Response of Upper Jurassic (Malm) Limestone to Temperature Change: Experimental Results on Rock Deformation and Permeability

Abstract

The present study aimed to quantify the thermally induced deformation and the associated evolution of permeability in Blaubeuren limestone, an outcrop analogue of the Upper Jurassic (Malm) carbonate formation, providing references for hydro-thermo-mechanical responses of the reservoir rock to temperature changes within future enhanced geothermal systems as located in the Southern German Molasse Basin. Experiments deriving the changes in the primary (water-accessible) pore volume and the bulk volume of three water-saturated rock samples were conducted via loading the samples to defined stress states ( $${\sigma}_{3}$$ of 15 MPa; $${\sigma}_{1}$$ of up to 75 MPa ), respectively, and then cycling sample temperature between 30 °C and defined levels up to 120 °C at a sustained pore pressure of 0.5 MPa. Permeability was measured under isothermal conditions at each attained temperature. The primary voids dilated upon heating and partly contracted via cooling in each applied temperature cycle yielding thereby residual dilation. However, the concomitant bulk sample deformation manifested residual compaction. The permeability increased with increasing temperature and showed residual decreases by the end of the temperature cycling tests. Flow of pore fluid from the primary voids into the secondary voids at increased temperatures due to the decreased fluid viscosity mimicked irreversible dilation in the primary voids. The interplay between pressure solution-driven compaction and thermal expansion in rock solids was considered to account for the sample deformation and the development of permeability. It is presumable that the pressure solution-driven compaction and the contraction in rock solids would influence the long-term hydro-thermo-mechanical behavior of the Upper Jurassic carbonate reservoir rocks in the cooling process related to fluid injection at the geothermal systems located therein.