Introduction to the Science and Engineering of Deep Geothermal Heat

The rocks below our feet grow hotter at greater and greater depths, yet the subsurface temperature profile varies a lot across Earth. In some places moving subsurface water carries heat along specific paths, causing irregularities in the temperature profile. Earth’s geothermal heat is a fascinating research topic for reasons that range from an interest in the flow of subsurface water along zones of fractured rock, to the search for strategic mineral resources, to the wish to extract heat from the rocks as an alternative energy source.

The geology of studying and extracting deep geothermal heat: CUBO

Iceland or Yellowstone can use volcanic heat near the surface for energy, but the rocks near the surface in Central New York are far cooler. This is true for a large part of the world’s land masses. Earlier research indicates that rock hot enough to provide the heat needed for Cornell University will likely be found at a depth of about 10,000 feet – close to two miles down! The Cornell University Borehole Observatory (CUBO) project aimed to drill a hole deep enough to test this hypothesis. An animation describing the drilling of this borehole can be found at the Earth Source Heat website.

Through the borehole, we gained access to the rocks, and could measure numerous properties. Geologists continue to investigate variables such temperature, flow rate of fluids through the rock, rock type, the distribution of fractures in the rock, and the subsurface stress conditions. That information will be used to figure out which rock layers will be best for extracting heat. CUBO researchers are studying the existing network of natural fractures in the rock through which naturally occurring water in the subsurface could absorb heat from the rock and flow into a well. CUBO will allow researchers to get a better understanding of the characteristics of the rock layers deep below the surface.

CUBO passed through 9400 feet of sedimentary rocks and then entered basement rock. Basement rock is the thick foundation of ancient metamorphic and igneous rock that dominates the crust of continents. Sedimentary rock may have large enough pores and cracks to allow water to flow through it. These pores and cracks determine the rock’s permeability. In general, metamorphic rocks are not porous. For water to pass through these basement rocks, there must be cracks which act as tiny channels. (Digging deeper: More about porosity, permeability, and transmition of water)

During the drilling process, rock ground up by the drilling process was conveyed to the surface in the drilling mud. Samples of small pieces of rock (“cuttings”) were collected throughout the length of the borehole, along with many types of signals that measure the density of the rock, the electrical conductivity of the fluids in the rock, the composition of the rock, and the temperature. The rest of the cuttings were disposed of; radioactivity of natural occurring radioactive materials (NORMs) in the crushed rock was low and did not need special handling (Digging deeper: More about the geology of NORMs). The New York State Museum and Paleontological Research Institution (PRI) are the archival homes of the samples of cuttings.
Cylinders of intact rock, referred to as cores, were collected at 25 depths of special geologic interest. Cores provide more geological information but are more difficult and expensive to take. The cores were taken from layers of sedimentary rock that are about 500 million years old (from the Ordovician and Cambrian periods) and from the shallow part of the metamorphic basement rock.
diagram of boreholes through over two miles geologic layers

The diagram to the left shows a simplified view of the Cornell University Borehole Observatory, CUBO (the vertical yellow line) through layers of rock under the Cornell campus. This exploration well was drilled in summer of 2022 to investigate the geology, in order to develop engineering designs for extracting deep geothermal heat at this location. The gray rocks at the base are igneous and metamorphic rocks in the “basement” of the crust. The layers above (about 2 miles, or 3000 m, in thickness) are various kinds of sedimentary rocks. The scale (in degrees Celsius) on the right shows the temperature predicted before CUBO was drilled, which rises steadily to values around 80 to 90°C near the bottom of CUBO. The true temperatures provided to be near these values, and will updated soon.

The engineering of deep geothermal heat

In brief, the Earth Source Heat project intends to heat the Cornell campus from water harvested from deep in the Earth’s crust. The technology to extract the heat is similar to that of shallow geothermal heat systems: the heat will be transferred to a separate supply of water flowing within Cornell’s heating distribution pipeline to warm most of the Ithaca campus buildings. However, the heat source is much deeper and far hotter.

Hot geothermal water will be pumped from deep below the surface. The heat from the production-well water will be transferred across a series of thin layers of steel to different water that is passed through a loop of pipes on campus. Only the heat is exchanged between the water loops, while the two types of water are kept separate. After giving up its heat, the geothermal water will then be injected through a second well deep into the ground, to reheat. That water will circulate through the naturally hot rocks via a network of underground pores and crevices, and the water’s temperature will rise again as it is drawn back to the first well.

The rate at which water from below flows upward in the well determines the viability of a geothermal heating system like is envisioned. The achievable flow rate between the production and injection wells will depend on the ability of water to flow through pores and cracks (permeability) of the rock, the volume of the rock holding the pores and cracks, and the difference in pressure between the two wells. The CUBO project has helped figure out the permeability and the necessary volume. If a flow rate between 30-70 liters (8 to 18 gallons) per second can be achieved, then one set of production-injection wells would likely be capable of heating a large part of campus, but not all. Later, to heat the whole Cornell campus, several pairs of wells may be needed.

An Earth Source Heat building will house the structure over the top of the wells known as “wellheads,” in addition to water pumps, heat exchangers, heat pumps and most other visible components of the system.

The Cornell University Borehole Observatory (CUBO), however, which is the hole that was drilled in 2022, is not being used for generating heat. Rather, it has served as a means to collect data to learn more about the viability and engineering needed for deep geothermal energy. Later, it will be used to collect more data, as is needed to refine the design for a geothermal reservoir. Then, if a reservoir is successful, is will be used to monitor the geothermal energy system.

Study of seismic activity 

Earth’s interior rocks move very slowly, usually imperceptibly, driven by forces ranging from the tug of the Moon’s gravity to the shifting of tectonic plates. When the movements occur in jolts rather than slowly, bursts of energy travel as waves through the rock. If the waves are weak, a sensitive instrument called a seismometer is needed to detect the small jolt. If the wave is large and people feel the ground move, the jolt is referred to as an earthquake. Central New York rarely experiences any earthquakes, and even sensitive seismometers identify only a low level of weak seismic activity.

In some parts of the United States and in other parts of the world, human activities have led to changes in the rocks and produced earthquakes. Usually, those activities involved pumping a lot of water into the rock while not pulling water back out. However, tens of millions of wells similar to the CUBO project borehole have been drilled without creating earthquakes, because drilling those routine wells did not involve forcing water deeply into the rock outside of the borehole wall. During the CUBO project, low pressures and small quantities of water will be used, and only for short times.

For an Earth Source Heat project in the future, the idea is that water will circulate deeply through the rock, moving both into and out of the rock, in balance. That design is intended to keep the risk of an earthquake extremely low. To monitor and minimize any risk of triggering small earthquakes, Cornell deployed over the last decade networks of seismometers.

Many studies have already been made of the geology, hydrology, and natural seismicity using past wells and surface instruments. These include analyzing the “background” rate of seismic activity in Tompkins County resulting from natural forces as well as human construction activity, and identifying the locations of existing bedrock faults. The exploratory well (CUBO) enabled up-close study of the geology, hydrology, and existing fracture networks. Seismic activity was monitored throughout the CUBO project, to track any changes in degree of rock movement below ground during all phases of the project. If Cornell advances with additional steps to test and develop the Earth Source Heat project, a seismometer installed deep in a borehole can monitor the rocks long into the future. Such monitoring will allow project scientists and engineers to carefully plan, and for everyone to monitor. Data collected with CUBO is providing critical information about the bedrock geology that is needed to design a drilling program for a future geothermal heat extraction system which minimizes the risk of induced seismicity.