Study On Plastic Zone Of Borehole Wall In Straight Wells

Wellbore instability is always an urgent problem to be solved in drilling.Now the drilling has reached the deeper stratum,most of the deep buried rocks show a certain degree of plasticity because of the high pressure,that is,the rock will not be directly damaged,but the deformation is plastic before the failure,not the previous theory of brittle failure.In some deep wells,when the mudstone layer is just drilled,the wall rock often does not collapse,but appears plastic deformation,which results in complex drilling situations such as hole shrinkage and pipe sticking.In this paper,the finite element software ABAQUS is used to simulate the problem of wellbore necking.(1)Firstly,the stress balance equation,physical equation and Mohr Coulomb failure criterion in the cylindrical coordinate are combined to establish the elastic-plastic model of the shaft wall,the plastic radius of the plastic zone under the non-uniform in-situ stress is deduced,and then the shape of the plastic zone is determined.(2)The rock mechanics parameters of the wells in Ledong block of Nanhai oilfield are calculated by combining the laboratory test and well logging data calculation.The rock mechanics parameters of the plastic formation are selected to prepare for the finite element software simulation.(3)Based on ABAQUS,the elastic-plastic finite element model of wellbore is established to simulate the plastic zone shape and expansion rule of wellbore under different horizontal in-situ stress difference,cohesion,internal friction angle and drilling fluid column pressure.The maximum radial displacement of wellbore under the above conditions is studied,and the necking problem of wellbore in plastic formation is further studied.The results show that the shape and displacement of the wellbore plastic zone are obviously affected by the horizontal in-situ stress difference,rock cohesion,internal friction angle and drilling fluid density.When the drilling fluid density is below a critical value,the increase of drilling fluid density can effectively inhibit the plastic necking of the surrounding rock of the plastic formation wall,so when drilling the plastic formation,the appropriate drilling fluid density should be selected.