Abstract: Mudstone is widely used as the host rock medium in oil and gas reservoir. The creep behavior is extremely complex and directly influences the failure of casing under high temperature and high confining pressure. This study focuses on the gypsum mudstone in deep oil and gas reservoir engineering. Firstly, under 130 ℃ temperature and 30 MPa confining pressure, triaxial creep tests are performed under different deviatoric stress on multiple samples with a single-stage loading method. The mechanical behaviors of mudstone exhibit significant time-dependent and the creep stress threshold is small. The stress and duration has significant effects on creep behavior of mudstone. The steady creep and accelerated creep failure can be obtained under a long duration even in a small deviatoric stress. The start time of accelerated creep can be described as an exponential function of deviatoric stress; and the start time decreases with an increase in deviatoric stress. Secondly, the accelerated creep and strength damage characteristics of mudstone are discussed in detailed. The threshold of start time of creep damage is proposed. It is considered that the internal creep damage variable of mudstone can be described as an exponential function. Finally, based on the creep damage variable and the start time, the improved Burgers creep model is established. The model is nonlinear and can describe the accelerated creep stage. Then the creep parameters are identified and analyzed. The model can accurately describe the creep characteristics of the deep gypsum mudstone. The research results provide dependable reference for the safety evaluation and long term stability analysis of deep oil and gas reservoir engineering.

Key words: oil and gas reservoir, gypsum mudstone, triaxial creep tests, accelerated creep failure, improved Burgers creep model