Abstract
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 CO2 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 (Tlow) of −10°C and a reservoir temperature (Tin situ) of 70°C. The results showed that the tensile strength σt, compressive strength σc, and fracture toughness KIC of the SS were all higher at Tlow than at Tin situ, although to different extents. The KIC of the SS increased slightly more than σt at the lower temperature, while both σt and KIC of the SS increased significantly more than σc 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 Tlow than at Tin 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 KIC values of the SS. The HSB at the crack front was notably narrower at Tlow than at Tin situ, suggesting that low temperatures suppress the plastic deformation of rocks and are therefore beneficial to reservoir stimulation.
Highlights
Tight sandstone gas is widely distributed across China, with favorable exploration zones totaling 32 × 104 km2 in area (Jia et al, 2012)
Temperature has a significant impact on the tensile mechanical properties of rocks
The following conclusions can be drawn from these experiments: 1) The average tensile strength σt of the SS increased by approximately 36.09% from 3.43 to 4.67 MPa when the temperature decreased from Tin situ (70 °C) to Tlow (−10 °C)
Summary
Tight sandstone gas is widely distributed across China, with favorable exploration zones totaling 32 × 104 km in area (Jia et al, 2012). Hydraulic fracturing is widely applied to stimulate tight gas sandstone reservoirs to enhance their gas production. This common fracturing technique requires an extremely large amount of water and causes severe environmental pollution (Gallegos et al, 2015; Thomas et al, 2019). Supercritical CO2 fracturing (Wang et al, 2012; Middleton et al, 2015) and liquid nitrogen (LN2) fracturing (Cai et al, 2014; Huang et al, 2020) are the most representative techniques For these two methods, the contacts between low-temperature fluids and reservoir rocks form low-temperature zones within reservoirs, and during the fracturing process, some rocks are fracked in a low-temperature frozen state and develop cracks (Zhang et al, 2018). Insight into the mechanical properties of rocks at low temperatures is of great significance for a better understanding and implementation of stimulation methods with low-temperature liquids
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
