Human activities have an intense impact on the Earth’s deep underground fluid flow

The impact of human activities – such as greenhouse gas emissions and deforestation – on the Earth’s surface is well studied. Now, hydrology researchers from the University of Arizona have investigated how humans influence the Earth’s deep subsurface, a zone that lies hundreds of meters to several kilometers below the Earth’s surface.

“We looked at how the rate of fluid production with oil and gas compares to the natural background circulation of water and showed how humans have had a major impact on the circulation of fluids in the subsurface,” said Jennifer McIntosh, a professor at the UArizona department. of Hydrology and Atmospheric Sciences and senior author of an article in the journal The future of the earth detailing the findings.

“The deep subsurface is out of sight and out of mind for most people, and we thought it was important to provide some context to these proposed activities, especially when it comes to our environmental impacts,” said lead study author Grant Ferguson , an adjunct professor in the Department of Hydrology and Atmospheric Sciences at UArizona and professor in the School of Environment and Sustainability at the University of Saskatchewan.

According to the study, these human-induced fluid flows are expected to increase in the future with strategies proposed as solutions to climate change. Such strategies include: geological carbon sequestration, which captures carbon dioxide from the atmosphere and stores it in underground porous rocks; geothermal energy production, in which water circulates through hot rocks to generate electricity; and lithium extraction from underground, mineral-rich brine to power electric vehicles. The study was conducted in collaboration with researchers from the University of Saskatchewan in Canada, Harvard University, Northwestern University, the Korea Institute of Geosciences and Mineral Resources and Linnaeus University in Sweden.

“Responsible management of the subsurface is central to any hope for a green transition, a sustainable future and limiting warming below a few degrees,” said Peter Reiners, professor in the UArizona Department of Geosciences and co-author of the study.

In oil and natural gas production, there is always some amount of water, usually salt, that comes from deep underground, McIntosh said. The underground water is often millions of years old and obtains its salinity through evaporation of old seawater or through reaction with rocks and minerals. For more efficient oil extraction, more water is added to the salt water from near-surface sources to compensate for the amount of oil removed and maintain reservoir pressure. The mixed salt water is then reinjected into the subsurface. This becomes a cycle of producing fluid and reinjecting it deep underground.

The same process occurs in lithium extraction, geothermal energy production and geological storage of carbon, which involves the remaining salt water from the subsurface that is re-injected.

“We show that the fluid injection or recharge rates of these oil and gas operations are greater than what occurs in nature,” McIntosh said.

Using existing data from several sources, including measurements of fluid movements related to oil and gas extraction and water injections for geothermal energy, the team found that current fluid movement rates caused by human activities are higher compared to how fluids moved before human intervention.

As human activities such as carbon capture and storage and lithium extraction increase, the researchers also predicted how these activities might be recorded in the geological record, which is the history of the Earth as recorded in the rocks that make up its crust.

Human activities have the potential to change not only the deep underground fluids, but also the microbes that live down there, McIntosh said. As fluids move, microbial environments can be altered by changes in water chemistry or by bringing new microbial communities from the Earth’s surface to the subsurface.

With hydraulic fracturing, a technique used to break underground rock with pressurized fluids for the extraction of oil and gas, a deep rock formation that previously lacked any detectable number of microbes can suddenly exhibit a bloom of microbial activity.

Much remains unknown about Earth’s deep subsurface and how it is affected by human activities, and it is important to continue working on those questions, McIntosh said.

“We must use the deep underground as part of the solution to the climate crisis,” McIntosh said. “Yet we know more about the surface of Mars than we do about water, rocks and life deep beneath our feet.”