Geothermal Implications For Fracture-Filling Hydrothermal Precipitation

Abstract

Fluid circulation in geothermal reservoirs is greatly dependent on the geometry and hydraulic properties of fractures. The soultz-sous-forêts site located in the upper rhine graben in alsace, france, consists of a granitic reservoir overlain by a 1.4 km-thick sedimentary succession. This is a site of significant geothermal potential thanks to the high thermal gradient in the first 1km and the abundance of natural brines.These brines circulate overseveral kilometres,facilitating heat transfer. Exploiting this natural heat source involves using deep wells, the fracture network in the triassic sediments and granitic basement acting as a heat exchanger. Temperature profiles show that in the sediments, the main regime for heat transfer is conductive as opposed to the convective regime in the base rock below. This makes the interface between the two a zone of particular interest with regards to fluid flow. Core analysis and borehole wall imagery collected from reconnaissance well eps1, drilled to a depth of 2230m, reveal an extensive fracture network throughout the granite and overlying sediments, including both open fractures and fractures filled through mineral precipitation (Primarily quartz, barite, calcite, and galena). These fractures have a preferred orientation; The two major fracture sets strike n005° and n170°, dipping 70°w and 70°e respectively. Here we present an experimental study that aims to provide insights into the impact of sealed or partially-sealed fractures on permeability anisotropy in the triassic buntsandstein sandstone (1000-1400 m depth). We then propose a time scale for sealing through mineral precipitation.