Measurement and analysis of geochemical signals – information that lies buried in the liquids, gases, and mineral deposits of the earth – are how researchers at the National Energy Technology Laboratory (NETL) kept tabs on groundwaters and produced waters at Texas oil fields where carbon dioxide (CO₂) was injected as part of enhanced oil recovery (EOR) operations.
Across the U.S., when efforts to extract oil with conventional means have diminished, one commonly employed strategy involves enhancing oil recovery with CO₂ injection. NETL researchers study the effects of CO₂-EOR thousands of feet below ground by examining the chemistry of oilfield produced waters, in this case within the Permian Basin in Texas.

Produced waters are typically salty waters that travel up the well to the surface along with oil and gas.

The geochemical information NETL uses lies hidden in the chemistry of the water. It is useful because the geochemical signals record evidence of processes that cannot be directly observed. The NETL team also uses geochemistry to keep close tabs on near surface fresh groundwaters that are used for irrigation and drinking water within the Permian Basin.

NETL geochemists examined the composition, structure, process and other physical aspects of the earth by working with the distribution of chemical elements in rocks and minerals and the movement of those elements in subsurface water systems.

James Gardiner is an NETL research scientist working on a team that studied geochemical signals to understand the ways carbon dioxide injection can change oilfields and to understand which geochemical signals are most useful to scientists trying to understand fluid flow in the subsurface.

“Geochemical signals are important because CO₂ injection could cause unwanted fluids, specifically CO₂ or produced water, to migrate into nearby groundwaters,” he explained. “This site in the Permian Basin provides NETL researchers with the opportunity to determine which geochemical signals are sensitive indicators of unwanted fluid migration. NETL’s geochemistry team applies these signals to ensure that no fluids are migrating into overlying groundwaters at this site and based on the results, the team can suggest applying these signals at similar fields across the world.”

The NETL geochemistry team analyzes general chemical parameters, like pH, and concentrations of dissolved calcium, magnesium and other elements. The team also characterizes a range of stable isotopes within the water.

Gardiner said the isotopes that NETL investigates, including oxygen, carbon, strontium, and lithium, are naturally occurring in all groundwaters. The isotopes often reveal key information about the water, such as the water’s carbon or salt sources, and downhole chemical reactions, which are often not readily revealed by the general parameters.
The NETL project in the Permian Basin in West Texas began in June 2013 before CO₂ injection began as part of an EOR project and continued through 2018. NETL is trying to describe and predict how injecting CO₂ affects the producing formation and to ensure that changing field practices are not causing unwanted migration into groundwaters and that changes in the baseline geochemistry from normal operations are not confused as signals of leakage
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The work reached four key conclusions:

• CO₂ injection has observable effects on the alkalinity, total dissolved solids and sodium values in produced waters in the carbonate reservoir.
• Only some groundwater parameters are distinct and therefore useful to constrain pathways of leakage between produced water and overlying groundwater.
• No evidence of produced water intruding into overlying groundwaters was observed during the study.
• Geochemical signals can be a significant way to monitor the environment. NETL’s work in the field is just one more way it pursues its mission of discovering technology solutions to enhance the nation’s energy foundation while protecting the environment for future generations.

NETL’s geochemical monitoring under the CO₂ storage portfolio is coordinated by Christina Lopano and Alexandra Hakala. Team members conducting field sampling and sample analysis also include Burt Thomas, Mengling Stuckman, Thai Phan, Richard Spaulding, Daniel Lipus, Harry Edenborn, Rodney Diehl, and Dennis Stanko.