Fluid Sampling

Geochemical monitoring of shallow groundwater may be carried out for several purposes: (1) to comply with expectations of the Class VI permit; (2) to provide assurance to local stakeholders that injected CO₂ has not been released to near-surface formations; (3) as part of an accounting protocol to demonstrate that CO₂ is not migrating out of the storage reservoir through the groundwater and into the atmosphere; and (4) to assure local landowners that geologic storage is not affecting residential drinking water or livestock water sources.¹

Monitoring techniques can be designed to look for gas phase CO₂, or CO₂ speciation from dissolving into water, such as dissolved CO₂, bicarbonate or carbonate ions, indicators of speciation such as a decrease in pH, or indicators of mineral interactions with CO₂, such as mobilization of reactive metal cations, alkalinity, and elevated electrical conductivity.²

Specifically, geochemical measurements may include pH, alkalinity (both lowered by dissolution of CO₂), electrical conductivity, and various cation (e.g., Na⁺, Ca²⁺, Mg²⁺, Fe²⁺, Fe³⁺) and anion (e.g., HCO₃ ^–, CO₃ ^2-, Cl^–, SO₄ ^2-) compositions.³

Typical shallow groundwater monitoring wells are less than 100 meters deep, though deeper wells may be required in locations where potable water sources occur at greater depths.⁴

Geochemical Sampling of Shallow Groundwater Summary

Description: Geochemical sampling of shallow groundwater above CO₂ storage reservoir to demonstrate isolation of the reservoir from underground sources of drinking water. Chemical analyses may include pH, alkalinity, electrical conductivity, major and minor elements, dissolved gasses, tracers, and many other parameters. Sensor probes/meters, as well as titration test kits, can be used to test/ sample in the field.
Benefits: Mature technology, samples collected with shallow monitoring wells. Sensors may be inserted into the aquifer. Address major regulatory concern regarding migration reaching underground sources of drinking water, and may have value in responding to local concerns, which typically elevate concerns about groundwater.
Challenges: Significant effort for potential lack of significant results. Signal may be retarded and attenuated. Many factors other than fluids from depth can change or damage aquifer water quality, and detailed assessment of aquifer flow system may be needed to attribute a change to signal either to migration or to other factors. Gas solubility and associated parameters (pH, alkalinity) are pressure sensitive, so that obtaining samples representative of the aquifer fluids requires careful sampling. Carbon isotopes may be difficult to interpret due to complex interactions with carbonate minerals in shallow formations.⁵

Application

The first benchmark of any monitoring system is the establishment of a baseline from which to define natural variations in the system prior to injection. Shallow groundwater monitoring at the CO₂CRC (Cooperative Research Centre for Greenhouse Gas Technologies) Otway Project in rural southern Victoria, Australia, was initiated in June 2006, nearly two years prior to the onset of CO₂ injection at the site.⁶ Carbon dioxide is injected into a depleted gas field and saline formations as part of the storage project.⁷

Location of the Otway Project in Australia.⁸

Monitoring the groundwater was considered of particular significance in demonstrating the ongoing integrity of natural resources to the community.⁹ A baseline was established by monitoring seasonal water levels and bi-annual groundwater chemistry in a shallow aquifer that lies approximately 2,000 meters above the CO₂ storage reservoir.¹⁰ Pre-injection baseline measurements, when compared with injection and post-injection monitoring results, indicated no significant fluctuations in the shallow aquifer chemistry as a result of CO₂ injection.

Time series data for three stations (bores) from one of the groundwater aquifers (Port Campbell Limestone) at the Otway Project pre-, during and post-injection of CO₂. Of the 21 bores completed in this aquifer, all are located within 10 km of the CO₂ injection well. No significant or systematic trends were seen in the data from these monitoring wells.¹¹