Study on shale creep experiment and fractional creep constitutive model after dynamic load impact

The combustion and detonation fracturing technology of methane in shale reservoir is conducive to improving the production of shale gas, but the combustion and detonation impact will cause damage to the wellbore, and the shale creep around the wellbore under the action of in-situ stress will lead to wellbore deformation and shaft wall instability. In order to analyze the impact of impact strength on the creep characteristics of shale, single and multiple impact tests of shale samples under different impact strength conditions were carried out in this paper, and then the shale tests with different impact damage degrees were subjected to graded loading creep tests. Finally, a new nonlinear fractional-order damage creep model for shale was proposed. The model is composed of Hooker, viscous element, fractional-order element, fractional-order improved Kelvin model and nonlinear fractional-order viscoplastic model in series. The experimental results show that the total shape variable of creep increases with the increase of impact strength. The impact strength increases from 1. 6 J to 25. 6 J, and the strain variable of impact sample increases by 7.3%, 16.1% and 31.5%, respectively. At the same time, the increase of impact strength will reduce the creep failure stress level, enhance the accelerated creep effect, and increase the creep rate and duration in the deceleration stage. The nonlinear fractional-order damage creep model can reproduce all stages of the creep process well. Compared with the classic Burgers model, the new model fits the experimental data better. The research results can provide theoretical support for analyzing the creep characteristics of shale wellbore after blasting. © 2024 China University of Mining and Technology. All rights reserved.