The potential for in-situ CO2 sequestration was analyzed experimentally for one basaltic hyaloclastite sample from the Nesjavellir geothermal reservoir in Iceland and three metasedimentary rock samples from the Kızıldere geothermal field in Turkey. Based on batch reaction experiments, this paper demonstrates the interaction between a CO2 gas-charged fluid and rock samples from these reservoirs. The experiments were conducted at 260 °C and 0.8 MPa, and 105 °C and 17 MPa for the basaltic and metasedimentary rocks, respectively. The experimental results indicate that CO2 sequestration within the glassy basaltic rocks is hampered by zeolite, chlorite, and anhydrite, which compete with carbonate minerals to uptake divalent cations at the P-T conditions applied. In contrast, the carbonation process for the metasedimentary rocks is inhibited by their mineralogical composition. Generally, these rocks are less reactive and provide an insufficient supply of divalent cations. The batch reactor experiments were numerically simulated with the PHREEQC geochemical modeling program. The simulations indicate that CO2 sequestration is feasible at the tested P-T conditions, provided that silicate and SO4 mineralization is suppressed for the basaltic rocks and that there is an effective source of divalent cations for the metasedimentary rocks.@en

Interpretation of tracer tests in geothermal reservoirs is carried out by fitting the measured data either with simplified two-dimensional (2-D) analytical solutions or with complex numerical models. Available analytical solutions commonly only describe isotropic conditions in 1-D or 2-D, which is generally unsatisfactory to construct realistic reservoir models. Moreover, due to the large spatial and temporal scale of a tracer test in deep reservoirs, the concentration levels measured in the field are relatively low due to dispersion that complicates the assessment of breakthrough curve tailing and residence time. Fitting tracer data with fully resolving 3-D numerical models thus may be more appropriate, but even with a rich set of field data, reliable calibration is often compromised by the high computational effort and data hunger of such models. In this study, an advanced workflow is presented for evaluating tracer test data. Firstly, a 3-D analytical model in which solute transport is considered in anisotropic porous media is developed. Green's function method is applied to obtain a moving line source solution of the convection-dispersion-diffusion equation for solute transport in a 3-D porous medium. In addition, Green's function equation is analytically convoluted with a rectangular pulse function, which represents tracer injection. Secondly, the analytical model results are fitted to the tracer test data by Monte-Carlo simulations to obtain feasible ranges of flow velocities, as well as longitudinal and transversal dispersivities. Finally, the derived parameter values are implemented in a 3-D numerical model to evaluate the solute residence time in a large-scale reservoir. The results applied to field data from the Kızıldere field in Turkey demonstrate that the workflow provides robust estimates of effective parameters from well-to-well data in complex reservoir systems with anisotropic flow paths. Thus, despite the higher effort of applying convolution and stochastic parameter estimation, the preceding analytical step of the workflow substantially eases numerical model set-up.@en

Geothermal energy is commonly recognized as an environmentally friendly source of energy. However, geothermal fluids have unusually high CO2 content, particularly, in carbonated geothermal reservoirs. An efficient method to mitigate the CO2 emissions of geothermal power plants is to re-inject the captured CO2 with the effluent fluid to mineralize mainly into calcite under reservoir conditions (e.g., reservoir temperatures ranging from 200 to 220 °C). One of the major concerns about the re-injection of the captured CO2 is to predict the geochemical interaction between the injected fluid-CO2 and rock, and the corresponding alterations that occur due to the re-injection in the reservoir parameters. For this study, we have selected one of the largest geothermal fields, which is located in the western part of Turkey. A predictive 3-D reactive transport modeling is carried out using TOUGHREACT v1.2 for potential fluid-CO2 injection into deep metamorphic formation rocks consisting of marble, quartzite, and schist. Each rock type with different mineral constituents is examined in three different scenarios. Moreover, in each scenario, three annual injection rates, 500, 2000, and 4000 tons of CO2 for ten years of continuous injection are tested. The aims of the study are first the evaluation of the dynamic fluid-rock interactions with maximum possible CO2-charged fluid injection and second the identification of the mineralization process. The modeling results indicate that the maximum CO2-charged fluid mixture remains stable as a single-phase in all scenarios. The mineralization process of CO2 in the reservoir is limited due to the different mineral contents of the considered metamorphic rocks and the differences between the CO2 mass fraction of the injected fluid and the reservoir. In addition, the injected fluid temperature, the pH, the convective dispersive fluxes between the wells also affect the chemical process. These results show differences compared to the CarbFix project carried out in basaltic Icelandic geothermal reservoirs where the CO2 mineralization process was successful.@en