Speaker: Dr. David Dewhurst
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This presentation will comprise two short talks highlighting different aspects of the use of geomechanics as part of the process of site evaluation for sub-surface CO2 storage. The first part of the talk will deal with experimental geomechanics and the impact of ageing shales in supercritical CO2, while the second part will deal with geomechanical modelling of an offshore storage site with scant data availability.
Two mudrock seals from a site in SE Asia being evaluated for geological storage of CO2 were tested to understand their geomechanical properties, mineralogical composition and geochemical make up before and after immersion in super-critical CO2 (sCO2) at high pressure and temperature (150˚C and 4200 psi) representing close to in situ conditions. Physico-chemical properties such as cation exchange capacity and specific surface area were also evaluated. Samples for mineralogy, geochemistry and physico-chemical properties were tested stepwise from their initial state through to 1, 4 and 6 months immersion in sCO2. Samples for geomechanical testing were compared in their initial preserved state and after ageing for 6 months in sCO2. No change in composition was noted for either mineralogy or major element geochemistry and the physico-chemical properties measured were also unchanged. However, geomechanical properties did change, with compressive strength, tensile strength, friction coefficient and elastic stiffness all increasing after exposure to sCO2. As there was no change in mineralogy or geochemistry, it is most likely that the loss of water during exposure to sCO2 resulted in the strength increase observed in these mudrocks.
The Petrel sub-basin has been assessed as potentially suitable for the geological storage of carbon dioxide. One of the elements required in characterising an area for geological CO2 storage involves geomechanical modelling of the region. A preliminary set of simple geomechanical models were performed to evaluate the risk of fault reactivation and potential degree of uplift for CO2 injection in the Petrel Sub-basin. A number of injection scenarios were run, ranging from likely in situ injection rates (1-5 million tonnes of CO2 per year) to very large injection rates (~80 million tonnes of CO2 per year). Overall model results suggest that injection at 1 to 5 million tonnes/year does not result in fault reactivation or host rock failure. Partial fault failure can occur at unrealistically high injection rates of 20 million tonnes/year or above. No fault reactivation occurs at any injection rates under strike slip stress conditions, fault reactivation only happens when the stress regime is on the normal fault-strike slip fault boundary. Varying fault permeability by up to two orders of magnitude changes modelled flow patterns, uplift and pore pressure distributions slightly but does not significantly affect fault reactivation potential.
Dr. David Dewhurst is a chief research scientist at CSIRO Energy in Perth. He holds a BSc (Hons) in Geology from the University of Sheffield (UK) and a PhD in Physics from the University of Newcastle Upon Tyne (UK). He previously held post-doctoral positions at the University of Birmingham (UK), the University of Newcastle upon Tyne (UK), the Institut Français du Pétrole near Paris and Imperial College, London, before moving to CSIRO in 1998. He works on mechanical and physical properties of rocks for petroleum exploration and development, specialising in overburden and gas shales, as well as reservoir and seal evaluation for geological storage of CO2 and other gases.