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

Abstract

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.

The physics of fault friction: insights from experiments on simulated gouges at low shearing velocities

“…The direct-shear assembly has proven especially useful for tests employing corrosive pore fluid compositions such as reservoir brine or CO 2 (e.g. Pluymakers et al, 2014;Pluymakers and Niemeijer, 2015;Bakker et al, 2016;Hunfeld et al, 2017Hunfeld et al, , 2019.…”

“…The CNS model has been strikingly successful in reproducing mechanical behaviours observed in laboratory fault-slip experiments (Figs. 7a, b) (Chen and Spiers, 2016;Chen et al, , 2019Chen et al, , 2020Hunfeld et al, 2019Hunfeld et al, , 2020a. In order to reproduce experimental data, the parameters appearing in Eqs.…”

Microphysical Model Predictions of Fault Restrengthening Under Room‐Humidity and Hydrothermal Conditions: From Logarithmic to Power‐Law Healing

“…While this fitting method does not validate the full model, it allows us to verify the important “frictional” component of the model, that is, the time‐dependence of frictional healing associated with gouge densification. Recently, using an empirical creep law derived from compaction data under 100 °C and brine‐saturated conditions, Hunfeld et al (2019) have reproduced their velocity‐stepping tests for a carbonate‐anhydrate‐rich gouge sheared at the same conditions. Future work should include better characterization of the relevant creep mechanism.…”

“…By considering the microscale physics operating in a shearing fault gouge, Chen and Spiers (2016) have previously developed a microphysical model elaborating on the work of Niemeijer and Spiers (2007) (hereafter referred to as the Chen‐Niemeijer‐Spiers or “CNS” model). Using independently measured parameter values, the CNS model is able to simulate a range of laboratory friction tests, including constant‐velocity, velocity‐stepping, and SHS tests, and produce results that capture the main features and trends seen in the experiments, with reasonable quantitative agreement (Chen et al, 2017; Hunfeld et al, 2019). Note that the present model is not derived to account for clay‐rich fault gouges (i.e., with a matrix‐supported structure), which may require a more microstructurally complex model such as the one presented by den Hartog and Spiers (2014).…”

Healing Behavior of Simulated Fault Gouges From the Groningen Gas Field and Implications for Induced Fault Reactivation

“…Zechstein gouges are much more prone to nucleating unstable slip via slip weakening, potentially aided by velocity-weakening in the case of Basal Zechstein material or its mixtures with sandstone gouge [Hunfeld et al, 2017[Hunfeld et al, , 2019. Note here that the seismogenesis-prone nature of the Basal Zechstein and Slochteren sandstone applies not only to first reactivation of long inactive faults but also to repeated reactivation due to gas production, as substantial healing already occurs within 3 months (see Figure 5)-which is rapid on production timescales.…”