Physical Modelling of Oil Wells During Drilling Using Designed Triaxial Cell

Behavior prediction of oil well during drilling is important to prevent of instability problems and lots of cost spending. One of the problems can be pointed is failure of oil well wall during drilling. Thick-walled hollow cylindrical specimens can be able to modelling failure of oil well wall during drilling. Different triaxial cells exist for this test in world. Cell was used is designed based on Hoek cell. The benefits of this cell are ability of oil well during drilling and hydraulic fracture modelling. Also it is possible to measure the tangential strain in the center hole. This feature simultaneously is absent in most triaxial cells. To evaluate the performance of this cell, gypsum and concrete hollow cylinder specimens were made to modelling of oil wells during drilling. Literature studies and experimental results showed that, depending on the magnitudes of the applied internal pressure, external pressure and axial load, any of the radial, tangential and axial stresses induced in the cylinder considered as minor, intermediate or major principal stress. Hence, in this paper, two different stress conditions were used that consist: ϭϴ = ϭ1 ϭr = ϭ2 ϭz = ϭ3¡. ϭϴ = ϭ1 ϭz = ϭ2 ϭr = ϭ3 =0. Results showed that, in condition that tangential stress induced of lateral stress is maximum stress, shear failure occurred toward at around well. So, failure of wall of thick wall hollow cylinder is caused by the deviator stresses between lateral stress and inner pressure. By increasing deviator stresses, the plastic zone will increase around wall of thick walled hollow cylinder specimens. It is noteworthy that, presence of internal pressure reduces the breakouts propagation.