WorldWide Drilling Resource

A New Era of Geothermal on the Horizon Compiled by Caleb Whitaker, Associate Editor, WorldWide Drilling Resource ® Geothermal is one of the least expensive forms of energy and is highly regarded as a renewable energy source, which has lower environmental affects than methods producing carbon dioxide (CO 2 ). The temperatures beneath the earth get hotter as the depth gets deeper. At a depth of 1-3 miles, deep heat resources appear more often, but cost-effectively mining them is difficult. This causes traditional geothermal energy projects to have geological constraints because they must be located near places where the earth is hot relatively close to the surface. Because CO 2 is produced as a by-product of many energy pro- ducing methods, it is a readily available, disposable commodity, which can be captured and injected into the subsurface for storage, thus less- ening CO 2 atmospheric emissions while preserving water resources. After being stored in a geothermal reservoir, the CO 2 can then be used like water to absorb heat from the earth’s interior and produce energy through a Carbon Dioxide Plume Geothermal (CPG) system. TerraCOH, Inc. has developed patented technology, which aims to efficiently unlock thermal energy, aid the environment, and inno- vate the geothermal field by harnessing compressed CO 2 . In this state, the CO 2 is dense and has a lower viscosity than water, so it flows easy. Oil companies often use compressed CO 2 to procure the last bits of recoverable petroleum from conventional wells. In TerraCOH’s system, CO 2 can either be provided via an emit- ter or captured from a power plant, compressed into liquid form, and pumped to the well site via pipelines. As the CO 2 is pumped lower into the earth to be stored in porous sedimentary rock reservoirs, it heats up. When it reaches the reservoir, its relative lightweight density causes a plume to form above the other fluids. This allows the CO 2 to pool just below the caprock where it stays contained underground. Furthermore, CO 2 has a high mobility and high thermal expansibility compared to water, resulting in the formation of a strong thermosiphon, a system where a liquid is circulated by convection caused by a difference in density between the hot and cold portions of the liquid, which allows the hot CO 2 to be drawn back up to the surface, reducing subsurface pumping re- quirements and significantly increasing the electric power production efficiency of the geothermal system. Once at the surface, the CO 2 will spin a turbine to create electricity. For a geothermal system, the heated fluid in the pro- duction well has high pressure, while the cool fluid in the injection well has lower pressure. The amount of energy production for the system is dependent upon the difference in pressure between the two wells. Heated CO 2 produces a greater pressure change than heated water, yielding substantially greater energy production over traditional geothermal systems. This means electricity generation can be conducted at shallower, cooler depths. TerraCOH is eyeing existing oil and gas fields for its early projects, since they’ve already been drilled. The company has plans for a small power plant at a conventional oil well in northwest North Dakota, which will produce electricity for the site. For this project, TerraCOH will harness the geothermal energy provided by oil and gases coming up the well, heating CO 2 in an aboveground tank, which will then power a turbine; however, long-term goals will involve the drilling of new wells as the company envisions placing TerraCOH plume geo- thermal systems near coal-fired or gas- fired power plants, directly capturing CO 2 emissions, pumping them into the ground for eventual use in larger CO 2 fired- power plants. These plants would initially generate up to 15 megawatts, but could eventually be up to 300 megawatts. There are also goals to use the geothermal system as a sort of battery for solar and wind power. The excess wind and solar power could be transmit- ted to a TerraCOH plant where it will be used to power pumps, which would inject CO 2 into the earth, where it could eventually be turned back into electricity when needed. Whereas many renewable energy sources are intermittent, produc- ing when the weather is sunny or windy, geothermal in general and plume geo- thermal specifically are not intermittent, so they can run on demand. An illustration of a CPG power plant attached to a power plant emitting CO 2 , courtesy of TerraCOH. 23 WorldWide Drilling Resource ® DECEMBER 2017