## 1 Introduction  
The total worldwide installed geothermal capacity in 2019 was 15,406 MWe. At the time, the European installed geothermal capacity reached 2960 MWe. The main European players in this field are Italy, Iceland, Turkey, Germany, France, and Portugal, with known and excellent capacities in Hungary, Poland, Romania, and Switzerland [1, 2]. With the new plants introduced in 2018, the installed geothermal capacity in Turkey has increased to about 1500 MWe, mainly in the Denizli and Aydin provinces [2, 3, 4]. The bulk of the aforementioned installed capacity in Turkey is represented by power plants located along the Buyuk Menderes graben. Although it is widely assumed that geothermal energy is a clean, i.e., zero-emission and renewable energy [5, 6], most geothermal energy plants emit carbon dioxide (CO2) as a part of the produced steam. In Turkey, the non-condensable gases that are released from geothermal plants within the country consist of 95 to 98% of CO\({}_{2}\). This leads to total CO\({}_{2}\) emissions from geothermal power plants of roughly 900 to 1300 gr/kWh [7]. The main reason for this is that nearly all geothermal reservoirs in Turkey are producing from carbonate rocks. The study presented here is part of the ACT2 CCS SUCCEED project where the aim is to investigate, and demonstrate, the feasibility of utilising produced CO\({}_{2}\) for re-injection in a geothermal field. The overall goal of the project is to provide the geothermal energy sector with the means to address the climate change challenge through CO\({}_{2}\) utilisation, to enhance reservoir pressure for geothermal deployment, as well as permanently storing the injected CO\({}_{2}\). This study focusses on one active geothermal power generation site: Kizildere in Turkey. An existing well will be used to inject both produced and captured CO\({}_{2}\), in supercritical state, into the geothermal reservoir. The idea of combining CO\({}_{2}\) storage with geothermal energy is not new [8; 9; 10; 11; 12; 13; 14], and the approach shows similarities with the concept of a CO\({}_{2}\)-plume geothermal (CPG) system, proposed by [PERSON] and [PERSON] [15; 16]. Key difference is that in a CPG system, CO\({}_{2}\) is treated as the working fluid, providing energy for electricity generation [15; 16; 17; 18; 19], whereas in our approach produced CO\({}_{2}\) is utilised for re-injection to maintain and enhance reservoir pressure as the driving mechanism, and, thus, improving geothermal performance. This provides a cost-effective and low-environmental impact coupled geothermal-CO\({}_{2}\) storage technology. This study presents an extensive and detailed laboratory investigation for which the reservoir and CO\({}_{2}\) injection conditions that prevail at the Kizildere site are mimicked. Using reference samples from the geothermal reservoir, acoustic measurements are carried out at varying stresses and temperatures to obtain a baseline seismic-response characterisation. Subsequently, in a later phase of the project, the acquired acoustic data will serve as an input for simulating seismic wave propagation using a detailed subsurface model that represents the Kizildere site. The simulation results will help the design of the active seismic surveys for the monitoring of CO2 injection performance. Acoustic wave velocities in different rock types as a function of confining pressure [20; 21; 22; 23], pore-pressure [24; 25; 26; 23], and temperature [27; 28; 29; 30; 31; 32], were studied by many researchers in the past. Bulk of these studies were either done on sedimentary rocks [20; 21; 26], igneous rocks [23], or on a combination of both [22; 24]. Most laboratory studies, aiming at investigating temperature-dependent velocities, were conducted under extremely high pressures (up to 600 MPa) and temperatures (up to 1000 \({}^{\circ}\)C). Few researchers have assessed the effect of temperature on seismic velocity in a more realistic range, for geothermal reservoirs of up to 250 \({}^{\circ}\)C, utilising saturated samples [31; 32].