Geothermal System Modelling - Basic Model

Geothermal System Modelling

Report Submitted by Group Fuji

Basic Model

1.0       Introduction

The Basic Model  parameters (basicmodel.in) was used to calculate the transient behaviour of the hydrotherm system up to 100,000 years. Team Fuji analysed the calculation results in the numerical model by changing one of the parameters in the initial model and run the simulation using HYDOTHERM. In this case, the team changed the size of the heat source while keeping the other parameters constant in the model. The calculation results were run at 20000,40000,60000,80000 and 100000 years.

The physical modes of each scenario are demonstrated in the following model diagrams (Fig. 1-5) below. Heat Source is shown at the centre at 4km x 4km x 2km for the basic model which is represented in red cubical color. The size of the heat source is decreased by 3km x 3km x 2km and then increased to 6km x 6km x 2km in that order. Two different input file  with the different  sizes in X and Y direction  (heat source dimensions only) were run using  Jupiter post-processor (Hydrotherm program). After the simulation in the series of years mentioned above, temperature and flow variation were used to explain the trends in cooling rate of the heat source and temperature variation with time, corresponding analysis is illustrated in the discussion section.

Fig. 1 Heat source at the deeper layer
 of the model (2km thick) 

  Fig. 2 Section View of the initial
 model

  

Fig. 3 Overview of the initial block model

  Fig. 4 Section view of the block model when heat source decreased to 3km x 3km

    

Fig. 5 Sectional view of the block model when increasing the
 size of the heat source by 6km x 6km

                               

Note: everything else is kept constant except the size of heat source changed for the next two models.

2.0    Discussion

1.1 Heat source

The trend of the cooling equations (below) illustrate the differences in the thickness of the heat sources. Therefore, the larger the areal extent of the heat source is inverse proportional to the cooling rate.  The bigger the heat source, the longer it takes to for it to cool down.

Figure 6: Cooling rate of the heat source

The cooling equations for the model with 3kmx3kmx2km, 4kmx4kmx2km and 6kmx6kmx2km heat sources are shown below:

respectively.

1.2  Rate of cooling of the reservoir

The graph below portrays the cooling rate of the reservoir, approximately 1km above the heat source where the convective heat transfer currents are mostly upwelling.

4kmx4kmx2km heat source

Figure 7: Cooling rate of the reservoir

The reservoir cooling curves in Fig.7 above have near - similar trend except for the model with 6kmx6kmx2km heat source which has a kink upwelling at 40,000 years.

1.3 Interstitial steam and water flow

1.3.1        3kmx3kmx2km heat source model

At 20,000years, the hot water rises from the center of the model and travels upward towards the surface as interstitial water moves slowly to recharge the reservoir. At 40,000 years, the rising hot water together with the conduction heat transfer heats a larger area above the magma thus expanding the reservoir area (region in which hot water rises upward).  From 60,000 to 100,000 years, the model cools to below 200°C and convective currents carrying hot water upward weakens over time.

Figure 8: Simulation of 3km x 3km x 2km heat source after 20000 years.

1.3.2        6kmx6kmx2km heat source model

At 20,000years, we have two convective upflow regions which may form two reservoirs about 1km on either side of the center of the model (approx. 9000m and 11000m from LHS of the model).

At 40,000yrs, the two reservoirs merge into one as the heat source cools with convective currents weakening as the model ages all the way to 100,000years.

Figure 10: Simulation of 6km x 6km x 2km heat source after 40000 years.

3.0     Conclusion

In this study, only the heat source dimensions were varied without any change in other parameters.  The results were then evaluated and discussed using that assumption.

The areal extent of the heat sources directly influences the convective flow of fluids and temperature. However, transient temperature evaluation indicates that the rate of cooling of the heat source is inversely proportional to the size of the heat source. The larger size (6km x 6km x 2km) of the heat source allows for a longer period of high-temperature fluid convection. 

     

Figure 12 : 3X3 Heat source       Figure 11: 6X6 Heat source

Source: Groupwork Hydrotherm Basic Model Assignment Report -

Contributions to Group Fuji:

Islomove Sunnatullo-Rock Engineering, Koskey Philemon Kiprotich- Geothermics, Gilbert Bett Kipngetich-Geothermics, Gutierrez Donaire Kevin Yamil - Geothermics, Haissama Osmanali - Geothermics, Kuri Las - Rock Engineering, Lim Pagna-Economic Geology, Mwangi Samuel Muraguri -Geothermics, Ngethe John-Energy Resources, Omondi Philip Omollo-Geothermics, Samod Yuossouf Hassan - Economic Geology