The effect of carbonate content on the mechanical behaviour of simulated clay-rich fault gouges

CO₂ capture and storage (CCS) in depleted oil and gas reservoirs is seen as one of the most promising large-scale CO₂-mitigation strategies. A key issue here is prediction of the effect of fluid-rock interaction on the mechanical integrity and sealing capacity of faults that cut clay-rich caprocks, on timescales of the order of 10³ or 10⁴ years. However, chemical interactions in rock/CO₂/brine systems are so slow, that the long-term effects of fault rock composition, microstructure, mechanical properties and transport properties cannot be reproduced in laboratory experiments. One way to overcome this problem is to use simulated, caprock fault-gouges in experiments, with a composition similar to natural material that has been altered by long-term reactions with CO₂ and brine. In CCS settings, where leaching or else carbonation reactions are expected to be widespread in the long-term, changes in the clay:carbonate content of claystone-derived gouges may significantly affect fault rock properties. This idea is supported by the contrasting behaviour of phyllosilicates, which show stable slip behaviour at the crustal P-T conditions associated with geological storage of CO₂, as opposed to carbonates, which have been shown in previous studies to become prone to seismogenic (velocity-weakening) slip at temperatures above about 80°C. However, very little is known about the mechanical and transport properties of carbonate/clay mixtures. We have investigated experimentally how clay:carbonate ratio affects fault friction, fault reactivation potential and slip stability, i.e. seismic vs. aseismic behaviour, as well as transmissivity evolution during and after fault reactivation. Simulated fault gouges were prepared by crushing natural Opalinus clay material (Mont Terri, Switserland). We used three types of starting compositions: i) crushed natural claystone samples, consisting mainly of phyllosilicates (60%), quartz (~20%) and calcite (15-25%), ii) 'leached' samples, consisting only of phyllosilicates (65%) and quartz (35%) and thus representing the maximum possible effect of CO₂, and iii) carbonate/phyllosilicate mixtures with intermediate compositions. We performed triaxial direct shear experiments at relevant in-situ temperatures (60-120°C) under fluid-saturated conditions (P_p = 25 MPa), using demineralized water as pore fluid, at an effective normal stress (P_e) of 50 MPa and shear velocities of 0.22 -10.9 μm/s. Preliminary results show that the shear strength (friction coefficient, μ) of the fully leached samples decreases by ~10-15% with respect to the crushed natural samples. Typical steady-state μ-values obtained for the natural samples are 0.27-0.33, whereas μ for the leached samples is 0.24-0.27. These values are much lower than for pure calcite (0.62-0.71). All samples show velocity-strengthening behaviour and only minor effects of temperature on the stability (rate and state friction) parameter (a-b) for the compositions investigated to date, suggesting that fault slip should be stable provided the calcite content remains less than 25% in CO₂ storage situations.