Experimental study on mudstone's creep behavior under different water contents and its effect on casing damage

Strength and creep experiments of mudstone from Daqing Oil Field under different saturated conditions are performed through the XTR01 servo-controlled compression machine. The creep characteristics and the change of mechanical parameters with the increment of water content are analyzed. The results indicate that the rock strength and elastic modulus decrease with the increment of water content, while the creep strain and steady state creep strain rate increase. Through the experiments, the nonlinear creeping constitutive equation of mudstone considering the change of water content is established as compared with the creep characteristic curves. In water injection reservoir, based on the relationship between the water cut in the mudstone and the mechanical property of the mudstone, the mechanism of casing failure is analyzed, and the casing-cement sheath-mudstone creeping model is built in order to simulate the creep pressure of mudstone with different water contents. The simulation results show that under heterogeneous stress, the increasing water content accelerates the incremental rate of the creep pressure of mudstone, so the time of reaching yield state reduces significantly, which means service time of case becomes much shorter. During the process of water injection production, high injection pressure cut in mudstone should be prevented to reduce casing damage.