Transport properties of faulted clay-rich caprock: implications for CO2 storage

Abstract:

Faults are weak features within reservoir-caprock systems and may affect the sealing integrity of suitable CO₂-storage reservoirs. Changes in stress state due to CO₂-injection may induce fault slip movement and potentially result in the formation of leakage pathways to fluids stored in the reservoir. Therefore, it is important to investigate the effect of changes in stress state on the slip tendency and transport properties of fault zones. However, flow properties of a fault zone are not only controlled by the fault core, but also by the surrounding damage zone. Therefore, it is important to understand the behaviour of both the fault core, consisting of fine-grained fault gouge, and the damaged but intact surrounding caprock.

Fault slip behaviour depends on both the stress state and the mineralogical composition of the fault plane. We investigated the effect of stress state on fault friction behaviour, fault reactivation, slip stability and flow properties. Three types of a natural clay-rich caprock (Opalinus Claystone (OPA), Mont Terri, Switzerland) were used: i) natural clay-rich simulated (crushed) fault gouge, ii) intact claystone samples and iii) intact clay-rich fault core material from the main fault in the OPA. The OPA consists of phyllosilicates (60%), quartz (~20 %) and calcite (~15-25%).

We performed triaxial, direct shear experiments at room temperature (σ_n^eff = 5-50 MPa, P_p = 2 MPa, V = 0.054 -10.9 µm/s). For the simulated fault gouge samples, across and along fault (argon) permeametry were measured both prior and during deformation to investigate the effect of shear displacement on transport properties. For the intact samples, only across fault permeability measurements were made.

Preliminary results show that permeability decreases with shear displacement. The initial permeability of intact OPA is in the order of 10^-20 m². The permeability of simulated fault gouges, both along and across fault, is sensitive to the applied normal stress, with permeability decreasing gradually with normal stress from ~10^-17 to 10^-19 m² for across fault permeability and from 10^-16 to 10^-18 m² for along fault permeability. Across permeability is generally 1-2 orders of magnitude lower than permeability measured along the fault plane. This suggests that flow along the fault will be easier than across fault.