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 behaviour 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 that faults cross-cutting the Basal Zechstein caprock and juxtaposing the caprock against the reservoir sandstone may show strongly velocity-weakening behaviour, specifically in the gas-filled upper portion of the reservoir/caprock system. From a RSF point of view, these faults thus have an increased potential for nucleating seismogenic slip in comparison with faults in the underlying formations.
The data is provided in a folder with 15 subfolders for 15 experiments/samples. Detailed information about the files in these subfolders as well as information on how the data is processed is given in the explanatory file Hunfeld-et-al-2019-Data-Description.pdf. Contact person is Luuk Hunfeld - PhD student - l.b.hunfeld@uu.nl
