A CO2-enriched brine, and two tracers will be injected in two intervals, one in the core of the fault one in the damaged zone outside of the core, at pressures just below the minimum pressure to open rock bedding planes (32 bar). The injections will be running over a period of approximately eight months.
At the beginning, a few pulse injections at pressures up to the fault opening pressure (40 bar) will be conducted in order to test the response of the fault. The same procedure will be repeated at the end of the eight month to compare the results.
In particular, we will be interested in the activation of aseismic vs seismic slip along the fault, before and after the prolonged exposure to CO2 brine.
We estimate to inject a maximum volume of a few deciliters of brine per week. Just for comparison, at Sleipner (North Sea), the world's first commercial CO2 storage project, approximately 0.85 million tonnes of CO2 is injected each year. Since its inception, over 17 million tonnes have been injected. At another project, Chevron Australia plans to inject between 3.4 and 4 million tonnes of reservoir carbon dioxide per year.
The monitoring system, placed in the six boreholes surrounding the injection borehole, will observe all operations using different techniques: Active and passive seismic sensors will monitor the variations of seismic velocities around the injection, and register possible micro-earthquakes.
Strain sensors will detect micro-slips in case of fault activation. Electric conductivity and pH-sensors will observe the break-through of the brine in the monitoring holes, thus indicating the movement of the injected fluids.
In addition, sampling of rock before and after the injection will provide cores for petrophysical and geomechanical investigation and observation of geochemical changes in the fault rocks. Fluid samplings during the whole injection time will provide material for rock-fluid interaction investigation. The monitoring activities will begin about two months before the injection starts and will end four months after the injection has ended.
Small scale – but better understanding
In contrast to a real, significant CO2 disposal, our experiment only investigates potential leakage on a very small scale. Nonetheless, its findings will contribute to a better understanding of the relevant processes in case of leakage through faults and contribute to an enhanced site characterization.
The experiment involves nine professorships from five academic institutions in Switzerland, all grouped in the Swiss Competence Center for Energy Research - Supply of Electricity.