Energy, Resources and the Environment

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Pluymakers, A. M. H., J. E. Samuelson, A. R. Niemeijer, and C. J. Spiers (2014), Effects of temperature and CO₂ on the frictional behavior of simulated anhydrite fault rock, J. Geophys. Res. Solid Earth, 119, 8728–8747, doi:10.1002/2014JB011575

Depleted oil and gas reservoirs form attractive CO₂-storage sites, where prerequisites to efficient and safe CO₂-storage include no leakage and no (additional) seismicity. It is thus of importance to understand the possible effects of CO₂ not only on the geomechanical behavior of the reservoir- and caprock, but also of the crosscutting faults. Many of these potential storage reservoirs are topped by anhydrite caprocks (CaSO₄), and thus reservoir-bounding faults are likely to contain anhydrite-derived damage material, or ‘fault gouge’. To better understand the frictional properties of anhydrite fault gouges, we have performed friction experiments on simulated anhydrite fault gouges, including effects of short-term CO₂ exposure. Our main research questions were:

1. How easy or how difficult is it to initiate movement within anhydrite fault gouges, i.e. how strong are they? What is the effect of CO₂?
2. Does anhydrite fault gouge show the potential to nucleate earthquakes (‘seismogenic potential’) at CO2-storage conditions? What is the effect of CO₂?

In these experiments the samples were under a pressure and temperature similar to those at 2-4 km depth. This means an effective normal stress of 25 MPa and temperatures between 80 and 150°C. We also used different pore fluids, namely lab air, water, CO₂ and CO₂-saturated water. For those experiments that contained a pore fluid, the fluid pressure was 15 MPa, so the CO₂ was in its supercritical phase.

Our results indicate that at these conditions, anhydrite exhibits a friction coefficient of 0.5 to 0.7, i.e. a friction coefficient typical for most rocks. However, it is important to note that the weakest samples were those containing CO₂-saturated water. To avoid fault reactivation, this small (up to 15%) weakening effect should be taken into account when determining the maximum allowable injection rates and pressures into a reservoir. Furthermore, with respect to the possibility of earthquake nucleation, these results show that the presence of supercritical CO₂ does not influence seismogenic potential. Only dry gouges are capable of nucleating earthquakes, and at this pressure, this only occurs at temperatures exceeding 120°C. Such high temperatures are in most areas only expected at depths exceeding 4 km, i.e. deeper than most targeted CO₂ storage reservoirs. Within the investigated temperature-range, gouges that contain water (with or without CO₂) exhibit very little seismogenic potential.

This research has been performed at the HPT lab of Utrecht University, the Netherlands, within the framework of CATO-2, the Dutch research program of CO₂ capture, transport and storage. It has been shown as a poster presentation at EGU 2013, in the ERE division, and has been published in December 2014 in the Journal of Geophysical Research – Solid Earth.