Scientists at the U.S. Geological Survey and their partners have completed a comparison study of deep-groundwater sampling techniques to provide guidance on the best available methods to accurately reflect the effectiveness of the carbon dioxide storage. Combustion of fossil fuels remains the predominant source of energy production in the world, and their use makes up the largest single source of global greenhouse gas emissions. Carbon capture and storage is an approach to mitigating these greenhouse gas emissions, and accurate monitoring, including tracking important changes in groundwater chemistry and detecting any carbon dioxide leakage, is necessary to evaluate how the carbon capture is working.

The results of a recent comparison study by the USGS and partners at the Electric Power Research Institute and Lawrence Berkeley National Laboratory demonstrate the difficulty of preserving dissolved gases in groundwater samples, particularly in the form of dissolved carbon dioxide. Loss of CO₂ causes a decrease in acidity and the precipitation of minerals dissolved in the water, changing the chemistry of the sample.

Collecting groundwater from a well that goes thousands of meters below the surface of the Earth has many challenges. The temperature and pressure differences between surface and depth are dramatic, and changes in these conditions can cause samples to lose dissolved gases, such as CO₂, and minerals can precipitate causing chemical changes in the sample. The sampling of deep groundwater is an important part of many projects, including monitoring and verification of deep groundwater during carbon storage and exploration of geothermal energy resources. A newly completed research paper, published in the International Journal of Coal Geology, details the results for a variety of chemical constituents from deep groundwater samples collected using four different sampler types.

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The four sampling methods used in the study are gas lift, electric submersible pump, a down-hole vacuum sampler, and a U-tube. Gas lift injects pressurized gas into the well, reducing the density of the groundwater, causing it to flow from subsurface pressure. An electric submersible pump is lowered into the well and pushes fluids to the surface. A down-hole vacuum sampler is a tool that is lowered into the well with a thin wire cable and consists of a sample bottle with locking valves and a timer. The U-tube, designed and built by researchers at Lawrence Berkeley National Laboratory is a fixed loop of narrow stainless steel sample tubing extending from the surface to the sampling depth coupled with a pressurized source to drive the groundwater sample through the tubing.

Among the four sampling methods tested, the down-hole vacuum sampler and U-tube system, both of which can maintain the pressure at which the sample is collected, perform best at preserving sample integrity until an analysis can be done. Although the effects of sampling devices on sample chemistry are well known in relatively shallow groundwater studies, this study shows a more extreme version of some of these effects resulting from the greater temperatures and pressures associated with deep sampling. The importance of cleaning the well before sampling was also shown in the study, as contamination by fluids left over from well drilling or maintenance was evident in some samples. Although the gas lift method substantially affected groundwater chemistry, it was essential in moving a large volume of groundwater to clean the well.

This research was carried out at the Citronelle injection site in Alabama, which is part of the Southeast Regional Carbon Sequestration Partnership Anthropogenic Test project, an integrated pilot project for carbon capture and storage that is funded by the United States Department of Energy and managed by the Southern States Energy Board in partnership with Southern Company, the Electric Power Research Institute, and Advanced Resources International, Inc., and the oil field is operated by Denbury Onshore, Inc.

Published in the International Journal of Coal Geology, the full report, “Comparison of geochemical data obtained using four brine sampling methods at the SECARB Phase III Anthropogenic Test CO₂ injection site, Citronelle Oil Field, Alabama,” by Christopher Conaway and others, is available online.