Porosity and Thermal Conductivity of the Soultz-sous-Forêts Granite

The success of the Soultz-sous-Foreˆ ts Hot-Dry-Rock project depends on the ability to maintain fluid circulation in a fractured granite. Fractures represent the main fluid pathways. To understand the behavior of this granite in respect to thermal fluid-rock interaction the important aspects are (1) the porous network around these fractures and (2) the thermal conductivity of the rock. This granite is altered and composed of different weathered facies. Variations of porosity and thermal conductivity take place in regard to the alteration and fracturing of the granite. Two types of porosity measurements were performed, mercury injection and water porosity on two samples sizes. The two methods give similar porosity values between 0.3% and 10%. Thermal conductivity measurements were performed in two perpendicular directions to look at anisotropy with two methods at different scale and value ranges from 2.3 to 3.9 W.m)1.K)1. Optical scanning provides us with a good knowledge of local increase of thermal conductivity due to sealed fracture or quartz-cemented matrix. The relationship between porosity and thermal conductivity is not obvious and has to be studied in details, and results show three cases: (1) a relationship between conductivity and porosity (increase of conductivity with a decrease of porosity), (2) a relationship between conductivity and sealed fractures (increase of conductivity related to an increase of fracture density), (3) and a combination of the two previous ones. The results are carefully compared for different types of granite: alterated, fractured or both. These first results indicate that parameters such as thermal conductivity are linked to the porous medium, the structure and the mineralogy of the rock.