Permeability creation and damage due to massive fluid injections into granite at 3.5 km at Soultz: 1. Borehole observations

The process of porosity and permeability creation in rock masses through increased pore pressure is important in many areas of geoscience, particularly for engineered geothermal sysytems. In this paper, we analyze an unusually complete data set to determine the hydraulic and mechanical changes that occurred about a 3.6 km deep borehole in previously undisturbed granite because of massive fluid injections. The hole is open for 750 m and intersects a relatively transmissive fault near the bottom at 3.5 km. The equivalent porous medium permeability of the rock mass in the 650 m above the fault was very low (∼10−17 m2), and focused at 17 naturally permeable fractures that lay within hydrothermally altered zones. During injection, some 95% of the flow entered the rock mass at just 10 “major flowing fractures,” most of which were naturally permeable. Following the injections, the transmissivity of the section above the fault increased 200‐fold, and the number of permeable fractures increased to ∼100, the distribution being clearly organized, with major flowing fractures each surrounded by clusters of weakly‐flowing, newly permeable fractures. These zones of permeability creation/enhancement correlate with the presence of hydrothermal alteration, which in turn reflects the intersection of the borehole with extensive, hydrothermally altered, cataclastic shear structures. Thus permeability creation/enhancement occurred primarily within these structures, the major flowing fractures representing the core of the structures and the clusters of newlypermeable fractures denoting the damage zone. Comparison of sonic televiewer logs run before and after the injections showed that all permeable fractures had suffered damage and major flowing fractures had suffered dislocations of millimeters to centimeters.