Improved Understanding of Carbon Storage Risk Via Controlled-Release Experiments

Two issues are often recognized as main risk drivers for carbon storage projects. The first is the possibility of pressure-induced slip of pre-existing faults, which can lead to breaching of seals to CO₂ storage reservoirs. Although the mechanics of induced seismicity are well known, the characteristics of such slip events are poorly constrained and significant questions remain. The second is the potential impact of leaking CO₂ on the quality of shallow potable groundwater. While several studies have been conducted using laboratory tests, natural analogues, and numerical models to evaluate the water quality changes induced by elevated CO₂ concentrations, predictive understanding of these coupled processes remains limited in realistic field settings.

We discuss in this invited contribution two controlled-release field experiments targeting remaining science gaps associated with induced seismicity and groundwater chemistry. The first experiment is a planned active fault slip experiment conducted in an underground research laboratory (URL) in a hardened shale formation that serves as a caprock analog. The critically stressed fault will be perturbed by the injection of fluid under pressure to simulate the influence of CO₂ overpressure. The in situ reactivation experiment will use a novel borehole deformation tool that assesses the magnitude of overpressure required to cause slip, defines the mode of this slip as creeping (aseismic) or rapid (seismic), and measures the evolution of permeability on the fault.

The second controlled-release field experiment was conducted in 2011/2012 to simulate the release of CO₂ from a geologic storage site and study the transport as well as the chemical mechanisms leading to the CO₂-induced mobilization of trace elements in a shallow aquifer. The field test involved a dipole system in which the shallow groundwater was pumped from one well, saturated with CO₂ at the pressure corresponding to the hydraulic pressure of the aquifer, and then re-injected into the same aquifer using a second well. Innovative monitoring techniques were used to measure water quality changes and to detect the dissolved CO₂ plume. For both field studies, we will discuss related coupled-processes modeling work.