4. Enhanced Geothermal Systems (EGS)
In principle, a temperature of 130C could be reached at any location of the planet at a sufficient depth. For instance, temperatures of this level can be found at depths between 5 and 10 km in half of the surface of USA. Based on this fact, enhanced geothermal systems (EGS) aim at utilizing the heat available at high depths by injecting a heat transfer fluid through a well. Although today few geothermal wells are deeper than 3 km given the high drilling costs and the reduced permeability at these depths (Moore & Simmons, 2013), EGS aim at increasing permeability at high depths by using techniques inherited from the oil and gas extraction industries.
Because in most of the regions where the temperature in the subsurface is above 150C the porosity is insufficient to accommodate significant volumes of fluid, it is necessary to enhance the porosity and permeability by hydrofracturing. Once a determined volume of rock has been stimulated, production wells can be drilled into the zone (Ghassemi, 2014). Hydrofracturing commonly reactivates pre-existing fractures and therefore, enhanced permeability is not randomly oriented. By monitoring the microseismic events occurring during the perforation, it is possible to estimate the shape and location of fractures, so that the injection wells can be drilled across the most permeable zones. The hydrofracturing fluid is composed of water and artificial proppants, such as sand and other materials of selected size and hardness which main task is to keep fractures once they are created. The development of EGS started in late 70’s and early 80’s with the Hot Dry Rock project in Los Alamos National Laboratory (USA). Although the project was a technical success, a series of challenges prevented it from being commercially viable. Since then, EGS projects have been developed in Australia, France, Germany, Japan, Sweden, Switzerland and UK, but none of these projects is working commercially today (Ghassemi, 2014). Figure 4: EGS schematic diagram. 1: Reservoir, 2: Pump house, 3: Heat exchanger, 4: Turbine hall, 5: Production well, 6: Injection well, 7: Hot water to district heating, 8: Porous sediments, 9: Observation well, and 10: Crystalline bedrock. By Geothermie_Prinzip.svg: *Geothermie_Prinzip01.jpg: "Siemens Pressebild" http://www.siemens.com derivative work: FischX (talk) Geothermie_Prinzip01.jpg: "Siemens Pressebild" http://www.siemens.com derivative work: Ytrottier (Geothermie_Prinzip.svg Geothermie_Prinzip01.jpg) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons Among the main challenges that affect the viability of EGS projects the following can be enumerated: i) Keeping the circulation and integrity of the drilling fluid. …show more content…
Sometimes the drilling fluid is absorbed into zones of high permeability. Also, the proppants used in oil and gas applications have problems in EGS because they loose viscosity at the elevated temperatures of EGS. ii) Equipment failure at high temperatures. Although the oil and gas industry has developed technology that operates up to 175C, EGS needs working temperature of 225-250C. this high temperatures affect the integrity of the drilling equipment, the packers (which are used to seal the high permeability zones) and the drilling muds. iii) Controlling the fracture properties and geometry. Knowledge of the likely response of the rock mass to changes in pressure, pumping rate, or fluid properties with enough degree of certainty is today beyond the available technology. Despite these technical challenges, the deployment of EGS has been claimed to be an important part of the future base load energy supply (Huenges, 2010), and therefore significant effort should be put for its development.
In principle, a temperature of 130C could be reached at any location of the planet at a sufficient depth. For instance, temperatures of this level can be found at depths between 5 and 10 km in half of the surface of USA. Based on this fact, enhanced geothermal systems (EGS) aim at utilizing the heat available at high depths by injecting a heat transfer fluid through a well. Although today few geothermal wells are deeper than 3 km given the high drilling costs and the reduced permeability at these depths (Moore & Simmons, 2013), EGS aim at increasing permeability at high depths by using techniques inherited from the oil and gas extraction industries.
Because in most of the regions where the temperature in the subsurface is above 150C the porosity is insufficient to accommodate significant volumes of fluid, it is necessary to enhance the porosity and permeability by hydrofracturing. Once a determined volume of rock has been stimulated, production wells can be drilled into the zone (Ghassemi, 2014). Hydrofracturing commonly reactivates pre-existing fractures and therefore, enhanced permeability is not randomly oriented. By monitoring the microseismic events occurring during the perforation, it is possible to estimate the shape and location of fractures, so that the injection wells can be drilled across the most permeable zones. The hydrofracturing fluid is composed of water and artificial proppants, such as sand and other materials of selected size and hardness which main task is to keep fractures once they are created. The development of EGS started in late 70’s and early 80’s with the Hot Dry Rock project in Los Alamos National Laboratory (USA). Although the project was a technical success, a series of challenges prevented it from being commercially viable. Since then, EGS projects have been developed in Australia, France, Germany, Japan, Sweden, Switzerland and UK, but none of these projects is working commercially today (Ghassemi, 2014). Figure 4: EGS schematic diagram. 1: Reservoir, 2: Pump house, 3: Heat exchanger, 4: Turbine hall, 5: Production well, 6: Injection well, 7: Hot water to district heating, 8: Porous sediments, 9: Observation well, and 10: Crystalline bedrock. By Geothermie_Prinzip.svg: *Geothermie_Prinzip01.jpg: "Siemens Pressebild" http://www.siemens.com derivative work: FischX (talk) Geothermie_Prinzip01.jpg: "Siemens Pressebild" http://www.siemens.com derivative work: Ytrottier (Geothermie_Prinzip.svg Geothermie_Prinzip01.jpg) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons Among the main challenges that affect the viability of EGS projects the following can be enumerated: i) Keeping the circulation and integrity of the drilling fluid. …show more content…
Sometimes the drilling fluid is absorbed into zones of high permeability. Also, the proppants used in oil and gas applications have problems in EGS because they loose viscosity at the elevated temperatures of EGS. ii) Equipment failure at high temperatures. Although the oil and gas industry has developed technology that operates up to 175C, EGS needs working temperature of 225-250C. this high temperatures affect the integrity of the drilling equipment, the packers (which are used to seal the high permeability zones) and the drilling muds. iii) Controlling the fracture properties and geometry. Knowledge of the likely response of the rock mass to changes in pressure, pumping rate, or fluid properties with enough degree of certainty is today beyond the available technology. Despite these technical challenges, the deployment of EGS has been claimed to be an important part of the future base load energy supply (Huenges, 2010), and therefore significant effort should be put for its development.