Page 178 - Geothermal Energy Renewable Energy and The Environment

164 Geothermal Energy: Renewable Energy and the Environment

0.35 3.5
Fluid velocity
0.30 (dia. = 0.2 m) 3.0

0.25 2.5
Pipe diameter (m) 0.20 2.0 Flow velocity (m/s)

0.15
Pipe diameter 1.5
0.10 (vel. = 2.0 m/s) 1.0

0.05 0.5
400 450 500 550 600 650
Flash depth (m)

FIGUre 9.9 The dependency of the flash depth on the well diameter and flow rate. The curves are calculated
allowing only the indicated variable to evolve.

where P is the reservoir pressure, P is the bottom hole pressure, and m is the mass flow rate. The
R
v
B
P , P , and m are determined by conducting flow tests of the well.
R
B
v
Once the conditions in the reservoir are known, the pressure at which the geothermal fluid will
flash to steam can be readily determined from data on the thermodynamic properties of water. For
the systems we are considering (water temperature of 235°C), the pressure at which flashing will
occur, P , is 3.97 MPa (39.7 bars). The distance above the top of the reservoir at which flashing will
f
occur, X , can then be computed using
f
2
X = (P – P – C × m)/[(ρ × g) + ( Γ × m )] (9.12)
f
f
R
D
and Γ is defined as
Γ ≣ (32 × f)/(ρ × π × D ). (9.13)
2
5
The depth below the surface is then computed as X – X . Figure 9.9 shows how the flash horizon
R
f
depth varies as a function of well diameter and mass flow rate.
The relationship between well diameter and depth of flashing is intuitively reasonable. Changes in
well diameter affect the factor Γ through an inverse relationship to the fifth power. Hence, increases
in well diameter will strongly diminish Γ, resulting in an increase of X and a decrease in the depth
f
below the surface. The effects from increasing fluid velocity at a fixed well diameter are more com-
plex, in that fluid velocity is a factor in determining the magnitude of the Reynolds number, draw-
down coefficient, and friction factors, all of which figure directly in the flash horizon expression.
The net effect is that increasing the fluid velocity increases the mass flux, resulting in an increase in
the depth at which flashing will occur.
Whether flashing will occur in the well, or between the wellhead and the turbine is determined
by the site characteristics and the management strategy for the reservoir. Particularly important in
this regard is the rate of pumping and its influence on reservoir quality and lifetime. In either case,
however, the quality of steam that is available for useful work strongly influences the overall gen-
erating capacity and will be an important consideration when determining how best to manage the
well flow and flashing conditions.