第29回日本情報地質学会講演会 講演要旨集 013-014頁 2018年

Simulation of Vapor and Liquid Reservoir Evolutionin Geothermal System
Nurita Putri Hardiani* and Katsuaki Koike**Graduate School of Engineering, Kyoto University, Katsura C1–2–215, Kyoto 615–8540, Japan.

E–mail: koike.katsuaki.5x@kyoto–u.ac.jp (K. Koike)Key words : Geothermal systems, 3D Tough2 Simulation, Vapor-dominated, Permeabilitystructure

1. Introduction
Occurrence of geothermal systems, in which
vapor-dominated conditions are found, are less frequent
than liquid-dominated conditions. However, how vaporconditions may evolve in geothermal systems is worth toinvestigate.
Thewell-known
conceptual
model
illustrating how the vapor-dominated conditions areformed in the natural state are tested using the 3Dmodel TOUGH2. The model represents a system in amoderate
topografic
reliefwith an extensivenear-vaporstatic vapor-dominated zone and a possiblelack significant liquid reservoir throughflow underneaththe vapor-dominated zone. This condition may be
developed due to increasing heat or decreasing
permeability from the upper boundary which causes
more water to be boiled off than is replaced by recharge.
The purpose of this study is to build a numerical modelin order to understand the development of vapor andliquid reservoir in geothermal systems based on thedescription of the proposed well-known geothermal
conceptual models.

2. Model Development
2.1. Model Preparation
Before a numerical model being set up, a conceptualmodel must be developed. The numerical model wasconstructed based on the conceptual model. TOUGH2
computer code was used to carry out the natural state
simulation (Pruess et al., 1999). In model preparation,
the rock properties such as permeability, rock density,
specific heat capacity were assigned to each block inevery layers in the model. Initial and boundary
conditions were also defined according to measured data.
In order to have a reliable model which closely
represents the real condition of reservoir, a model must
be validated through the matching process between
observed data and simulation result. This process
involves an iterative process which requires running themodels to steady state until a good matching oragreement between the simulation result and theobserved data obtained.
2.2. Grid system and layer
The model was constructed with size of 4000 x 4000 x
3000 cubic meters which is divided into 8000 cells. In

vertical, the model is divided into 20 layers, whilelaterally each layer is divided into 20 x 20 cells.
The entire model domain has a density of 2600 kg/m3,
porosity of 10%, thermal conductivity of 2.5 W/mK, and a
specific heat of 1 kJ/kg.
2.3. Initial and boundary conditions
The initial condition of the model is fully liquidsaturated at a normal geotherm (30°C/km) withhydrostatic pressure.
During the period of time, the boundary condition has
to be set up to maintain the system inside the reservoir
which allows the system to be equilibrated.
The side boundary was set to be conductive heat andmass flow with relatively tight permeability. The initialstate of the simulations is a normal thermal gradientand a hydrostatic pressure gradient.
A number of grids at the bottom layer was set as heatsource with very high temperature and huge volume
factor to indicate the magnitude of intense heat source
which drives the pressure temperature profile in theupper layers.

Fig. 1: Illustration of the model.

The illustration of the model configuration is as shownin Fig. 1. At the top is the surficial layer represents thesurface layer underlying caprock. The reservoir
permeability is set to be fixed at 10-13 m2. While theother permeability values were assigned using trial anderror method.

3. Results and Discussion
As mentioned earlier, only permeability value forreservoir is set to be fixed while others were assigned
using trial and error approach to be able to replicate aprobable geothermal system condition. After severaltimes simulation, the configuration of permeability

14settings for cap rock, side, and bottom boundary is 10-16
m2, while for surficial, reservoir, and heat source is 10-13
m 2.
To understand the process happening in a geothermal
system, the model was run for a long period of time. We
run a series of simulation. Each stage of simulation
series is run for 10 kyr. The choice of 10 kyr for theduration of the high basal heat flow phase wasinfluenced by the time for vapor-dominated conditions toform as stated in Raharjo et al. (2016).
In the natural-state, the simulation should be carriedout over long period of time. This suggests that thesimulation in natural state condition requires running
the simulation until it reaches steady state condition.
Even though there is no rule as to how long is the time
to be set to create natural state condition, somemodellers tend to set the simulation time as long as forexample 300,000 years. But, since the aim of this study
is not to understand the condition after it reaches theequilbrium, instead we use a series of simulation time inorder to understand the evolution of geothermal systemin every slice of time.
aThe development of heat body becomes less and less innear boundary and more intense in the center of themodel. The temperature becomes hotter after 20,000
years suggesting more intense heat added into thereservoir.
While, the vapor formation cannot be seen clearly after
10 kyr (Fig. 4) suggesting that additional time needed tocreate such condition. However, two phase condition at
the top of the reservoir can be formed after 10 kyr. After~20 kyr, vapor starts to appear and develop at the top of
the reservoir as shown in Fig. 5.

Fig. 4: Vapor (left) and liquid (right) reservoir development after
10.000 years simulation time.

bcdFig. 5: Vapor (left) and liquid (right) reservoir development after
20.000 years simulation time.

4. Conclusion

Fig. 2: Temperature distribution development result after
10,000 years simulation time. a = sliced in the middle part ofthe model (x = y = 2000 meter), b = at 2500 meter, c = at 3000
meter, d = at 4000 meter (boundary).

abcdFig. 3: Temperature distribution development result after
20,000 years simulation time. a = sliced in the middle part ofthe model (x = y = 2000 meter), b = at 2500 meter, c = at 3000
meter, d = at 4000 meter (boundary).

As shown in Figs. 2 and 3, the temperature growth canbe seen after 10 and 20 kyrs. The temperature
distribution is also strongly influenced by the location.

It has been simulated the evolution of vapor and liquid
reservoir formation in a geothermal system. As thesimulation time increases, it can be seen the growing of
the heat in the entire model and vapor formation at the
top of the model.
Such condition can be achieved in a geological settings
where reservoir is overlain by low permeability value
body (cap rock) as indicated by the conceptual model.
The formation of vapor is detected after 20 kyr
suggesting that with certain geological properties as
used in the model, additional heat and time is needed tomake the evolution of geothermal system from liquid
reservoir to two-phase and finally vapor reservoir at the
top of the model.
It is also found that the convergence issue in TOUGH2
is the main challenge to simulate a behaviour of themodel. Even though the model seems to be a simple
generic model, the issue cannot be easily overcome.
Sometimes it can take many hours only to solve theconvergence issue. Therefore, for the modellers, theconvergence issue has been more challenging than thesimulation itself.

References
Pruess, K., C. Oldenburg, and G. Moridis. (1999). TOUGH2
user’s guide. Version 2.0. Rep. LBNL-43134. Lawrence
Berkeley Natl. Lab., Berkeley, CA.
Raharjo I.B., Allis R.G., and Chapman D.S. (2016).
Volcano-hosted vapor-dominated geothermal systems inpermeability space. Geothermics, v. 62, 22–32.