Abstract: Mudstone, as a host rock medium widely distributed in oil and gas reservoirs, exhibits unusually complicated creep behaviour under conditions of high temperature and high pressure. Moreover, this creep behaviour is an important factor causing the failure of casing. In this study, gypsum mudstone from a deep oil and gas reservoir was chosen to investigate the creep characteristic. Triaxial compression tests and triaxial creep tests were conducted to characterise the instantaneous mechanical behaviours of mudstone at 130℃ temperature and with 30 MPa confining pressure. It was found that the mudstone presented characteristics of soft rock, such as no obvious heterogeneity, great plastic compression and ductility dilation strain, and low peak strength. Both axial and lateral peak strains reached more than 1.5%, with a corresponding peak strength of 74 MPa. The mudstone showed obvious time-dependent behaviour in mechanical properties, and had a low creep stress threshold. Stress and duration had significant effects on creep characteristics. Under a long-term low stress, the mudstone also exhibited clearly a steady-state creep and accelerated creep failure, furthermore, the start time of acceleration creep decreased exponentially with the increase of stress. The damage and accelerating creep characteristics of mudstone were discussed as well. It is suggested that the creep damage variable in mudstone can be represented as an exponential function, and the creep deformation is the binary exponential function of stress and time. Therefore, the results provide a valuable reference for the establishment of rock creep model and the analysis of long-term stability in deep oil and gas reservoir engineering.

Key words: rock mechanics, high buried deep reservoir, gypsum mudstone, triaxial creep tests, accelerated creep behaviour