Laboratory experiments as a tool to determine optimal measurement setups for monitoring carbon capture facilities.

Sustainable subsurface usage for carbon capture sequestration (CCS) operations requires a comprehensive understanding of geological structures and stress conditions. The fibre optical method enables on-demand data acquisition and, as a result, facilitates near-real-time updates of subsurface deformation risk models based on real-time data. These measurements can be used as input data for subsurface stress analysis and, consequently, aid in managing storage containment risks. Although distributed acoustic sensing is widely applied for the imaging of the reservoirs and monitoring of the subsurface operations, current research challenges include understanding the influence of the geometry of the acquisition set-up on the capability to resolve the changes in the reservoir during active seismic surveying, to detect induced seismicity and resolve the source mechanisms of the recorded events.

To better understand the optimal acquisition geometry, limitations due to cable positioning, and the influence of the coupling conditions, we created measurement setups in a well-controlled laboratory environment using large-scale samples (height: 0.47, diameter: 0.39 m) of basalt, marble, sandstone, and fibre optical cables for acoustic data recordings. In the first test, due to the limitations of the gauge length parameter (minimal value of 2 m) of the interrogator unit, we coiled marble and basalt samples with telecommunication fibre to achieve dense sampling of receivers (0.01 m) along the sample length using distributed acoustic sensing (DAS) technology and placed the source on top of the samples. In our next experiment, we used an interrogator unit, which allowed us to record the data in the mm gauge length range. Therefore, we tried to reproduce more field-like experiments with dense spatial sampling along the sample. The eight cables were placed around the sample with an azimuthal distance of 45 degrees and, in addition, a cable inside the metal tube which imitates the borehole and a cable in the form of a zigzag, which models the possible installation of the cable on the surface for the monitoring.

With our experimental active acoustic setups, we recorded laboratory-scale vertical seismic profile (VSP) data, allowing to create a 3D image of different types of samples using DAS. Furthermore, based on the interpretation of acquired fibre optics data, we were able to locate the fracture plane in marble and basalt samples and show the differences in the responses for natural and artificial fractures in the different rock samples. Additionally, a successfully created realistic acquisition setup on a laboratory scale using cables placed on top of the sample and inside a borehole allowed us to test acquisition geometry, typically used during CCS monitoring.

With these developed lab setups, we aim to better understand the effects of CO2 injections, fracture behaviours in the reservoir areas, and microseismicity detection thresholds and improve seismic monitoring methods. The developed new approach will allow for the improvement of the quantification of detection thresholds for both changes in the reservoir, event detection, and characterisation inside the reservoir.