Diffusion driven barite front nucleation and crystallisation in sedimentary rocks

Abstract

The need to investigate mineral precipitation in heterogeneous sedimentary rock with complex pore network and mineral composition arises with problems like pore clogging by barite precipitation during sulfate-rich water injection in geological reservoirs, the durability of long term storage of nuclear waste or the damage induced by crystallisation. At the LFCR (Laboratoire des Fluides Complexes et leurs Réservoirs, Anglet, France), we aim to reproduce geological objects in laboratory conditions. In this frame, we built a contra diffusive set up to precipitate a barite front in two sedimentary rock samples, a Lavoux oolitic limestone and a Vosges Sandstone with a fraction of clays. Two reservoirs filled respectively with BaCl2 and Na2SO4 dissolved in milli-Q water are in contact with a porous sample so that the ions diffuse through it. X-ray tomography shows that a barite front is precipitated in both samples at different positions depending on the diffusion of the different ions. SEM/EDS microanalysis on polish sections highlight both the barite front location and its connectivity. In the Lavoux limestone, a connected barite front is present. Fine barite aggregates preferentially precipitate in the smallest pores of the Lavoux sample, whilst the crystals precipitating in large macropores (>20 μm) show a preferential orientation. We propose that the Gibbs free energy barrier for barite heterogeneous nucleation in the limestone is lower in contact with micrite in small pores than in contact with euhedral calcite in large pores. Finally, the Gibbs energy barrier for barite homogeneous nucleation in large pores is the highest. In the Vosges sandstone, the barite front is scattered with well-crystallized barite precipitating in large pores, and a more striking thin layer of barite is precipitated in the interfoliar space of chlorite-smectite complex. Consequently, we propose that smectite can concentrate barium by adsorption. Then because sulfate and sodium concentrations increase, ultimately barium is desorbed and barite precipitate.