Chemo‐mechanical behavior of clay‐rich fault gouges affected by CO₂‐brine‐rock interactions

Abstract

The impact of long‐term CO2‐brine‐rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO2 storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre‐existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO2‐charged formation water and fault gouges (1–1000 years residence time, i.e. 10–106 pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σneff = 50 MPa; Pf = 25 MPa; V = 0.2‐100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay‐rich caprocks overlying potential CO2 storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault‐gouge mineralogy imposed a stronger control on clay‐rich fault behavior than the extent of CO2‐brine‐rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO2‐brine‐rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain). © 2018 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons, Ltd.

CO2‐water‐rock interactions in undeformed and sheared claystone caprocks from Northern Europe

“…The initial Opalinus claystone contained a relatively high amount of quartz (44%), with major components including kaolinite (24%), calcite (16%), clinochlore (8%), and illite (8%). The composition found in this study differs from the composition of the larger formation found by, 21 which presents a much lower quartz (28%) and higher calcite content (25%).…”

“…Fracturing not only may lead to an increase in porosity and permeability (facilitating CO 2 penetration), but also the shearing movement may lead to grain size reduction, causing the material to react faster. 21 The aim of this study was to evaluate and compare, via batch experiments and numerical modeling, the mineral alterations in sheared and undeformed samples of clay-rich caprock formations from Northern Europe after exposure to a CO 2 -rich brine. The caprock formations were selected based on their potential suitability to function as a seal for geological storage of CO 2 and likeliness to react within the experimental timeframe.…”

Fault Stability Perturbation by Thermal Pressurization and Stress Transfer Around a Deep Geological Repository in a Clay Formation

“…When accounting also for the slip‐strengthening behavior of the possible composition of fault gouge in a clay formation (Orellana et al, ), it is evident that the canisters emplacement should stabilize normal faults in the host formation. Accordingly, rheological studies show that a rupture nucleating in the argillaceous formation would be likely aseismic because of intrinsic velocity‐strengthening behavior of clay‐rich fault material saturated with water (Bakker et al, ; Tembe et al, ). In contrast, deeper sections of a normal fault may be destabilized by the combined effect of thermoelastic and poroelastic stress and by pore pressure increase.…”

Chemical‐Mechanical Impacts of CO2 Intrusion Into Heterogeneous Caprock

“…Zhang et al, 2016). A limited number of experimental studies assess coupled chemical-mechanical behavior of caprock with CO 2 intrusion (Bakker et al, 2019;Ilgen et al, 2018;Spokas et al, 2018;Y. Zhang et al, 2019).…”

“…Research on long‐term coupled chemical‐mechanical simulations in the caprock is very recent, with emphasis on coupled processes in GCS reservoirs and wellbores (Carroll et al, 2017; Iyer et al, 2020; R. Zhang et al, 2016). A limited number of experimental studies assess coupled chemical‐mechanical behavior of caprock with CO 2 intrusion (Bakker et al, 2019; Ilgen et al, 2018; Spokas et al, 2018; Y. Zhang et al, 2019). Most of these studies use carbonate‐rich caprock samples with excessive CO 2 partial pressure to observe obvious chemical alterations within limited experimental time.…”