Electrical resistivity tomography (ERT) gauges underground adjustments by measuring electrical conductivity within the rock. ERT then produces 4D—that's, 3D plus time-lapse—pictures of the subsurface. Credit score: Tim C. Johnson and Mike Perkins | Pacific Northwest Nationwide Laboratory
The circulation of electrical present alerts adjustments in fractures deep beneath the earth
Scorchingly scorching granite deep underground could be tapped for vitality by opening up cracks within the rock. This potential useful resource, often called enhanced geothermal vitality, requires a transparent sense of adjustments taking place within the rock over time—a fancy image that may be tough to seize.
A crew led by researchers at Pacific Northwest Nationwide Laboratory (PNNL) has demonstrated a brand new solution to monitor deep subsurface fractures. The approach, electrical resistivity tomography (ERT), gauges underground adjustments by measuring electrical conductivity within the rock. ERT produces 4D—that's, 3D plus time-lapse—pictures of the subsurface.
What's an enhanced geothermal system?
Standard geothermal techniques rely on pre-existing water and circulation pathways which might be already inside scorching rock. By injecting water and cracks, an enhanced geothermal system captures warmth trapped inside dry rock. Operators drill two underground wells 1000's of ft beneath the floor, then pump high-pressure fluid into the rock between the wells to fracture it. The fracturing course of to acquire warmth is corresponding to “fracking” shale rock to launch oil and fuel.
Temperatures at this stage can attain past 200 ºC (392 ºF). Water pumped from one effectively to the opposite and again as much as the floor collects warmth from the rock, producing steam that may drive a turbine for electrical energy.
Enhanced geothermal techniques might present an estimated 100 gigawatts of electrical energy—sufficient to energy 100 million houses. However such techniques contain costly drilling, and so they want higher monitoring and prediction of underground adjustments to cut back the uncertainty and danger related to a given challenge.
Like every underground setting, enhanced geothermal techniques change over time. Fractures within the rock open and shut in response to stresses attributable to high-pressure fluid injections, altering the system’s warmth output. Seismic exercise is one indicator of subsurface stress, however info from microseismic monitoring is restricted.
“In these deep, scorching rocks, it’s too costly to drill sufficient monitoring wells to grasp what’s happening utilizing direct sampling,” mentioned Tim Johnson, a computational scientist at PNNL who co-authored the examine. “The first focus of this challenge is to raised perceive, and finally to foretell, how fractures are going to behave in a high-stress setting if you attempt to join them between two wells.”
Getting a clearer underground image
ERT entails inserting metallic electrodes into monitoring boreholes after which imaging the conductivity of the rock when an electrical present is shipped between them. Conductivity will increase over time point out the place fractures are opening; when fractures are smaller or closed, conductivity decreases. Johnson developed software program referred to as E4D which runs on supercomputers and turns all of this electrical info into a visible that resembles a warmth map, displaying fluctuations in conductivity over time. In 2016, E4D was awarded an R&D 100 Award.
Time-lapse electrical resistivity tomography. Credit score: Tim Johnson, et al. | Pacific Northwest Nationwide Laboratory
“It’s much like medical imaging, besides that you simply’re doing a time lapse,” Johnson mentioned. “So that you’re watching how issues change, and normally the change pertains to how the fluid is flowing within the subsurface.”
Johnson and different researchers at PNNL have pioneered the usage of ERT as a 3D monitoring software, and E4D at shallower depths of as much as 350 ft, the place it has been used to detect and hint contaminants, for instance. To check it within the deep subsurface, the crew deployed it on the Sanford Underground Analysis Facility in Lead, South Dakota. The work, which is supported by the Division of Vitality (DOE)’s Workplace of Vitality Effectivity and Renewable Vitality by means of its Geothermal Applied sciences Workplace, is a component of a bigger collaborative effort throughout DOE to reinforce entry to pure assets and storage within the subsurface. Lawrence Berkeley Nationwide Laboratory leads the hassle, often called the Enhanced Geothermal Programs (EGS) Collab. Associate labs embody PNNL, Sandia Nationwide Laboratories, Lawrence Livermore Nationwide Laboratory, Idaho Nationwide Laboratory, and Los Alamos Nationwide Laboratory.
Pioneering a brand new subsurface imaging approach
The intent of the ERT monitoring at Sanford was to watch fluid circulation, as had been completed at shallower ranges. However the outcomes initially didn’t appear to align with these earlier makes use of.
“What we had been seeing with the adjustments in conductivity didn’t make sense when it comes to fluid circulation,” Johnson mentioned. But when the conductivity wasn’t reflecting the motion of fluids, what was it displaying?
The experimental testbed is situated in a mine tunnel 4,850 ft beneath the floor, within the Sanford Underground Analysis Facility. Credit score: Hunter Knox | Pacific Northwest Nationwide Laboratory
Johnson found the reply after years of looking out by means of scientific papers from the Nineteen Sixties and Seventies. Researchers on the Massachusetts Institute of Know-how and the Lawrence Berkeley Nationwide Laboratory had beforehand observed variations within the conductivity of crystalline rocks in response to emphasize—in lab experiments, compressing the rock made it much less conductive. This indicated that the ERT wasn’t simply following fluid underground. It was measuring how fractures opened and closed in response to emphasize.
“As soon as we made that hyperlink, all the things made sense when it comes to what the time-lapse pictures had been doing,” Johnson mentioned.
ERT provides a number of benefits. With no transferring components and electrodes put in exterior the effectively casing, the tools is low upkeep and might function whereas injections are taking place. And the imaging occurs in real-time, giving facility operators suggestions they'll use virtually instantly if wanted. Nevertheless, ERT can't be used with metallic wellbore casings, that are ubiquitous in deep subsurface initiatives.
There are methods round this hurdle, similar to utilizing fiberglass casing, coating the casing with a non-metallic epoxy, or utilizing a special, nonmetallic materials altogether. However for now, Johnson and the crew are persevering with to enhance and take a look at the usage of ERT on the Sanford facility.
Reference: “4D Proxy Imaging of Fracture Dilation and Stress Shadowing Utilizing Electrical Resistivity Tomography Throughout Excessive Strain Injections Right into a Dense Rock Formation” by T. C. Johnson, J. Burghardt, C. Strickland, H. Knox, V. Vermeul, M. White, P. Schwering, D. Blankenship, T. Kneafsey and the EGS Collab Group, 08 October 2021, Journal of Geophysical Analysis: Strong Earth.
DOI: 10.1029/2021JB022298
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