We performed ultrasonic reflection measurements on a basalt sample from the Hellisheidi geothermal field, Iceland, as part of the Synergetic Utilisation of CO2 storage Coupled with geothermal EnErgy Deployment (SUCCEED) project. We performed the measurements on a sample, representing the geothermal-reservoir rocks, placed in a cylindrical steel borehole simulator. The cylindrical basalt sample had a diameter of 40 cm and height of 48 cm. It was subjected to 23.3 MPa axial pressure and 14.7 MPa radial pressure. We performed measurements before CO2 injection and at different levels of injected CO2, where the injection was performed in the sample at an artificially constructed horizontal fracture. During CO2 injection, the temperature was 40°C. We performed the reflection measurements using Panasonic 1-MHz ultrasonic transducers. We used two fixed points distanced at 20 mm for source transducers. We deployed a construct with four receiver transducers with fixed distance between them of 25 mm. The construct was placed in-line with the two sources, and measurements were taken from each source. The construct was then slid 5 mm away from the closest source, and new measurements were taken. The construct was moved five times to obtain 21 separate receiver points spaced at 5 mm. In this way, we measured two common-source gathers each with 21 receivers. The transducers were placed on one of the plane sides of the borehole simulator.
To reach the target depth level of the artificial fracture and return to the receivers, the ultrasonic waves pass two times through the steel cap of the simulator (215 mm thickness) and an underlying aluminium plate (30 mm thickness), while the fracture is 67.5 mm below the top of the basalt sample. To eliminate the kinematic influence of the steel and aluminium, we apply seismic interferometry for retrieval of non-physical reflections. We do this by correlating at each receiver the reflection arrival from the basalt/aluminium interface with the reflection arrival from the basalt fracture, and them sum the 21 correlation results over the receivers. This retrieves a non-physical reflection arrival between the two sources (turning one of them into a virtual receiver), which in a kinematic way corresponds to a reflected wave that has propagated only between the basalt/aluminium interface and the fracture. In this way, we monitor directly for seismic-velocity changes inside the basalt. We show comparisons of the thus-estimated dry-measurement velocities and the velocities at the different CO2 injection levels.
Acknowledgements
The SUCCEED project is funded through the ACT – Accelerating CCS Technologies (Project No 299670) programme. Financial contributions by the Department for Business, Energy & Industrial Strategy UK (BEIS), the Ministry of Economic Affairs and Climate Policy, the Netherlands, the Scientific and Technological Research Council of Turkey (TUBITAK), Orkuveita Reykjavíkur/Reykjavik Energy Iceland (OR) and Istituto Nazionale di Oceanografia e di Geofisica Sperimentale Italy (OGS) are gratefully acknowledged.
