
20 years from 1995 to the year 2015, this time with an extension to 011<3.-
ria Nort.h East field with a station capacity of 64MvVe. The future status
of the reservoir in terms of fluid enthalpy and pressure changes were predieted
for the period 1995 - 2015. The results of this simulation indicate
that <:1 pressure drop of 15 bars is likely to occur over the latter production
period. The average production enthalpy is predicted to range from
1350k.J/kg in the year 1995 to 1780k.J/kg in the year 2015. The above
enthalpy range shows that the target of 64MWe power generation for 20
years may be met. without the enthalpy "rising too high( an indication of
the increase ill vapor-phase in the reservoir). However, a drop of 15 bars
in reservoir pressure may call for drilling of extra make-up wells to sustain
the 64MV/e power generation for 20 years. | en |