ERE
Energy, Resources and the Environment

rock mechanics

Words on Wednesday: Effects of temperature and CO2 on the frictional behavior of simulated anhydrite fault rock

February 4, 2015

Words on Wednesday aims at promoting interesting/fun/exciting publications on topics related to Energy, Resources and the Environment. If you would like to be featured on WoW, please send us a link of the paper, or your own post, at ERE.Matters@gmail.com.

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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.

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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.

… and the winner is: OSPA Winner ’14

January 28, 2015

Every year, young students have the opportunity to compete for the Outstanding Student Poster Award (OSPA) at the EGU General Assembly. The OSP Award is intended to further improve the overall quality of poster presentations and, most importantly, to encourage younger colleagues in presenting their work in form of a poster.

Last year’s OSPA Winner in the Energy, Resources and Environment Division was Elisenda Bakker M.Sc. She is currently doing her PhD at the High Pressure and Temperature Laboratory of the Faculty of Geosciences, Utrecht University (the Netherlands). Her work is part of the European ULTimateCO2 Program, aimed at increasing confidence in the long-term (i.e. after 1000’s of years after site closure) efficiency and safety of subsurface CO2 storage.

We at ERE Matters invited Elisenda to explain the research that won her the OSPA last year! If this will trigger your curiosity, she will be presenting her most recent work at this year’s EGU General Assembly as well. And to all the young ERE scientists: give it a go yourself this year! Prizes include a conference fee waiver for the next EGU General Assembly and a publication free of cost to one of the EGU Journals. 🙂

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BakkerI’m grateful for receiving the ERE OSPA Award 2014, thanks! The work that has earned me the award is part of my PhD-research on the long-term effects of CO2-exposure on the coupled chemical-hydro-mechanical behaviour of faulted clay-rich caprocks. My research is set in a larger consortium that investigates the feasibility of large-scale CO2 storage in the European subsurface. As storage can only be considered when the CO2 can be retained for the long-term, it is important to investigate the chemical effect, which is known to occur in the presence of CO2, on the mechanical integrity of a potential reservoir-caprock storage facility. And particularly on fragile features such as (pre-)existing faults, which are known to be present in many reservoirs. I performed so-called shear experiments in the HPT-laboratory at Utrecht University to simulate slip along a fault surface, which I can use to get insights into the processes active in reservoir-scale slip movement along faults. Movement along a fault will not always result in earthquakes, and we want to know what the requirements are for the unstable slip regime in which earthquakes might occur when they nucleate in clay-rich caprock, specifically.

Schematic diagram showing a CO2 storage reservoir and overlying caprock, several km's below the Earth's surface, which are cross-cut by a fault.

Schematic diagram showing a CO2 storage reservoir and overlying caprock, several km’s below the Earth’s surface, which are cross-cut by a fault.

This type of work really suits me as I felt, before I ended up doing this, that to me doing research was only valuable when the work that I would do, would contribute to our society. I could not do research for the sake of doing research, so when this research topic crossed my path I decided that this would be the perfect combination of doing scientific research and serving society by investigating the feasibility of a proposed solution to one of the major challenges mankind is currently facing.