Abstract
To investigate the impact of rock heterogeneity and flowrate on reaction rates and dissolution dynamics, four millimetre-scale Silurian dolomite samples were pre-screened based on their physical heterogeneity, defined by the simulated velocity distributions characterising each flow field. Two pairs of cores with similar heterogeneity were flooded with supercritical carbon-dioxide (scCO2) saturated brine under reservoir conditions, 50°C and 10MPa, at a high (0.5ml/min) and low (0.1ml/min) flowrate. Changes to the pore structure brought about by dissolution were captured in situ using X-ray microtomography (micro-CT) imaging. Mass balance from effluent analysis showed a good agreement with calculations from imaging. Image calculated reaction rates (reff) were 5-38 times lower than the corresponding batch reaction rate under the same conditions of temperature and pressure but without mass transfer limitations. For both high (Péclet number=2600-1200) and low (Péclet number=420-300) flow rates, an impact of the initial rock heterogeneity was observed on both reaction rates and permeability-porosity relationships.
Highlights
Understanding reactive transport in subsurface formations is important in many applications including carbon capture and storage (CCS) (Bachu, 2000), well acidization (Wang et al, 1993), contaminant transport (Zheng and Bennett, 1995), and leaching (Lin et al, 2016b)
Dynamic micro-CT imaging is used to study the dissolution of Silurian dolomite by scCO2 saturated brine at reservoir conditions at the millimetre scale
We investigated the effect of initial rock heterogeneity and flowrate on effective reaction rates, porosity-permeability relationship, and the nature of dissolution patterns by employing combined pore-scale imaging and direct numerical simulation modelling of flow on images
Summary
Understanding reactive transport in subsurface formations is important in many applications including carbon capture and storage (CCS) (Bachu, 2000), well acidization (Wang et al, 1993), contaminant transport (Zheng and Bennett, 1995), and leaching (Lin et al, 2016b). Field-scale flow modelling that incorporates reaction processes between the CO2, formation brine, and rock is a necessary precaution and may help control uncertainties which affect storage security (Sifuentes et al, 2009). This task proves challenging as there are large variations in reported values for parameters, resulting in predicted reaction rates that span orders of magnitude (Black et al, 2015). It is common to use a power law to predict permeabilityporosity relationships, calibrated by its exponent − denoted in this work by m (Carman, 1937; Kozeny, 1927) This relationship is needed to characterize the effect of dissolution on averaged flow properties as input into field-scale models. Pore-scale observations can help explain the mismatch between laboratory and field scale reaction rates (Li et al, 2006; Moore et al, 2012; Noiriel et al, 2004; Swoboda-Colberg and Drever, 1993; White and Brantley, 2003) and help elucidate the relationship between permeability and porosity for different initial pore structures
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