| Recent research indicates that CO2 is capable of inducing swelling in clay minerals in a
similar fashion to water, though to a more modest extent. It is therefore of importance for
feasibility studies of the geological storage of CO2 to understand if the addition of CO2 to
clay rich fault zones has the potential to cause significant frictional weakening, similar to that
associated with water. We conduct velocity-stepping direct shear experiments on pre-pressed
plates (49 mm long x 35 mm wide x ~1 mm thick), of montmorillonite and muscovite. An
effective normal stress of 35 MPa is used in all experiments, which is roughly equivalent to
the effective overburden stress expected in many storage projects. Temperature was held
constant at ~48 °C, consistent with previous experiments which indicated CO2 induced
swelling in montmorillonite. Pore fluid conditions are the main variable in this suite of
experiments, in which the frictional strength of each clay mineral is analyzed oven-dry
(attached to vacuum), saturated with deionized (DI) water, and oven-dry saturated
with supercritical CO2. Pore pressure is maintained at 15 MPa for the water and
CO2 saturated experiments (Ïăn=50 MPa, PH20-CO2=15 MPa). Shearing velocity
is varied systematically from approximately 11 μm/s to 0.2, 1.1, 11, 1.1, and 0.2
μm/s in order to determine the rate and state friction parameters, a, b, and DC.
Additionally, microstructural analysis of the post-shear clay gouges is conducted in an
effort to understand the rheology behind changes observed in frictional properties.
Preliminary results of experiments on montmorillonite show an overconsolidation
peak at strains of approximately 0.3 for each of the oven-dry and water and CO2
saturated experiments. Peak friction (μP) for oven-dry montmorillonite is 0.53,
decaying to a steady state friction (μSS) of 0.51. For DI-saturated montmorillonite
μP=0.11 and μSS=0.10. CO2-saturated montmorillonite displays frictional strength
between that of dry and DI-saturated montmorillonite with a peak friction of 0.44,
and steady state friction of 0.34. These early results suggest that saturation with
supercritical CO2 may induce frictional weakening of fault zones rich in swelling clays,
however such weakening is not as significant as that associated with water saturation. |