Carbon Dioxide potential use to boost Geothermal Power Generation.

Conventional geothermal power plants works on a water based system, using hot water in underground reservoirs to produce electricity. (Brown, 2000) suggested that the rate of geothermal energy production using super critical CO2 as a heat extraction fluid would be about 60% that of water based system. The concept of using CO2 as the fluid for hydro fracturing the reservoir (reservoir creation) and heat extraction can help in solving energy and global warming problems.

The concept of Sequestered Carbon dioxide (SCCO2)-HDR suggested by Brown (2000) is a novel approach for increasing the efficiency of a hot dry rock production (also known as Enhanced Geothermal system EGS) and the sequestration of CO2 in a deep reservoir. In the SCCO2-HDR concept supercritical CO2 acts as a heat transport fluid, the heat contained in SCCO2 is then transferred to the secondary fluid which drives an expansion turbine in a binary cycle to produce power. Working on the concept of (Brown, 2000), (Pruess, 2010) studied the operation of enhanced geothermal systems (EGS) with CO2. (Pruess, 2010) numerical analysis concludes that CO2 would achieve a more favorable heat extraction rate than water and will also avoid unfavorable rock fluid interactions that can be encountered in water based system.

As (Brown, 2000) and (Pruess, 2010) focused their studies on Enhanced Geothermal Systems (EGS) but the draw back in the EGS process is that it may induce seismicity when the critical fracture stresses of geological formation are exceeded during hydro fracturing, so (Randolph & Saar, 2011) instead of using hydro fracturing, used the existing reservoir with high permeability and porosity for his study, His approach is known as CO2-plume geothermal system (CPG). (Salimi & Wolf, 2012) came up with another concept of co-injecting CO2-water mixture in the porous reservoir and gave one possible numerical solution for this kind of problem. This concept uses the extended gas saturation to numerically overcome the problem of phase appearance and disappearance. In their work they analyzed the effect of reservoir characterization (permeability and porosity heterogeneity) on the heat extraction and CO2 storage.

 (Buscheck, Chen, Sun, Hao, & Elliot, 2012) Introduced a hybrid two-stage energy recovery approach to sequestrate CO2 and produce geothermal energy. The hybrid two stage approach is carried out in the two steps. In the first step brine as a heat extraction fluid can also provide pressure relief for CO2 injection. The produced brine is used for fresh water production through desalination or is used as a working fluid for a neighboring reservoir. The second step begins when CO2 reaches the production well, from this time the coproduced brine and CO2 act as working fluids.

FEATURES AND BENEFITS OF GEOTHERMAL POWER GENERATION SYSTEM WITH CARBON DIOXIDE SEQUESTRATION:

• Geothermal power generation is environmentally friendly, uses renewable resources, and provides reliable energy

• Carbon dioxide has a larger heat extraction efficiency and better subsurface fluid mobility compared to water

• Uses CO2 gases accumulated from carbon dioxide sequestration and could use CO2 produced in existing coal or gas fired power plants for additional revenue

• Avoids hydrofracturing which is a potential earthquake risk in current water-based geoexchange systems

• Geothermal power plant could be used in conjunction with CO2-based enhanced oil recovery systems by allowing recovery of heat energy along with the recovery of oil

• Allows for geothermal power generation in lower temperature areas not suitable for water-based systems

Future Possibilities.

1.     SCCO2 as working fluid in EGS system is more favorable then water, with an anticipated thermal performance approximately 60% that of a water-based EGS system for equivalent operating conditions ( (Brown, 2000)).

2.     In SSCO2-EGS system the rock-fluid interactions may also be more favorable for than with water, but little information is available about chemical interactions on high temperatures between supercritical CO2 and rock minerals (so it has to be still studied in detail. (Spycher & Pruess, 2010).

3.     In SSCO2-EGS system there is a benefit of CO2 sequestration which makes it economically more feasible then water-EGS system.

4.     (Pruess, 2010) showed that the linear well arrangement has different heat extraction then typical 5 well arrangement, so more studies can be done to see the effect of well arrangement on the heat extraction in SSCO2-EGS system.

5.      SSCO2-EGS system has a problem of inducing the seismicity (during hydro fracturing process) so this process is less acceptable publicly.

6.     CO2-plume geothermal (CPG) system is more acceptable publicly because it does not produce seismicity.

7.     CPG system can sequestrate more amount of CO2 then EGS system (i.e 2% more) and is more economically feasible due to low construction and maintenance costs ( (Randolph & Saar, 2011)).

 REFERENCES

Bielinski, A. (2006). Numerical simulation of CO2 sequestration in geological formations. Inst. f{\"u}r Wasserbau.

Brown, D. (2000). A hot dry rock geothermal energy concept utilizing supercritical CO2 instead of water. Proceedings, (ss. 233-238).

Buscheck, T., Chen, M., Sun, Y., Hao, Y., & Elliot, T. (2012). Two-Stage, Integrated, Geothermal-CO2 Storage Reservoirs: An Approach for Sustainable Energy Production, CO2-Sequestration Security, and Reduced Environmental Risk.

Davison, J. (2007). Performance and costs of power plants with capture and storage of CO< sub> 2</sub>. Energy, 32(7), 1163-1176.

Fouillac, C. B., & Czernichowski-Lauriol, I. (2004). Could Sequestration of CO2 be Combined with the Development of Enhanced Geothermal Systems? Third Annual Conference on  Carbon Capture and Sequestration.

MIT. (2006). The Future of Geothermal Energy. Tech. rep., Massachusetts Institute of       Technology, Cambridge, MA.

Pruess, K. (2007). Enhanced Geothermal Systems (EGS) comparing water with CO2 as heat transmission fluids.