Effect of Pore Pressure Rate on Rate and State Frictional Slip In Experiments

Injection of fluid into the Earth’s crust can cause earthquakes. Such risks exist in the context of geothermal stimulation, energy storage and recovery, and carbon sequestration. A dramatic example is the rapid increase of earthquakes in the central US.

Understanding induced seismicity is critical to assess and mitigate the risks. Yet, it is challenging because of the complex geology of field sites and uncertainty on material properties. Laboratory experiments provide essential information about how injecting fluids may trigger earthquakes.

Here, the researchers use theoretical analysis and numerical simulations to study the effects of fluid pressurization on rock frictional slip. Comparing simulations with experimental observations, they examine how pore pressure changes affect fault sliding. Computer simulations allow testing parameters at conditions unattainable in the laboratory.

Here, they are used to extrapolate experimental results to the time and scales of field observations. The project also supports the training of graduate and undergraduate students at Northwestern University (IL). Its outcomes improve the assessment of hazards associated with induced seismicity.

More specifically, the project examines the effects of imposed rate of pore pressure change on the stability of rate and state frictional slip. The work is motivated by recent experiments that impose pressure at different rates. These show that the slip velocity and shear stress drop of accelerated slip events correlate with pore pressure rate rather than the magnitude of the pore pressure.

Numerical simulations for a simple spring – slider model assuming sliding is governed by rate and state friction - show that the pressure rate can control the frequency of rapid slip events. Refinement of the model indicates how the features of slip events observed in the laboratory depend on frictional parameters, rate of loading, rate and magnitude of pore pressure increase, and diffusivity.

This work provides a fundamental basis for understanding the effects of pore pressure rate accompanying injection of fluids in laboratory tests and at field sites.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.