ALBERT

Description: Application of the Debye-Hückel theory for chemical reaction modeling of geothermal brines does not yield sufficiently accurate results. Thus, for the development of a new chemical reaction module for the numerical simulation model SHEMAT (Clauser and Villinger, 1990), the Pitzer formalism (Pitzer, 1991) is used to calculate aqueous speciation and mineral solubilities. It is based on an extended code of PHRQPITZ (Plummer et al., 1988). Using temperature dependent Pitzer coefficients, the system Na- K-Mg-Ca-Ba-Sr-Si-H-Cl-SO4-OH-(HCO3-CO3-CO2)-H2O can be modeled with sufficient accuracy for temperatures from 0° to 150°C. The incorporated carbonic acid system (set in parentheses in the list above) is valid for temperatures from 0 to 90°C, only. Flow, heat transfer, species transport, and geochemical reactions are mutually coupled for modeling reactive flow. Changes in porosity and permeability influence the flow and transport properties of the reservoir. These changes are taken into account by a relation derived from a fractal model of the pore space structure (Pape et al., 1999). A conceptual case study of the injection behavior of a geothermal installation focuses on the immediate vicinity of the well. The injection of cold water has a great influence firstly on the hydraulic conductivity of the aquifer indicated by continuous head pressure increase at the well and secondly on the equilibria between the minerals of the formation and the geothermal fluid. Reservoir changes are studied for the two cases of temperature dependence of solubility, prograde (i.e. barite) and retrograde (i.e. anhydrite). Dissolution of anhydrite induced by cooling down increases the permeability of the formation in a growing region around the borehole and precipitation at the temperature front decreases it. During the initial period of reinjection considered in this study, the negative effect on the injectivity by the colder water is partially compensated by the dissolution of anhydrite. Precipitation of barite around the borehole does not alter the permeability of the formation significantly because the volume of relocated mineral is too small.

Description: Simulating complex flow situations in hydrogeothermal reservoirs requires coupling of flow, heat transfer, transport of dissolved species, and heterogeneous geochemistry. We present results of simulations for typical applications using the numerical simulator SHEMAT/Processing SHEMAT. Heat transfer is non-linear, since all thermal fluid and rock properties depend on temperature. Due to the coupling of fluid density with both temperature and concentrations of dissolved species, the model is well suited to simulate density-driven flow. Dissolution and precipitation of minerals are calculated with an improved version of the geochemical modelling code PHRQPITZ, which accurately calculates geochemical reactions in brines of low to high ionic strength and temperatures of 0–150°C. Changes in pore space structure and porosity are taken into account by updating permeability with respect to porosity changes due to precipitation and dissolution of minerals. This is based on a novel relationship between porosity and permeability, derived from a fractal model of the pore space structure and its changes due to chemical water — rock interaction. A selection of model studies performed with SHEMAT completes the review. Examples highlight both density-driven and reactive flow with permeability feedback. With respect to the former, the thermohaline free convection Elder’s problem, and density-driven free convection in a coastal aquifer with geothermal exploitation, are considered. Mineral redistribution and associated permeability change during a core flooding experiment; reaction front fingering in reservoir sandstone; and long-term changes in reservoir properties during the operation of a geothermal installation, are all considered in relation to reactive flow with permeability feedback.

Description: Numerical simulation of reactive transport was validated in a core flooding experiment simulating conditions in a managed geothermal reservoir. Permeability was measured along a sandstone core prepared with anhydrite and subjected to a temperature gradient. Anhydrite was dissolved and precipitated in the cold upstream and hot downstream regions of the core, respectively. The numerical code SHEMAT was used to simulate coupled transport and chemical reactions at the temperature front. It comprises an extended version of the geochemical speciation code PHRQPITZ for calculating chemical reactions in brines of low-high ionic strength and temperatures of 0-150 °C. Permeability is updated to porosity via a novel, calibrated power-law based on a fractal pore-space model resulting in a large exponent of 11.3. Simulation results agree well with measured permeability. This both validates the model and demonstrates that the fractal relationship is crucial for a successful simulation of this type of reactive transport.

Description: The subsurface flow and hydrogeothermal simulation system SHEMAT (Bartels, J., Kühn, M., Pape, H., Clauser, C., 2000. A new aquifer simulation tool for coupled flow, heat transfer, multi-species transport and chemical water-rock interactions. In: Proceedings World Geothermal Congress 2000, Kyushu - Tohuku, Japan, May 28 - June 10, pp. 3997-4002) is used to investigate a typical hydrothermal sandstone reservoir situated in the North German Basin. This study focuses on the prediction of long-term behavior of reservoir properties for the entire operation time with reinjection during heat exploitation for district heating. The Stralsund location in NE Germany and the Detfurth sandstone horizon (Buntsandstein) are chosen due to the combination of its already confirmed geothermal potential and the availability of a complete data set. An installation of two production wells and one well for reinjection implements heat exploitation. Reinjection is required due to high salinity of the water. In order to quantify injectivity changes and allow the separation of thermal from chemical effects, changes in the hydraulic parameters of the reservoir are at first studied without chemical reactions. Reinjection of cooled water of higher viscosity than the natural reservoir fluid leads to a continuous reduction of the injectivity. This effect is partially balanced by thermally induced mineral reactions. Dissolution of anhydrite in the vicinity of the injection well dominates the effect of anhydrite precipitation at the propagating thermal front leading to a net increase of injectivity. Observed calcite precipitation around the injection well and dissolution at the thermal front are too small to alter reservoir properties significantly. Coupled numerical simulation indicates that the injectivity of the reservoir is influenced primarily by the viscosity effect, but that mineral reactions weaken this negative trend. Operation of a geothermal heating plant at the Stralsund location would not be restricted by a long-term reduction in the injectivity of the reinjection well.