In an article first published in the internationally renowned geoscience journal JGR Solid Earth on October 8, 2021, a research team led by the Pacific Northwest Laboratory (PNNL) in introduced a deep underground crack monitoring method called " resistivity tomography" (ERT for short). shows that by measuring the conductivity in the rock, the ERT scheme can generate 4D (i.e., 3D + time-lapse) images underground.
experiment was conducted in a 4850-foot mine tunnel underground in Sanford (Photo from: PNNL)
In order to better utilize geothermal energy, geological inspectors need to use cracks opened in the rock to explore the energy contained in the scorching granite deep underground.
However, if you want to clearly understand the changes in rocks over time, the technical difficulty of setting up an enhanced geothermal system will be so complicated that it is beyond ordinary people's imagination.
ERT Image example
It is reported that traditional geothermal systems rely on liquids and their flow paths that already exist inside hot rocks. And by injecting water and cracks, enhanced geothermal systems can capture heat trapped in dry rocks.
Operators can try drilling two underground wells thousands of feet below the surface and then pumping high-pressure fluid into the rocks between the wells to break them—a way of heat acquisition, much like the “fracting” method of oil/gas.
Delay resistivity tomography diagram shows that
Because the temperature is as high as 200℃ (392°F), the liquid that travels to and from the ground between the two wells can collect the energy of steam from the rocks, thereby driving the turbine to generate electricity.
Data says that the enhanced geothermal system can provide about 100 GWh of electricity — enough to power hundreds of millions of households. However, such systems involve expensive drilling processes, and require better monitoring and prediction of underground changes to reduce the risk of uncertainty that projects may face.
Research diagram - 1: The relationship between the resistivity and confining pressure of several saturated crystalline rock samples
Specifically, cracks in the rock need to be opened or closed in response to the stress caused by high-pressure fluid injection, thereby changing the heat output of the system. Earthquake activity remains a monitoring indicator that cannot be ignored, but what can be achieved so far is relatively limited. "It is too expensive to drill enough monitoring wells in deep and hot rocks. The focus of the new project is to better understand and predict how cracks between the two wells perform in high stress environments," said Tim Johnson, a co-author of the study of
.
Research diagram - 2: Plan of test bench and monitoring line
For this purpose, they envisioned inserting an ERT metal electrode into a monitoring drill hole and then 3D imaging of the conductivity of the rock. If the data increases over time, it means the crack is opening. And when the crack is smaller or closed, the conductivity will decrease.
In addition, Tim Johnson developed an E4D software running on a supercomputer and presented all electrical signals and fluctuations over time with a heat map-like visual effect. Back in 2016, the software won the RD 100 Award.
Research diagram - 3: Prestimulation images of conductivity and natural crack patterns
Johnson added: The principle is similar to medical imaging, except that the delay parameters are added. As a 3D monitoring tool, you can use ERT to see how things change, which is often related to how fluid flows underground.
The PNNL research team has experimented with E4D software analysis in a shallower depth range of 350 feet, but to carry out relevant tests at a deeper level, we still have to wait for the latest progress at the Sanford Underground Research Facility in South Dakota.
Research diagram - 4: Resistivity tomography data related to interlayer pressure and injection/production flow rate
It is reported that as part of the U.S. Department of Energy (DOE) strengthens its greater cooperation to obtain underground storage of natural energy, the study was also supported by the Office of Energy Efficiency and Renewable Energy and other Offices of Thermal Technology.
In addition, this enhanced geothermal system (EGS) collaboration project led by Lawrence Berkeley National Laboratory (LBNL), which includes national laboratories such as PNNL, Sandia, Lawrence Livermore (LLNL), Idaho, and Los Alamos (LNAL).
Research diagram - 5: Areas with increased conductivity indicate increased porosity
Like the early experiments conducted by the research team at shallower depths, the Sanderford ERT project is also committed to monitoring the movement of fluids—although their purpose was not in the beginning.
Johnson said: "If the conductivity change we observed has nothing to do with fluid movement, then what does it reveal?" After searching many scientific papers from the 1960s to 1970s, they finally found an answer.
Research chart - 6: Areas with reduced conductivity indicate reduced porosity
MIT (MIThings Institute of Technology (MIThings 3) and Lawrence Berkeley National Laboratory (LLNL) researchers have noticed that the conductivity of crystalline rocks will change under stress.
Laboratory studies show that compressing rocks can reduce their conductivity—which suggests that ERT not only follows underground fluids, but also measures how voids open and close under pressure. And once this connection is established, everything becomes very meaningful in terms of the role of time-lapse images.
(Journal of Geophysical Research: Solid Earth)
In addition, without installing mobile parts and electrodes, the ERT device has extremely low maintenance costs, can be put into operation immediately upon infusion, and real-time imaging can provide operators with good feedback. Unfortunately, ERT does not work with common metal wellbore casing schemes.
As for the solution, the project team may use the outer layer of the glass fiber wellbore sleeve, coat the shell with non-metal epoxy resin , or even completely replace with new non-metallic materials.
