1. COPRODUCTION OF GEOTHERMAL
POWER FROM OIL AND GAS FIELDS
by Youssef TLEMÇANI
OIL & GAS AND GEOTHERMAL ENERGYOIL & GAS AND GEOTHERMAL ENERGYOIL & GAS AND GEOTHERMAL ENERGYOIL & GAS AND GEOTHERMAL ENERGY

2. EXECUTIVE SUMMARY
ENHANCED GEOTHERMAL SYSTEMS (EGS)
CONTENT
CONTENT
ENHANCED GEOTHERMAL SYSTEMS (EGS)
RESERVOIR ENERGY PRODUCTION AND INJECTION POWER REQUIREMENT
RESERVOIR ENGINEERING APPROACH OF GEOTHERMAL SYSTEMS
CONCLUSION
BIBLIOGRAPHY

3. EXECUTIVE SUMMARY
Significant effort, by using reservoir is made to study the effect of reservoir properties (such as
permeability, fracture intensity and distributions, reservoir pressure and temperature…), and well
parameters (injection rate, injection pressure, production intervals..) on the geothermal energy
production. In the literature, most of these studies are performed on homogeneous models. They
contrast between the uses of CO2 and water as circulating fluids.
The aim of my project is to re-investigate, by simulations and thermodynamic calculations, the
effect of the parameters mentioned above on the energy produced. This started by generating 3D
aquifer model, and using STARS to run more than thirty sensitivities on the abovementioned
parameters for the case of CO2 and water injections. CO2 and water injection base cases were
assessed and compared economically. Then, a study on a heterogeneous reservoir model was
completed to compare the energy produced in cases of CO2 and water injection.
Results show that injection rate and initial reservoir temperature are the two common parameters
affecting the energy produced in CO2 and water injection cases. In addition to that, energy
produced from water injection is affected by injection temperature; whereas energy produced in
CO2 injection is also affected by reservoir permeability. The economic analysis that was restricted to
compare the cost of injection against the cost of energy produced shows that the case of water
injection in an aquifer is economically viable, whereas CO2 injection case is not. Finally, the
simulation on a heterogeneous model shows that fractures are crucial to extract geothermal energy
from the reservoir. In addition to that, the simulations on the Tea Pot Dome confirm the fact that
water injection allows a higher energy production than CO2 in a water reservoir.
Further analyses were recommended to validate these results and to investigate more in the
importance of fractures in EGS, and in areas of similar interest to this present study.

4. ENHANCED GEOTHERMAL SYSTEMS
(EGS)
Geothermal energy is stored in the Earth crust. Thermal energy is contained in the host rock and
natural fluid that at high temperatures. The thermal energy that is trapped in geothermal fluids
has been used as a source of energy at a small scale such as domestic power generation. At the
beginning of the 21st century, up to 10,000 MW of electric power and up to 100,000 MWt (thermal
power) have been generated from thermal sources.
The transport mechanisms of geothermal heats consist of:
• Upward convection and conduction of heat from the Earth mantle and core
• Heat generated by the decay of radioactive elements such as uranium, thorium and Potassium
In principle, EGS consists of circulating a
fluid through an injector to a producer.
The fluid is brought into contact with the
Concept of Enhanced Geothermal Systems
(https://energyscience.wordpress.com/category/renewableenergy/
geothermal-energy/)
The fluid is brought into contact with the
rock where a heat exchange takes place
between the injected fluid and the rocks
in subsurface. Then the fluid is pumped to
surface through the production well and
could serve as an input to a power plant
to generate electricity or to generate
thermal power. As shown in the figure
below, the cooled fluid produced by the
power plant is recycled by re-injecting it
into the reservoir.

5. RESERVOIR ENERGY PRODUCTION
AND INJECTION POWER REQUIREMENT
Increased downhole temperature is favorable for geothermal energy production. However, this will
result into a reduction of pressure increase with depth in the injection well. Therefore the pressure
drive that transport CO2 in the hot dry rock reservoir requires a high electrical power to maintain fluid
circulation. The similar effect happens in the production well. In fact, the isenthalpic decompression of
CO2 when the gas is flowing up the well leads to a temperature decline. The temperature drop is
stronger for smaller downhole pressure. Temperature decrease becomes smaller if the downhole
temperature is increased.
The temperature drop along the producer is desired to be reduced as much as possible. Therefore this
will also require high electrical power of the fluid circulation system. It can be conclude, that a trade-
off should be find between high power requirement for the fluid circulation system and the energy
production.

