Seismic Velocity Characterisation of Geothermal Reservoir Rocks for CO2 Storage Performance Assessment

Abstract

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.

Storing Co2 in Geothermal Reservoir Rocks from the Kızıldere Field, Turkey: Combined Stress, Temperature, and Pore Fluid Dependence of Seismic Properties

“…This work serves as a full extension of our earlier experimental work on stress-dependency of seismic velocities for a wide range of dry sedimentary and metamorphic rocks that form the different reservoir and caprock formations at Kızıldere (Janssen et al, 2021). Here, we present an extensive, well-controlled, and detailed laboratory study of the combined stress (up to 42 MPa), temperature (up to 100 ± 1 • C), pore pressure (up to 17 MPa), and pore fluid (brine or supercritical CO 2 ) dependence of the seismic properties in the two main metamorphic reservoir formations that make up the Kızıldere geothermal system: marble and calcschist.…”

“…Turkey has seen a significant growth in its installed geothermal capacity over the past five years, reaching a total of approximately 1710 MW at the end of 2021 (IEA, 2021). The majority of the installed capacity in Turkey comes from geothermal power plants operating along the Büyük Menderes Graben (Kaygusuz et al, 2019;Yamanlar et al, 2020;Janssen et al, 2021;Ozcelik, 2022). Practically all geothermal production wells in the Büyük Menderes Graben produce from carbonate reservoir rocks, leading directly to considerable amounts of dissolved carbon dioxide (CO 2 ) in the produced geothermal fluids (Akin et al 2020;Bustaffa et al, 2020;Parlaktuna et al, 2021;Ozcelik, 2022).…”

“…The work presented here is part of the ACT2 CCS SUCCEED project whose goal is to investigate the viability of re-injecting produced CO 2 back into a geothermal field, for permanent storage and enhancing reservoir pressure, instead of emitting it to the atmosphere. More detailed information on the SUCCEED project is given elsewhere (Janssen et al, 2021;Parlaktuna et al, 2021).…”

Seismic velocity characterisation and survey design to assess CO2 injection performance at Kızıldere geothermal field

“…The seismic source and receiver were placed at the top and bottom of the core sample, respectively. A more detailed description of the materials and equipment, the experimental procedure, and the experimental set-up utilised, is presented in Janssen et al, (2021). Most of the seismic velocity data presented in the top graphs of Figure 11 show gradual increase in velocities as a function of increasing σ1, and thus depth.…”

“…The initial seismic velocity data for the modelled formations were taken from the laboratory experiments described above (Janssen et al, 2021). Subsequent calibration at depth will come from real DAS VSP seismic data after the field surveys.…”