| Many of related references at home and abroad were studied in the paper, Finite Element simulation research method was put forward as the main research method on the mechanism of borehole undergauge induced by formation creep. A lot of rock creep data were gathered and tided up, as provided the rock mechanics parameters needed later in Finite Element Simulation. Regression analysis on the test data was done by use of creep material mode-Heard creep model. Meanwhile, the application conditions on this mode were argued.The paper mainly used high nonlinear Finite Element theory. Research work and achievement are as follow.(1)summed up the characteristic factors which influence rock salt creep.(2) put forward and testified the transformation relations between infinite region boundary conditions-infinite region in-situ horizontal stress conditions on the square body with circle hole and on the hollow cylinder.(3)deduced drilling fluid density function relation formula keeping the constant velocity of borehole undergauge.(4) compiled Heard creep material model into ADINA user subroutine by programming language-Fortran, and the ADINA software was extended by merging Heard creep material model subroutine into ADINA main program. By doing so, the limitation of lacking corresponding creep material model in the Finite Element software was made up(5) used Heard creep material model, set up the coupling Finite Element mode of non-creep formation and creep formation with different dip, solved the borehole undergauge problem induced by borehole creep deformation under the effect of complex three dimensional in-situ stress.(6) On the base of large numbers of model calculation and plot analysis, found the general rule of borehole shrinkage under the effect of formation dip, maximal principal stress orientation and maximal horizontal stress orientation.(7) discussed briefly the effect of rock mechanics characteristic parameter-elastic modulus and Boisson's ratio on formation creep speed. |
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