Temperature and Gas/Brine Content Affect Seismogenic Potential of Simulated Fault Gouges Derived From Groningen Gas Field Caprock

We investigated the rate-and-state frictional properties of simulated anhydrite-carbonate fault gouge derived from the basal Zechstein caprock overlying the seismogenic Groningen gas reservoir in the NE Netherlands. Direct shear experiments were performed at in situ conditions of 50–150 °C and 40-MPa effective normal stress, using sliding velocities of 0.1–10 μm/s. Reservoir pore fluid compositions were simulated using 4.4 Molar NaCl brine, as well as methane, air, and brine/gas mixtures. Brine-saturated samples showed friction coefficients (μ) of 0.60–0.69, with little dependence on temperature, along with velocity strengthening at 50–100 °C, transitioning to velocity weakening at 120 °C and above. By contrast, gas filled, evacuated and partially brine-saturated samples showed μ values of 0.72 ± 0.02 plus strongly velocity-weakening behavior accompanied by stick slip at 100 °C (the only temperature investigated for gas-bearing and dry samples). A microphysical model for gouge friction, assuming competition between dilatant granular flow and thermally activated compaction creep, captures the main trends seen in our brine-saturated samples but offers only a qualitative explanation for our gas-bearing and dry samples. Since the reservoir temperature is ~100 °C, our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal Zechstein anhydrite-carbonate caprock against the Groningen reservoir sandstone, specifically in the gas-filled upper portion of the reservoir system.