In the ACT Consortium funded project SUCCEED, researchers study the potential for monitoring the process of (re-)injecting produced and captured CO2 into the Hellisheiði geothermal field for the aid of enhancing geothermal deployment as well as permanently storing CO2 through mineralization. The Hellisheiði site provides an excellent opportunity for demonstrating an innovative seismic monitoring technique. Prior to conducting an active-source monitoring survey, we perform acoustic transmission measurements, on Hellisheiði rock samples, at field-representative stress conditions to obtain the seismic-response characteristics of all present formations. Subsequently, we use the acquired velocity data as an input for simulating 2D seismic surveys using a subsurface model representing the Hellisheiði site. Results show that the impact of increasing depth, i.e., stress, on seismic velocities is most apparent for the porous basalt layers due to their relatively large portion of open pore space, allowing for substantial compaction, increasing their bulk density and thus velocity. The poorly-consolidated hyaloclastites reveal a negligible effect of increasing depth on their velocity as the material already reached its maximum compaction at low stresses, thus at shallow depths. Comparison of synthetic and field geophone data reveal that the velocity profiles have to be updated for the shallow depths in the model.@en

Two 0.3 m × 0.3 m × 0.3 m shale blocks, one representing a homogeneous sample while the other representing a naturally fractured sample, are modelled using the lattice based DEM code, XSite. The synthetic rock mass approach (SRM), which assigns the smooth joint contacts (SJM) to the weakness planes, is used to represent the natural fractures in shale block-2. Firstly, the developed models are compared with the findings of previously conducted true-triaxial hydraulic fracturing experiments with acoustic measurements, and their subsequent computed tomography (CT) and seismic velocity tomography results. The 3D model results confirmed the curved shape hydraulic fractures, which propagated perpendicular to the minimum stress directions in both shale blocks. Model results also captured the natural fracture (NF) and hydraulic fracture (HF) interaction, particularly the arrest, the dilation of major NFs, followed by crossing with offset mechanism, in shale block-2. Secondly, the parametric studies are carried out to investigate the role of fluid flow rate (q), and fluid viscosity (µ) on different NF/HF interaction mechanisms. The effects of q and µ are discussed based on the total stimulated area including the tensile and shear microcracks, the pipe apertures, and the pressure evolutions within NFs.

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Two 0.3 m × 0.3 m × 0.3 m cubic blocks of shale and coal were used for hydraulic fracturing experiments under true tri-axial stress conditions. The shale block used was highly homogeneous and without visible fractures, while the coal block contained a host of natural fractures. The mechanical and hydraulic properties of both rocks were characterized through multi-stage triaxial tests, Brazilian disk tests, and porosity and permeability measurements. A true tri-axial rock testing machine equipped with loading, pump and acoustic systems was used in the experiment. The acoustic system uses 48 transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses to reveal fracture initiation and growth. Before the experiment, initial seismic response of both blocks was recorded under hydrostatic stress conditions to characterize anisotropy and heterogeneity of the blocks as reference. Silicon oil was injected centrally into both blocks to create a hydrofracture under deviatoric stress conditions and the load, displacement, pump pressure and volume, and seismic response during the injection process were recorded. Results from two blocks are being compared in terms of hydrofracture geometry and seismic features.@en