Experimental investigation on the effects of heating-cooling cycles on the physical and mechanical properties of shale.

The deep shale gas reservoirs are characterized by great burial depth, high formation pressure, high temperature. During the hydraulic fracturing process, the fracturing fluids will inevitably exchange heat with the surrounding rock, causing the shale in the near-well zone to undergo temperature alternation, the physical and mechanical properties of shale reservoirs are significantly affected, which brings technical difficulties to fracturing optimization design. In this paper, the Longmaxi Formation shale is used to pay attention to the rock physical and mechanical characteristics of the shale undergo temperature alternation, the microstructure and damage and fracture characteristics of the shale undergo temperature alternation are analyzed. The results show that: (1)As the heat treatment temperature increases and the number of heating-cooling cycles increases, the permeability of the shale increases, the P-wave velocity decreases, the tensile strength (TS) decreases, and the brittleness index rises. (2) Heat treatment temperature and the number of heating-cooling cycles have little effect on the triaxial compressive strength (TCS) of shale. However, it has a significant effect on the failure mode of shale. The increase of temperature and the number of heating-cooling cycles will enhance the degree of fracture of the shale failure surface. (3) Based on industrial CT scanning and scanning electron microscopy (SEM) observations, the thermal cracks caused by heating-cooling cycles are mostly inter-granular cracks and inter-layer cracks, which unfold on the surface of the sample and extend to the interior. Heating-cooling cycles also lead to an increase in the diameter of organic pores. The study results can provide a certain theoretical basis for the optimization design of staged fracturing parameters for horizontal wells in the process of deep shale gas development. • Damage of high-temperature shale subjected to heating-cooling cycles was investigated. • Damage induced by multiple heat-cooling cycles becomes more significant than a single air cooling. • Heating-cooling cycles can lead to initiation and propagation of shale cracks. • The number of heating-cooling cycles affects the frequency and width of cracks. [ABSTRACT FROM AUTHOR]