Calculation Of Casing Load In Salt Rock Formation

In our country, salt stratums are widely distributing in the oilfield such as Tarim and Jianghan, and casing damage occurred seriously in the salt bed due to the salt layer creep features such as triggering. This paper aims at the calculation of outside load of casing in salt bed, the test of collapse resistance and resistance to internal pressure of casing. On the basis of researching literature at home and abroad, the following work is carried out:(1) The calculation equation for outside load of casing is deduced on the premise that the formation is kelvin model and 3 elements or less, the influence factors and its effect on load of casing in salt bed are analyzed in detail. The analysis results show that:casing load in viscoelastic or salt formation stable value depends on the parameters of casing, parameters of cement loop, Poisson's ratio and long-term shear modulus of formation together, rather than simply equal to the pressure of overlying formation; with the increase of elastic modulus of cement sheath, outside load of casing increases first and then decreases, the peak of outside load of casing depends on the parameters of formation, casing and cement sheath together, rather than simply depends on the elastic modulus of cement sheath.(2) The influence laws of non-uniform crustal stress, internal pressure, decentration, angle of deviation and azimuth to the casing load in salt layer are calculated and analysed by method of finite element while 6 kinds of casing which are commonly used in salt bed in Tarim oilfield are taken as research objects, meanwhile, the associations between finite element calculation for load of casing in salt bed based on Heard model and theoretical calculation for load of casing in salt bed based on viscoelastic model are established. The calculation results can draw the following conclusions:uniform in-situ stress will greatly increase the risk of casing damage in salt bed; there is great casing damage risk in Tarim oilfield when the casing diameter is 250.83mm and the wall thickness is 15.88mm; the angel of well azimuth and the direction of maximum principal stress is 45° can reduce the incidence rate of casing damage.(3) Non-uniform extrusion experiment of casing is done, and the deformation law of casing whose external diameter is 177.8mm and wall thickness is 12.65mm in concentrated load is obtained.(4) A set of device used in testing anti internal pressure strength of casing is designed, which can simulate the temperature and restrict axial extension of casing under downhole conditions; a method for measuring the internal pressure strength and deformation of casing is formed. Anti internal pressure strength test of casing is conducted.