6. RESERVOIR ENGINEERING APPROACH OF GEOTHERMAL
SYSTEMS – HOMOGENEOUS WATER RESERVOIR
A simulation procedure is followed to study water
injection and CO2 injection in a ¼ five spot
homogeneous water reservoir model. The
simulation results will be used to analyze the
geothermal energy production rate according to the
different sensitivity tests.
The reservoir with the following dimensions 2000’ x
2000’ x 250’ represents the base case for the
simulation of water and CO2 injection. Up to thirty
scenarios were run to get a better understanding of
produced fluid temperature behavior, makeproduced fluid temperature behavior, make
predictions of the total energy produced and allow
final recommendations to be discussed. Homogeneous water reservoir model
Tornado Chart for CO2 injection case in a homogeneous water reservoirTornado Chart for water injection case in a homogeneous water reservoir

7. RESERVOIR ENGINEERING APPROACH OF GEOTHERMAL
SYSTEMS – HETEROGENEOUS WATER RESERVOIR
The Tea Pot dome, is the reservoir that was
targeted in this study because it is a heterogeneous
fractured reservoir. The Tea Pot Dome is originally
an oil reservoir. For the purpose of this study,
properties have been changed to generate 100 %
water saturation reservoir. The objective is to study
the energy produced according to three different
model properties that were generated from the
original Tea Pot model. Also, this study aims to
confirm the importance of fractures in EGS. The
three models that are generated incorporate two
scenarios each: water injection and CO2 injection.scenarios each: water injection and CO2 injection.
Tea Pot dome reservoir, 100 % water saturation
Energy produced by water injection from model 1 Energy produced by water injection from models 2 and 3
Energy produced by CO2 injection from model 1 Energy produced by CO2 injection from models 2 and 3
• Model 1: Fracture
permeability and porosity
• Model 2: Using matrix
permeability and porosity
• Model 3: Using the
arithmetic average and
porosity

8. CONCLUSION
According to the present study, we may draw the following conclusions on the most important
sensitivities:
• Comparing the base cases of water injection and CO2 injection in the homogeneous model
clearly shows that the cumulative energy produced by water injection is much higher than that
produced by CO2 injection (1098 MW for water injection against 72.038 MW for CO2 injection).
• The study shows that injecting at a high rate help to maintain a constant water production.
• The simulations show that a reasonable water injection allows constant water production for all
the period of simulation. However, very large volume of CO2 needs to be injected to maintain
constant water production.
• Initial reservoir temperature plays an important role in geothermal systems. High initial• Initial reservoir temperature plays an important role in geothermal systems. High initial
reservoir temperature allows high temperature fluid production; hence high thermal energy is
produced.
• Results show that injection temperature affects the time of thermal breakthrough at the
producer. The higher the injected temperature, the later the thermal breakthrough, hence the
larger the heat extracted.
• Sensitivities show that high permeable reservoir allows cold fluid to be transported faster to the
producer. Therefore the reservoir temperature drops quickly resulting in a low produced energy
rate.
• CO2 injection in high permeable reservoir shows in fact that the reservoir temperature stays
close to the initial reservoir temperature. However CO2 reaches the producer within few years,
resulting in heat energy drop earlier than in the water injection case.

9. FURTHER WORK
This section identifies the issues that are related to this study and that could be investigated
further:
• The present study assumes that reservoir temperature does not change with the reservoir
depth. A further study should consider the thermal gradient and look at its effect on the
produced energy.
• Further investigation on homogenous oil reservoir could consider again the cases of CO2 and
water injection. Same sensitivities discussed in this study should be used to study the change of
the reservoir temperature and the geothermal energy produced.
• In the economic analysis discussed above showed that geothermal energy extraction by
injecting CO2 in a water reservoir is not economical. An interesting scenario is to consider
geothermal energy extraction by injecting CO2 simultaneously with CO2 injection (CO2-EOR) ingeothermal energy extraction by injecting CO2 simultaneously with CO2 injection (CO2-EOR) in
an oil reservoir. This study should assess if this is economically and technically viable.
• During the CO2 circulation, the gas is not fully reproduced. Part of the injected CO2 is absorbed
by the formation (CO2 storage). Further investigation should study CO2 storage combined with
recovery of geothermal energy.
• At pressures and temperatures of interest, chemical interactions between CO2 the reservoir
rocks take place. A further study could look at the impact of these chemical reactions on the
flow and the heat transfer.
• Produced water from the subsurface could produce scale build up and corrosion, in the
production facilities as well as in the power plant components. Further studies could suggest
means to treat produced water, and production monitoring plans for the large volumes of water
that is produced in the surface.

10. Youssef TLEMÇANI
MSc. in Petroleum Engineering – Heriot Watt University
tlemcani.y@gmail.com
http://www.linkedin.com/in/tlemcaniyoussef