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GeoThermal Energy

GeoThermal energy along with hydroelectric are the only renewable energy sources that operate on a 24/7 basis.

GeoThermal energy is generally located in ideal locations to utilize the existing formation permeability and natural geofluids for power generation..

Alternative geothermal systems have been reseached since the early 70's especially hot dry rock as a geothermal source, known as Enhanced Geothermal System. The hot dry impermeable brittle rock is conventionally hydraulic fractured with invasive fluids and thus the potential for induced seismicity can be significant in EGS, especially due to the significant volumes of elastic brittle rock being fractured in the attempt to achieve fracture connection between injector and producer wells. The EGS geothermal field experimental trial in hot elastic brittle rocks in Basel, Switzerland was shut down due to induced seismicity during hydrofracturing activities and the plant was never completed.

After 40 years of field research and extensive field trials in the USA, UK, Australia, France, Germany, Japan, Sweden and Switzerland, EGS has not become a viable commercial option due to 1) the difficulity of connecting the injector and producer wells by hydraulic fracturing, 2) the inability to enhance hydraulic fracture connection to achieve commercial viable flow rates and heat flux, and 3) induced seismicity issues during construction and operation.

Conventional geothermal exploration and deployment can be both expensive and carry significant risk, since all three factors need to be present at the site:

  • Hot environment
  • Permeability
  • Fluids

An alternative is a geothermal energy system in hot low perm anelastic ductile formations - an engineered solution to create permeability, store captured carbon dioxide and sell desalinated extracted groundwater from the system.To utilize carbon dioxide as the subsurface working fluid requires a meaningful carbon tax on emitters; otherwise the carbon dioxide is simply released to the atmosphere and not captured and stored. If such a carbon tax is not in place, then the system uses recirculated groundwater as the subsurface working fluid, operating with a reduced efficiency and with no upfront carbon storage revenue from a cashflow perspective to offset the system's Capex.



Engineered GeoThermal Energy Systems

An alternative GeoThermal energy source is the hot anelastic ductile formations in sedimentary basins, that exist close to infrastructure in the USA and are located in many countries worldwide. Highly permeable inclusions can be installed in these anelastic ductile formations by the injection of a non-invasive fluid and garnet sand proppant, without giving rise to any potential induced seismicity. The system could be a single well with multiple azimuth propped vertical planes for a small standalone electrical power plant in a remote area.

A larger capacity system, injection and production wells, both consisting of multiple propped vertical planes, cycle the supercritical carbon dioxide through the hot ductile low perm formation for optimum heat extraction. The highly permeable installed vertical planes are propped with a garnet proppant, and their high permeability enable high extraction and injection flow rates of the supercritical carbon dioxide. The closed cycle supercritical carbon dioxide loop is used as the thermal energy transfer medium for electrical power generation. The system has considerable advantages both economically and environmentally, especially its 24/7 electrical generation capability, close to infrastructure, and with construction and operation being minimally invasive and a small surface footprint. Also the system geologically stores carbon dioxide, and being a closed system has zero emissions.

There are extensive hot anelastic ductile formations in sedimentrary basins worldwide, and throughout the USA, with an ideal location of a first pilot being in the Imperial Valley area in southern California.

GeoSierra's geothermal electrical power generation system in enhanced hot ductile low perm formations is in final feasibility assessment study for field pilot deployment mid-2017 in Imperial Valley, CA. Revenue for the GeoThermal Energy system comprises of carbon storage and electrical energy supply, both base and peak load demand. Net operating and capital costs are comparable to a new 2016 natural gas fired electrical power generation plant, at US$ 2016 natural gas price and ignoring any carbon tax on the natural gas power plant emissions. Net operating cost is operating cost minus carbon storage revenue and desalinated extracted groundwater income.


GeoThermal Energy Hot Ductile Formations


Imperial Valley GeoThermal Energy Pilot

Geothermal energy pilot electric power generation consists of a production well surrounded by three injection wells. All wells are stimulated and propped with a garnet sand proppant. Due to sharing of the injection wells with neighboring cells, on aggregate there are 2 wells per cell. The 2 wellheads are located on a small surface pad due to directional drilling.

The wells are drilled to a depth of 6,000' and intersect a very thick hot anelastic ductile formation, which is overlain by a caprock. The temperature in the productive zone is in excess of 200°C. The cell is 80 acres in size and 3 cells have a electric power generation capacity of 10MWe for base load electricity. Based on the expected efficiency of a combined electric power generation cycle, the production and re-injection flow rates of supercritical carbon dioxide would need to be 700 metric tons per hour to achieve a power capacity of 10MWe in a 3 cell system. Each cell contains and stores 5 million metric tons of carbon dioxide. The significant volumes of extracted groundwater are planned to be desalinated and sold in the local market. Imperial County, CA has considerable water supply/demand issues and in particular the Salton Sea will not be replenished from the Colorado River after 2018 due to the 4.4Plan. 

The system is designated to cool the cell reasonably quickly, so that it is left to reheat, and another system of cells operated, so as to maximize the carbon storage and power generation capacity and revenue of the overall scheme and optimize it's net income. Optimum layout and cell operation depends on heat flux in the area and revenue from carbon storage. It may be optimum to operate alternate cells in a cyclic manner, i.e. a cell cluster surrounded by warmer neigboring cells. A cluster of 3 cells for 10MWe electric power generation capacity is shown opposite. Due to the internal symmetry of the cell, only the pie shaped region shown below is needed to be modeled to assess and optimize the geometry and operation of the system.

The first pilot is planned to be constructed in stages, initially as a single central production/injection well, with vacuum insulated tubing separating the two fluid streams in the well. Upon the functional performance of the single well, the pilot will be expanded to include circumferential injection wells with the central pilot well converted to a production well only. Subject to system performance, the pilot will then be expanded to 3 or more cells.


                Plan View of Pilot GeoThermal Cell

     GeoThermal Energy 10MWe Pilot, Imperial Valley, CA





                            Plan View of Operating 3 Neighboring Cells



Combined Power Generation Cycle - sCO2

The optimum power generation scheme is a combined upstream supercritical carbon dioxide turbine with a downstream organic Rankine cycle as shown opposite to utilize the excess pressure head available. Due to the different efficiencies of the two cycles, the power generation capacity is approximately equal for each cycle; i.e. 50% of the electrical power generation is from the supercritical carbon dioxide turbine and 50% from the organic Rankine cycle. A flow rate of 700 metric tons per hour of supercritical carbon dioxide is required for electric power generation in the combined cycle of 10MWe capacity.


Binary Cycle - Recirculated Groundwater

In the case of a lack of revenue for carbon storage or the unsuitability of the site for carbon storage, then the subsurface working fluid will be recirculated groundwater, and the proppant in the vertical permeable planes will be quartz sand rather than garnet. The optimum power generation scheme with the subsurface working fluid being recirculated groundwater is a binary Rankine cycle. A flow rate of 1,400 metric tons per hour of recirculated groundwater is required for electric power generation in the binary cycle of 10MWe capacity.







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