Plasticity analysis and constitutive model of salt rock under different loading speeds

Abstract

Salt rock is recognized as an ideal source for oil, natural gas and compressed air due to its good deformation and low permeability. The injection–production frequency/rate under different working conditions greatly influences various mechanical behaviours, such as the deformation and convergence of the reservoir. Therefore, it is crucial to analyse the rate correlation of the mechanical characteristics of salt rock and to establish the corresponding constitutive model; this model accurately evaluates reservoir safety and reasonably designs reservoir operating pressure. According to the experimental deformation characteristics of salt rock under different loading and unloading rates, the time correlation, stage characteristics and evolution law of salt rock deformation were analysed, and a viscoplastic constitutive model of salt rock in the whole process was constructed based on loading plasticity and rate-related plasticity. The prediction effect of the model was verified by graded creep tests and different rate cyclic loading and unloading tests, revealing the good adaptability of the model for salt rock under complex loading paths. The results showed that the plastic (unrecoverable) deformation of salt rock could be divided into loading plasticity caused by stress state change and creep plasticity generated over a long period by stress accumulation. The creep deformation characteristics of salt rock could be better described using hardened damping elements, classical Norton elements and fracture damage variables; the relationship between loading plasticity and stress path changes was linear. The model had a high accuracy in predicting the deformation of salt rock under complex cyclic loading paths. For radial strain and volume strain, the accelerating point that protended the rapid growth of microcracks was used to distinguish the accelerated creep stage from the steady state creep stage. The number of events with an average frequency in the range of 0–50 kHz after the accelerating point suddenly increased significantly, which could be regarded as a precursor to the primary fracture of the rock.