Analysing the microscopic creep characteristics of carbonate rocks using nanoindentation experiments

In recent years, the depth of oil drilling has exceeded 9000 m, and the testing and evaluation of mechanical properties of ultra-deep rocks face great challenges. Obtaining complete core samples has lots of serious problems through ultra-deep drilling, so it is difficult to process standard rock samples for general tests to obtain the mechanical properties. In order to study the creep behaviour of deep rocks, we conducted continuous step holding nanoindentation creep experiments on carbonate rocks from the Tarim Oilfield in Xinjiang, China. We first investigated the effects of load and holding time on the creep behaviour; Furthermore, by calculating the stress exponent, the deformation mechanism is revealed; At last, four traditional creep models and fractional order Maxwell model were used for regression to explore the applicability of models in describing the creep behaviour. The results show the creep displacement is positively correlated with the load and holding time. Creep curves with different shapes were obtained in nanoindentation (named A1 and A2 respectively). The Burgers and the two-dashpot Kelvin model are in good agreement with experimental data under long holding time. The stress exponents of A2 are greater than 3, indicating that the creep mechanism of A2 is dislocation creep.