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. This paper summarizes the different approaches for using CO2 as a working fluid in extraction of heat and producing electricity from geothermal reservoirs and gives future recommendations. The concept of 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. Different studies done till now suggest that the CO2 as a working fluid is feasible but still it has to be worked out that which configuration can make this process the most feasible and publicly acceptable. This is the time to apply this concept on research site and come up with more data set to encourage investors to commercialize this approach. Cost is the key factor in applying this approach, so more work can be done to find configurations which can be applied practically.

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