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The Carbfix process is based on solid scientific research and has been proven successful at industrial scale (e.g., Clark et al., 2020). To date, over 70,000 tons of CO2 have been successfully stored at the Carbfix injection site in Hellisheiði, Iceland. To ensure that mineralisation is taking place and that the CO2 is being safely stored, established monitoring techniques are employed. These include the injection of chemical tracers to track mineralisation and the flow of the injected fluids, regular fluid sampling in adjacent monitoring wells, CO2 soil flux measurements to confirm that the injection is properly managed, and reservoir modelling. Proper monitoring and verification are vital to ensure optimal CO2 injection operations for safe and permanent mineralisation. This leads to an increased economic benefit, transparency that is required to maintain public acceptance and a faster knowledge transfer that can lead to faster deployment of future projects.

Schematic of Carbfix injection with an injection well (right hand side) and a monitoring well (left hand side)

How do we monitor the injection of dissolved CO2?

The monitoring strategy of Carbfix applies to all parts of the CO2 transport, injection and mineralisation storage chain. It covers the quality control of the received CO2 and the CO2 prior to injection. It monitors the injection well network, the impact of the operations on the reservoir host, and verifies the mineralisation process of injected CO2.

Temperature and pressure meters monitor the physical state of the CO2 transported, preventing untimely vaporisation of liquids and condensation of gases and impurities. An online process-flow analyzer monitors the chemical composition (major gases and selected impurities) of the gas delivered to the injection system.

To ensure the safety of the injection system, any leak detection from surface installations and wellheads are closely examined. CO2 gas detectors are placed at strategic locations around the premises. In addition, all wellhead buildings are equipped with CO2 sensors and visually inspected for gas leaks.

Pressure sensors in the wellhead's gas pipes, as well as pressure sensors located above where CO2 is dissolved in water and at the wellhead will verify optimal injection conditions. Any formation of gas bubbles will cause a deviation of differential pressure enabling rapid intervention. A conductivity meter is placed at selected depths above the sparger in the mixing pipe, which could further indicate any lack of gas dissolution.

How do we verify mineralisation?

A suite of chemical and isotropic tracers have been employed to monitor the in-situ mineral carbonation at Carbfix.

Photograph of precipitated minerals (green-brown in colour) on a water sampling pump from a monitoring well. These minerals show physically that the injected dissolved CO2 and H2S mineralized through the Carbfix process. The diameter of the pump is 101 mm (Snæbjörnsdóttir et al., 2017).

During the Carbfix pilot-scale injection tests in 2012, over 95% of injected CO2 was verified to mineralise within two years (Matter et al., 2016). The CO2 was spiked with carbon-14 (14C) to monitor its transport and reactivity. 14C2 chemically and physically behaves identically to 12C2 and is only minimally affected by isotope fraction during phase transitions, thus it provides a means to accurately track the fate of injected CO2. In addition, volatile tracers were injected: SF6 during phase I and SF5CF3 during phase II. The CO2 and CO2/H2S mixtures were injected with the tracers into the target storage formation fully dissolved in water. Nearby monitoring wells were subsequently sampled and analyzed for tracers, 14C, dissolved inorganic carbon (DIC) and pH compositions. An increase in tracer content of the samples confirms the arrival of the injectate at monitoring wells.

Comparison of 14C concentrations in the target CO2 storage formation fluid showing a successful mineralization. Time series of expected (solid circles) versus measured (open squares) 14C (Bq/liter) in monitoring well HN04, showing >95% of injected CO2 to be converted to carbonate minerals. The shaded area indicates the phase I and II injection periods (from Matter et al., 2016).

The fate of the injected CO2 is quantified using mass balance calculations. The resultant DIC and 14C concentrations are much higher than those measured in the collected water samples, suggesting a loss of DIC and 14C along the subsurface flow path toward the monitoring well. The differences between calculated and measured DIC and 14C indicate that >95% of the injected CO2 was mineralized through water-CO2-basalt reactions between the injection and monitoring wells in less than 2 years.  (Matter et al., 2016).

In addition, calcium isotopes studies have been used both in pre- and post-CO2 injection to quantify extent of carbonate precipitation for carbon storage. Calcium isotopic studies have shown that up to 93% of dissolved Ca is transformed to calcite minerals during certain injection phases (von Strandmann et al., 2019).

Drill cores can be retrieved from the subsurface to assess if mineralization was successful. However, drilling can be expensive and time consuming as it needs to be from a significant depth and the minerals that form are fine-grained and spread out.

Seismic monitoring

Monitoring any seismic events during reinjection activities is vital to maintain public acceptance and provide transparency. Independent seismic risk assessments are conducted during the preparation phases of CO2 injection. At present, on-site seismic networks are deployed and combined with selected stations from the national permanent seismic network in Iceland to increase the detection level and location accuracy of detectable seismic activity.

Injection sites are monitored before and during the preparation phases of well drilling activities. In addition, storage sites are typically developed in a stepwise manner, where each development stage accounts for any possible induced seismicity due to previous stages. This approach minimizes the risk of seismicity felt at the injection site.

A seismic traffic light system is implemented during commission of injection activities. It is a proven method, developed to minimize risk of induced seismicity, and has been in operation for the Hellisheiði injection system since 2012. The system is based on controlling and adjusting flowrates in the injection system to keep seismicity low and within acceptable levels for the local region.