Urban Close-Loop Enhanced Geothermal Systems with Deep Multi-Lateral Wells

摘要

The enhanced geothermal systems (EGS) are considered as engineered subsurface heat capacitors with artificial fractures propagated in the geothermal rock, in order to increase formation permeability and thereafter a fluid rate as a function of increased permeability. The applied multi-stage fracturing process to create a fracture network in horizontal wells is widely implemented in the oil and gas industry, though it may be considered disadvantageous in geothermal wells for a number of reasons. When the formation rock is fractured, there is a) limited control over the fracture network distribution, b) a significant possibility of the caprock integrity failure, c) inability to control fluid flow distribution within the fracture network. In addition to these potential issues, the injected fluid can react with or dissolve minerals within the bearing rock, causing the production of unexpected mineral inclusions and other microelements. Correspondingly, this would require additional water treatment processing on the surface. The EGS engineering concepts are still under development, but most of them would be able to provide only indirect subsurface measuring and monitoring of the fracture network within a geothermal formation. The alternative to multi-stage fracturing is a well-controlled technology based on the drilling of multi-lateral wells, where the injected fluid is transported within an enclosed media of the wellbore. This fluid is heated inside the wellbore by a geothermal formation, where its flow can be monitored all the time by downhole sensors. As the injected fluid doesn't leave the wellbore enclosure, fluid rates can be controlled and measured back on the surface, with negligible change in water quality and therefore unnecessity for the installation of water treatment processing equipment. When the power plant requires a temperature increase in producing water, then additional lateral legs can be drilled from the same main wellbore. This will increase the overall volume exposure of the injected water to the enclosing wellbore walls. Thereby, a hot formation will conduct heat to the injected water in multiple laterals at a faster exchange rate. These geothermal systems with deep multi-lateral wells can be built within existing and densely populated communities. As a result, it will bring a power generation facility closer to a consumer without any impact on the ecological and environmental quality of the existing urban life.