Drilling techniques originally developed to unlock oil and gas from shale formations are increasingly being adapted to access and extract heat from deep underground rock formations, enabling the generation of electricity from geothermal sources. And while geothermal’s share of total U.S. electricity generation remains quite small, it may be poised to accelerate, opening up new opportunities for companies in the oil and gas sector. In today’s RBN blog, we explain the basics of geothermal energy, its main advantages and drawbacks, and how U.S. expertise in oil and gas comes into play.
To begin our discussion of geothermal energy, we need to start with a quick trip back to basic geology. Geothermal energy is produced by the slow decay of radioactive particles in the earth’s core, a process that happens in all rocks. The earth has four major sections: an inner core of solid iron, where temperatures can reach an astounding 10,800 degrees Fahrenheit (°F); an outer core about 1,500 miles thick; a mantle of magma and rock about 1,800 miles thick; and a crust of solid rock that is 15-35 miles thick under the continents and three to five miles thick under the oceans. Underground temperatures are relatively stable near the surface at 50°F to 60°F and generally increase by 72°F to 87°F for every mile (5,248 feet) of depth.
People have directly used geothermal energy for bathing and cooking for millennia — via hot springs — and have more recently used it to heat buildings and in certain industrial applications. (Geothermal energy can also make its way to the surface via volcanoes and geysers.) But there are also naturally occurring reservoirs of geothermal energy that can be found deep underground and are largely undetectable from the surface. Geologists use various methods to find them, although drilling a well and testing the temperature deep underground is the most reliable method. Those reservoirs can be tapped to generate electricity through an enhanced geothermal system (EGS), which is the step we want to focus on next.
A naturally occurring geothermal system, also known as a hydrothermal system, requires three key elements to generate electricity: heat, fluid and permeability, which is the ability of fluid to move freely through underground rock, similar to oil and gas drilling. (Much more on that comparison to come.) In many areas, the underground rock is hot enough but there is not enough natural permeability or fluids present. In such cases, an EGS can be used to create a reservoir to tap that heat for energy. In an EGS, water is injected deep underground under controlled conditions (#1 in Figure 1 below) to create new fractures and reopen pre-existing fractures, thereby enhancing permeability. Operators pump the hot water to the surface to produce steam (#2), where it then spins a turbine connected to a generator to produce electricity (#3). The steam then heads to a cooling tower (#4) and is pumped back below the surface (#5). It’s more complicated in real life, of course, and there are several variations of power plants, but that’s the basic approach.
Figure 1. Geothermal Power Plant Schematic. Source: Environmental Protection Agency
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