Effect of heating-cooling cycles on mechanical properties and microscopic characteristics of shale

摘要

Shale gas will become an important substitute of conventional fossil fuels in the future, and the reservoir characteristics of shale are closely related to its pore structures. In order to explore the change characteristics of pore structures and mechanical properties of shale under the cyclic heating and cooling effect, from the perspectives to analyze of physical and mechanical properties and microscopic pore structure, etc. First, the ultrasonic wave velocity test and triaxial compression test were carried out. Afterward, the change law of macroscopic physical and mechanical parameters was microscopically verified based on X-ray diffraction technology, Fourier infrared spectroscopy and low-field nuclear magnetic resonance technology. The research results showed that the specimens drilled perpendicular to stratification direction were more likely to be damaged under the effect of heating-cooling cycles; under the temperature shock brought about by heating-cooling effects, the physical parameters of shale would vary at different degrees; however, chemical properties of shale basically remained unchanged; temperature effect would promote the further propagation of primary fractures and the initiation of secondary fractures and the fractures coalesced to form larger ones. Through the change of T_2 spectra obtained based on NMR, it can be inferred that the number of micropores in shale declined while that of macropores increased; with the growth of heating-cooling cycles, the pores in rocks constantly developed under the effect of repeated temperature shocks, which resulted in the increase of the porosity. Additionally, the compressional wave velocity and uniaxial compressive strength generally decreased and the reduction amplitudes of the peak strength of shale treated through 5, 10 and 15 cycles were 4.12, 24.18 and 24.58, respectively. The research result provides effective reference for efficient mining of shale gas and reconstruction engineering of tight reservoirs.