The three-dimensional (3D) distributed acoustic sensing (DAS) vertical seismic profile (VSP) technique is an effective tool to characterize subsurface reservoirs, enabling the use of large and densely sampled borehole receiver arrays with many surface vibrator source points for onshore time-lapse monitoring. However, the processing of the DAS VSP signals for imaging purposes is based on a reliable wavefield separation, which may depend on the recognition and quality of the direct arrivals. To overcome this limitation for common-source gathers with poor signal-to- noise ratio or with interferences, we apply the dual-signal processing method, which allows us to estimate and separate the DAS wavefields by signals' combination without arrival picking. We present a case study of a 3D VSP DAS dataset recorded at a geothermal reservoir in Turkey, showing that the method, similar to a geophone and hydrophone combination, is robust and effective and can be advantageously integrated with the conventional processing. Supported by signal benchmarking, modelling and signal-to-noise ratio analysis, we treat common-source and common-receiver data. Our analysis shows the advantages and limitations of the proposed approach, valuable in the time-lapse perspective.

The noncondensable gases in most geothermal resources include CO2 and smaller amounts of other gases. Currently, the worldwide geothermal power is a small sector within the energy industry, and CO2 emissions related to the utilisation of geothermal resources are consequently small. In some countries, however, such as Turkey and Iceland, geothermal energy production contributes significantly to their energy budget, and their CO2 emissions are relatively significant. SUCCEED is a targeted innovation and research project, which aims to investigate the reinjection of CO2 produced at geothermal power production sites and develop, test, and demonstrate at field scale innovative measurement, monitoring and verification (MMV) technologies that can be used in most CO2 geological storage projects. The project is carried out at two operating geothermal energy production sites, the Kızıldere geothermal field in Turkey and the CarbFix project site at the Hellisheiði geothermal field in Iceland. Together with a brief description of the project, this paper presents the details of the two field sites and the progress made in seismic velocity characterisation and modelling relevant to the Kızıldere geothermal field in Turkey.

As part of a seismic monitoring project in a geothermal field, where the feasibility of re-injection and storage of produced CO₂ is being investigated, a P-and S-wave seismic velocity characterisation study was carried out. The effect of axial (up to 95 MPa) and radial (up to 60 MPa) stress on the seismic velocity was studied in the laboratory for a broad range of dry sedimentary and metamorphic rocks that make up the Kızıldere geothermal system in Turkey. Thin section texture analyses conducted on the main reservoir formations, i.e., marble and calcschist, confirm the importance of the presence of fractures in the reservoir: 2D permeability increases roughly by a factor 10 when fractures are present. Controlled acoustic-assisted unconfined and confined compressive strength experiments revealed the stress-dependence of seismic velocities related to the several rock formations. For each test performed, a sharp increase in velocity was observed at relatively low absolute stress levels, as a result of the closure of microcracks, yielding an increased mineral-to-mineral contact area, thus velocity. A change in radial stress appeared to have a negligible impact on the resulting P-wave velocity, as long as it exceeds atmospheric pressure. The bulk of the rock formations studied showed reducing P-wave velocities as function of increasing temperature due to thermal expansion of the constituting minerals. This effect was most profound for the marble and calcschist samples investigated.

Hydraulic fracturing of coalbed methane wells has been widely practised as an effective method to increase drainage efficiency in low-permeability, low-pressure and low-saturated coal seams. To investigate hydraulic fracture performance and associated seismic response in coal, hydraulic fracturing experiments were carried out on two cubic coal blocks containing a host of natural fractures using a true triaxial rock testing machine equipped with loading, injection and acoustic systems. The acoustic system uses transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses for characterising mechanical properties of the coal blocks and revealing fracture growth. Silicon oil was injected into the middle of coal blocks to create hydraulic fractures under deviatoric stress conditions, and the stress and displacement, borehole pressure and volume, and seismic response were recorded over the injection process. X-ray computed tomography (CT) was conducted before and after the experiments to identify the location and geometry of hydraulic and natural fractures. Results have shown that the fracturing behaviour, the drawdown period of borehole pressure and the intrusion of fracturing fluid are dominated by the complexity and insulation offered by internal natural fracture networks of coal blocks. In addition, seismic spectrograms captured both fracture initiation and its subsequent interaction with natural fractures, which indicates that the induced fracture and fracturing fluid interfere with the propagation of seismic waves and influence ultrasonic seismic characteristics. Seismic velocity tomography of ultrasonic acoustic signals recorded also provided the spatial information of fractures, such as approximate locations of pre-existing fractures and injection-disturbed regions.