The dynamics of porosity and reactive surface area changes controlling water-­‐rock interaction Supervisors: Dr Fiona Whitaker1, Prof Tom Scott2, Prof Chris Perry3, Prof Maurice Tucker1 1. School of Earth Sciences, University of Bristol 2. Interface Analysis Centre, School of Physics, University of Bristol 3. School of Geography, University of Exeter The challenge: Understanding reactive surface area is essential for quantitatively modeling water-­‐
rock interactions including chemical weathering and the fate of solutes. Porosity changes associated with mineral dissolution or precipitation are important in a range of geological settings and industrial applications, including diagenesis, karst and ore formation, well stimulation and sealing, and CCS. Associated changes in pore structure and volume affect both flow and transport. Porosity change can be highly localized, leading to pore coalescence and formation of conductive flow channels. Understanding interactions between reactions, pore geometry, mass and solute transport is a prerequisite to accurate prediction of sources and fate of solutes in all reactive systems. The problem: Numerical models coupling reaction kinetics and flow (RTM – reactive transport models) may fail to reproduce laboratory or field observations in heterogeneous porous media, especially where chemical and flow gradients are steep. This is in part due to the use of equivalent rock properties averaged at the scale of an REV, ignoring the pore network geometry and nature and disposition of minerals. Reactive surface area (RSA) is a key parameter because the mineral-­‐fluid interface area controls reaction rate in many systems. RSA is often approximated using geometric relationships based on idealized grain/crystal shapes which can bear little resemblance to reality (Fig. 1). However direct measurement of RSA may overestimate reactivity as it ignores the complex relationship between the mineral surface morphology, diffusive boundary layer thickness and reaction rate. To model reactive transport we need to characterize RSA changes during water-­‐rock interaction and to incorporate their effects into RTMs. The project: will identify the behaviour of RSA in natural systems and evaluate the impact on reactive transport. Using flow-­‐through experiments you will simulate dissolution and precipitation in materials with contrasting RSA. During the course of the experiments you will monitor bulk reactions via fluid composition and changes in permeability as they impact macroscopic flow behavior, and use XRT to directly image changes in pore geometry. Lab experiments could be complemented with field experiments in a natural system which has already been well characterized. You will directly examine changes in the surfaces and pore structure of experimental substrates using 3D imaging of minerals and pores via micro-­‐CT and measure RSA by gas adsorption. From this you will develop pore scale models of flow and reactions using a Lattice Boltzmann approach. Finally, you will use the data from these studies to develop novel approaches to upscale our improved understanding of feedbacks between porosity, permeability and RSA to a macroscopic scale, which can be incorporated into next generation RTMs. Training: This project offers a chance to develop an unusual breadth of expertise, spanning laboratory and field experiments, physical and chemical characterization of porous media, and modeling of water-­‐rock interaction from pore to macroscopic scale. This work is of direct relevance to the Oil and Gas industry as it is central to understanding diagenesis, an important modified of reservoir quality in many carbonate reservoirs, and may be run as an Industrial CASE studentship. Background Reading: Gabellone T. & Whitaker F., Secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems: insights from reactive transport modelling. Sedimentol. 63, 1233-­‐1259 (2016). Lai P. et al., Pore-­‐scale heterogeneity in the mineral distribution of reactive surface area of porous rocks. Chem. Geol. 411, 260-­‐273 (2015). Norriel C. et al., Changes in reactive surface area during limestone dissolution: and experimental and modelling study. Chem. Geol. 265, 160-­‐170 (2009). Steefel C. et al., Micro-­‐continuum approaches for modelling pore-­‐scale geochemical processes. Rev. Min. & Geochem. 80, 217-­‐246 (2015)