Hydrodynamics and Growth of Colloidal Silica in Geothermal Reinjection
Department of Mechanical Engineering, University of Canterbury Abstract:
Time & Place
Thu, 10 Oct 2019 10:30:47 NZDT in 129 Meeting Room 1 (next to lecture theatre E5), Engineering CORE
In geothermal energy production from hot aquifers, the cooled and condensed geothermal brine may be reinjected into the water-bearing strata to maintain reservoir pressure and sequester toxic minerals. The solubility of amorphous silica reduces due to loss of heat, and the concentration may increase due to loss of water. Deposition (scaling) of silica may result, narrowing the fluid pathways in the reservoir and reducing the injectivity and the economic lifetime of the well.
In the present work, a comprehensive model is developed, to predict the injectivity as a function of time under the effects of reactive transport of relevant chemical species (e.g. silica, calcite, anhydrite, etc.), using a finite volume Eulerian approach. The fluid pathways in the geothermal reservoir are modelled as parallel flat plates. 2D unsteady partial differential equations are solved to model the heat and mass transfer problems. The consumption of the reactants and the formation of the products are implemented using sink and source terms respectively. The surface chemical reactions are modelled using semi-empirical formulas developed from experimental results reported by other workers, with care taken to select data taken under conditions close to those in geothermal reservoirs where possible. The decrease in reservoir porosity due to accumulated scale is modelled by applying the porosity-permeability correlation proposed by Verma and Pruess (1988). The increase in porosity due to reservoir stimulation processes is also modelled.
The model outputs are validated by comparing to experimental results and real-world data at laboratory and full scales (Huminicki and Rimstidt 2007; Tamura et al. 2018; Carroll et al. 1998; Mroczek et al. 2017; Van den Heuvel et al. 2018; Xu et al. 2004): the thermal and chemical sub-models are validated independently, as well as validating the code as a whole against injectivity measurements. The validation results suggest that the model can be used to predict the lifetime of geothermal injection under varying conditions, but limitations exist due to gaps in current knowledge. These include how to accurately predict the rate of heterogenous nucleation; how to quantify the effects of multivalent ions on the nucleation and growth of silica particles; and sparse data on the interactions of silica particles under typical geothermal conditions. Future work should also study the effect of the varying inclination, roughness, and tortuosity found in natural fractures.