Reservoir  properties of fault-related hydrothermally altered granites in Cornwall: Implications  for geothermal energy prospectivity           

Abstract

Granite based geothermal systems are currently being explored in Cornwall for their potential to decarbonise energy production. Fault structures, known locally as cross-courses, are being targeted due to their potential to both host fluids at depth and create zones of enhanced permeability through fault related fractures. These structures have been transmissive in the past, evidenced by mineral veins, and surrounding host rock alteration. Hydrothermal alteration has the potential to affect the petrophysical properties of a rock, however these effects on potential geothermal systems has received little attention to date. In this study we measure the tensile strength, porosity and permeability of samples of Carnmenellis Granite which have been hydrothermally altered to different amounts. We find that hydrothermal (argillic) alteration leads to a weaker, more porous and permeable rock, which has implications both in terms of reservoir volumes and fluid production rates. The alteration of feldspars into clay minerals leads to microporous regions that are connected throughout the material, and consequently an increase in total porosity by an order of magnitude and matrix permeability by up to 4 orders of magnitude. However, fractures hosted in the altered material are more likely to close under higher effective stress than those hosted in comparatively unaltered material, which leads to a lower fracture permeability. Finally, we demonstrate that hydrothermally altered zones have the potential to host significantly greater amounts of accessible fluid than the fractures alone, and that they should be considered when assessing reservoir volumes in these types of geothermal systems.