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Insight into the difference between the mechanical properties of rocks at low and  in situ  deep reservoir temperatures is vital for achieving a better understanding of fracking technologies with supercritical CO 2  and liquid nitrogen. To address this issue, the fracking-related mechanical properties of the Shaximiao Formation sandstone (SS) were investigated through direct tension, uniaxial compression, and three-point bending fracture tests at a typical low temperature (T low ) of −10°C and a reservoir temperature (T in situ ) of 70°C. The results showed that the tensile strength  σ   t  , compressive strength  σ   c  , and fracture toughness  K  I C   of the SS were all higher at T low  than at T in situ , although to different extents. The  K  I C   of the SS increased slightly more than  σ   t   at the lower temperature, while both  σ   t   and  K  I C   of the SS increased significantly more than  σ   c   at the lower temperature. In addition to the strength, the stiffness (particularly the tensile stiffness) and the brittleness indices of SS were similarly higher at T low  than at T in situ .  In situ  monitoring using the digital image correlation technique revealed that a highly strained band (HSB) always appeared at the crack front. However, because of the inhomogeneous microstructure of the SS, the HSB did not always develop along the line connecting the notch tip to the loading point. This was a possible cause of the highly dispersed  K  I C   values of the SS. The HSB at the crack front was notably narrower at T low  than at T in situ , suggesting that low temperatures suppress the plastic deformation of rocks and are therefore beneficial to reservoir stimulation.

Mechanical Properties of a Typical Jurassic Shaximiao Sandstone Under Subzero and Deep in situ Temperature Conditions