Barite precipitation: consequences on fracture permeability and injectivity at the geothermal sites of the Upper Rhine Graben

Abstract

In geothermal reservoirs, fluid circulation is greatly dependent on the geometry, density, and hydraulic properties of fractures. Here we present a combined experimental and modelling study that aims to provide insights into the impact of mineral precipitation on fracture permeability anisotropy in the Triassic Buntsandstein sandstone (1000-1400 m depth) at the Soultz-sous-Forêts geothermal site in Alsace, France. For our study, we targeted borehole samples that best represented the variability of fractures within the Buntsandstein. Forty cylindrical core samples (40 mm in length and 20 mm in diameter) were prepared from the chosen borehole samples such that they contained healed or partially-healed fractures either parallel or perpendicular to their axis. These samples were then subject to porosity and permeability measurements, and thin sections were made for Scanning Electron Microscopy (SEM) to characterise the nature of the fractures and the precipitated minerals. SEM analysis suggests that prevalent pore-filling clays can explain the low permeability of the sandstone host rock. We found that fractures may present a conduit for or a barrier to flow, depending on the nature of the filling and the extent of sealing. We then modelled the crystal growth rate of barite with temperature to estimate a time scale for fracture sealing. (This extended abstract summarises the research reported in Griffiths et al., 2016).