Influence of Pyrolysis Degree On Oil Shale Anisotropy and Damage Deterioration Characteristics

To reveal the mechanism of organic matter loss on anisotropy and damage evolution during oil shale pyrolysis. The pyrolysis temperatures were set at 400 °C, 450 °C, and 500 °C. The pyrolysis degree is controlled by the pyrolysis time. The pyrolysis degree determines the organic matter loss, and the damage is represented by compressive strength, P-wave velocity, and energy. The relationship between micro-damage and reduced mechanical strength caused by oil shale pyrolysis was analyzed. The results show that there is a linear relationship between oil shale compressive strength and pyrolysis time. At 400 °C, 450 °C and 500 °C, the oil shale perpendicular and parallel to bedding compressive strength decreased by 84.18%, 88.81%, 90.93% and 77.94%, 81.76%, 85.67%, respectively. The ratio of peak dissipation energy to peak total strain energy ( \(D_d\) $D_d$ ) of oil shale perpendicular and parallel to bedding at different temperatures is 0.1–0.4 and 0.2–0.4, respectively. The anisotropy coefficient of compressive strength ( \({{\upsigma }}_{ac}\) ${\mathrm{\sigma }}_{\mathit{ac}}$ ), wave velocity ( \(v_{ac}\) $v_{\mathit{ac}}$ ), and peak energy ( \(E_{ac}\) $E_{\mathit{ac}}$ ) at different pyrolysis degrees is less than 4, 8, and 36. As the pyrolysis time increases, the oil shale energy absorption, storage, and release properties are weakened, while the energy consumption is enhanced. At 500 °C, the threshold time of the anisotropy coefficient is 75 min. At 500 °C, the internal structure evolves from multi-fractures to fracture networks. Oil shale damage is mainly caused by developing pores and fractures.