Fault friction and slip stability not affected by Co2 storage: Evidence from short-term laboratory experiments on North Sea reservoir sandstones and caprocks

Abstract

We conducted velocity stepping direct-shear experiments on 5 separate simulated fault gouges made using 3 caprock samples, 1 reservoir rock, and 1 mixture of reservoir and caprock. Our aim was to understand the frictional strength and velocity dependent slip behavior of faults within and lateral to a potential pilot CO2 sequestration reservoir situated in the Netherlands sector of the North Sea. To analyze fault mechanical properties prior to CO2 injection, and to gain an understanding of the potential short-term influence of CO2 after injection, experiments were conducted under a variety of pore fluid conditions; dry, dry-pressurized with supercritical CO2, brine-saturated, and brine-saturated plus pressurized with supercritical CO2. Experiments were conducted at in situ effective stress (35MPa) and temperature (115°C) for the potential pilot project. Results indicate that the coefficient of friction (μ) for the studied caprock material is highly dependent on mineralogy, with μ increasing as a result of increasing quartz content and concomitant decrease in clay content. Quartz-rich reservoir rock exhibited the highest value for μ, and a 50/50 (wt.%) mixture of caprock and reservoir rock showed μ values lying directly between those of the two end-member compositions (0.47 and 0.61). Saturation of clay-rich gouges with a brine solution led to weakening compared to dry gouges, whereas quartz rich gouges showed no clear weakening upon brine saturation. The addition of supercritical CO2 to dry and to brine saturated gouges had no clear influence on frictional strength. The friction rate parameter (a−b) as defined by the rate- and state-dependent friction laws was measured in low shear velocity steps (0.2–1–10–1–0.2μm/s) for all materials and pore fluid conditions, and indicates that all tested materials exhibit predominantly velocity strengthening behavior, and that neither the addition of brine nor supercritical CO2 has any clear, strong influence on friction velocity dependence, indicating that reservoir faults should behave aseismically both before and after the addition of CO2.