Study On Creep Behavior And Creep Model Of Deep Gypsum Mudstone

Mudstone is widely used as the host rock medium in deep oil and gas reservoir.Under the environment of high ground stress,the creep behavior is complex and can cause the failure of casing directly.Researches on the creep behavior of rock under high ground stress are mostly about granite and salt rock,which are used as energy storage or nuclear waste disposal field.Experimental study on creep damage evolution of deep mudstone in oil and gas reservoir engineering is still relatively rare.The study of creep behavior provides reference for the long term stability analysis of casing,which has important significance for deep oil and gas reservoir engineering.This paper focuses on the gypsum mudstone in deep oil and gas reservoir engineering.Triaxial conventional tests and triaxial creep tests are conducted,the concept of creep damage start time threshold is proposed,and an improved Burgers creep model considering accelerated creep is established.The main contents and conclusions are as follows:(1)Triaxial conventional tests are carried out to characterize the instantaneous mechanical behavior of deep gypsum mudstone.9 rock samples are divided into 3 groups,all of them are under 130℃ temperature while the confining pressure of three samples in each group are as follows: 10 MPa,20MPa,and 30 MPa.The basic mechanical parameters of mudstone are obtained,and the strength,deformation and failure characteristics are analyzed respectively.The results show that increase the confining pressure enhances the rock resistance to deformation and failure.The peak strength of mudstone is 74 MPa under 130℃ temperature and 30 MPa confining pressure.According to this,all the stress levels in triaxial creep tests are designed.(2)Triaxial creep tests are conducted to study the creep behavior of deep gypsum mudstone under 130℃ temperature and 30 MPa confining pressure.All the triaxial creep curves at different stress levels are consist of the initial attenuation creep phase,the steady creep phase,and the accelerated creep phase.The deviatoric stress and duration have significant effects on creep behavior of mudstone.The creep stress threshold of mudstone is small.Under a long duration,the accelerated creep can be obtained even at the lowest deviatoric stress level.The creep damage of mudstone increases sharply in the accelerated creep stage,and it can be expressed preliminarily by exponential function through statistical method.(3)According to the results of triaxial creep tests,the Burgers model is selected to simulate the linear deformation of mudstone in attenuation and steady creep phase.Then the concept of creep damage start time threshold is proposed,an exponential function to describe the creep damage variable is defined,and the Heaviside function is introduced.Finally the improved Burgers creep model considering the nonlinear deformation in accelerated creep stage is established.Consequently,the axial creep constitutive equation in triaxial conventional state is deduced.With the creep test curves,the algorithm of Universal Global Optimization is adopted to identify the model parameters,which shows that the proposed creep model is available and reasonable.