Multiscale seismic characteristics of fluids in natural fractures from the GCS pilot project, Kevin Dome, Montana

Novel subsurface engineering technologies such as geologic carbon sequestration (GCS), enhanced geothermal energy production, and unconventional hydrocarbon stimulation are increasingly utilizing seismic monitoring networks that allow characterization of subsurface perturbations with higher resolution than previously acheivable. To make the best use of high-resolution spatiotemporal data, it is essential to understand the physical processes influencing the geophysical behavior of reservoirs at a correspondingly fine scale. Time-lapse (4D) seismic monitoring is one of the main tools used to track dynamic changes in the subsurface related to the injection of fluids, for example, supercritical carbon dioxide (sCO₂). One environment where time-lapse seismic is being tested is low-porosity and low-permeability reservoirs, where acceptable injectivity can be provided by fractures. One of the challenges is that high-quality laboratory data on the seismic properties of natural fractures during such injection activity are sparse. Conventional models also suggest that these "stiff" reservoirs can be challenging to monitor seismically due to small fluid substitution effects.

We present laboratory seismic measurements conducted on naturally fractured dolomite samples acquired from the Kevin Dome GCS pilot site as part of the Big Sky Carbon Sequestration Partnership. Using samples from the Duperow Formation (dolostone) obtained from the Wallewein and Danielson test wells, we conducted multi-scale structural (SEM, interferometry, synchrotron micro-tomography, pressure sensitive film) and multi-frequency seismic (10 mHz - 1 MHz) measurements on naturally and artificially fractured core in order to characterize the effects of fluid substitution, effective stress, and chemical interaction with pore fluid on the seismic response of fractured reservoir rock. We focus in particular on the textural and seismic characteristics of natural fractures, which are often partially healed via mineral precipitation. These measurements provide insight into the basic physical processes controlling seismic behavior of fractured rocks in general and can also be used to directly inform GCS monitoring efforts for similar reservoirs.