Reactive transport model of kinetically controlled celestite to barite replacement
Authors: Tranter Morgan , Wetzel Maria , De Lucia Marco , Kühn Michael 
Published: March 2021

&lt;p&gt;Barite formation is of concern for many sustainable utilisations of the geological subsurface, ranging from oil and gas extraction to geothermal reservoirs, and also acts as a scavenger mineral for the retention of radium for nuclear waste disposal. The surface reaction-controlled nature of its formation in these dynamic systems entails a strong sensitivity of the host rock's permeability towards heterogeneities and boundary conditions. The impact of precipitation on effective flow properties can vary by many orders of magnitude as shown by barite scale formation and injectivity loss models for geothermal systems [1], emphasising the need for robust prediction models.&lt;/p&gt;&lt;p&gt;A relevant example case is the replacement of celestite (SrSO4) with barite (BaSO4), which was investigated for various barite supersaturations with flow-through experiments on the core-scale [2]. Three distinct cases were observed for supersaturations from high to low: (1) quick overgrowth and passivation of soluble celestite grains, (2) partial replacement of celestite with barite, (3) slow moving reaction front with complete mineral replacement. The authors presented heuristic approaches that include linking reactive surface area development to molar fractions to model the results. We provide a comprehensive, full-physics geochemical modelling approach using precipitation and dissolution kinetics as well as nucleation and crystal growth [3] for a more flexible representation of the problem. Additionally, the generation of a digital rock representation based on CT-scans of the granular sample is utilised to derive its inner surface area [4]. The experiments were modelled using core-scale reactive transport simulations. The three observed cases at varying supersaturations were reproduced with regard to evolution of sample rock composition and porosity.&lt;/p&gt;&lt;p&gt;In a next step, the characteristic values taken from the calibrated reactive transport models can be further integrated into the existing digital rock physics model [4], thus enabling the development of up-scaled relationships such as reactive surface area as a function of mineral fractions and porosity. The resulting models can then be applied to reservoir-scale simulations for various applications related to subsurface utilisation.&amp;#160;&lt;/p&gt;&lt;p&gt;---&lt;/p&gt;&lt;p&gt;[1] Tranter, M., De Lucia, M., Wolfgramm, M., K&amp;#252;hn, M., 2020. Barite Scale Formation and Injectivity Loss Models for Geothermal Systems. Water 12, 3078. https://doi.org/10/ghntzk&lt;br&gt;[2] Poonoosamy, J., Klinkenberg, M., Deissmann, G., Brandt, F., Bosbach, D., M&amp;#228;der, U., Kosakowski, G., 2020. Effects of solution supersaturation on barite precipitation in porous media and consequences on permeability: Experiments and modelling. Geochimica et Cosmochimica Acta 270, 43&amp;#8211;60. https://doi.org/10/ghntxn&lt;br&gt;[3] Tranter, M., De Lucia, M., K&amp;#252;hn, M., 2021. Numerical investigation of barite scaling kinetics in fractures. Geothermics 91, 102027. https://doi.org/10/ghr89n&lt;br&gt;[4] Wetzel, M., Kempka, T., K&amp;#252;hn, M., 2020. Hydraulic and Mechanical Impacts of Pore Space Alterations within a Sandstone Quantified by a Flow Velocity-Dependent Precipitation Approach. Materials 13, 3100. https://doi.org/10/ghsp42&lt;/p&gt;