Research On The Bearing Capacity Of Slurry Sheath Under Complex Engineering And Geological Conditions

Slurry sheath is one of the important barriers to the integrity of the shale gas well.The failure of Slurry sheath will cause sustained casing pressure,which will directly lead to the reduction of the bearing capacity of the casing and even the failure of the integrity of the wellbore.Most of the existing studies on the bearing capacity of Slurry sheath only consider the effects of non-uniform in-situ stress and eccentric annulus in a circular wellbore.In directional and horizontal wells,irregular circular wellbores(or elliptical wellbores)are formed due to non-uniform in-situ stress and anisotropy of formation rocks.The combination of non-uniform in-situ stress,elliptical wellbore and eccentric annulus can make the stress of the slurry sheath extremely complicated.In order to clarify the bearing capacity of slurry sheath under the combined action of WY shale gas filed,such as non-uniform in-situ stress,elliptical borehole and annulus eccentricity,a numerical model for predicting the stress of formation-slurry sheath-casing combination under the action of non-uniform in-situ stress,elliptical borehole and annulus eccentricity is established on the basis of elastic mechanics and finite element analysis theory.Finally,the finite element software ANSYS Mechanical APDL is used to carry out simulation.The bearing capacity of slurry sheath under multi-factors is analyzed systematically,and the influence rules of different factors on the bearing capacity of slurry sheath are clarified.The main understandings obtained through the research in this paper are as follows:(1)The increase of non-uniform in-situ stress,ratio of major and minor axis and eccentricity of casing will reduce the bearing capacity of slurry sheath,but it is beneficial to the bearing capacity of slurry sheath when the ratio of major and minor axis is about 1.1-1.2 under slim hole size condition;(2)The bearing capacity of slurry sheath increases firstly and then decreases with the increase of borehole ellipticity,which is beneficial to the increase of bearing capacity of slurry sheath when ratio of major and minor axis is 1.2-1.3.With the increase of casing eccentricity,the bearing capacity of slurry sheath decreases,that is,the eccentricity of casing towards short axis of elliptical borehole is beneficial to the increase of bearing capacity of slurry sheath;(3)When the non-uniform in-situ stress acts on the bearing capacity of slurry sheath,the influence of non-uniform in-situ stress on the bearing capacity of slurry sheath is the greatest,and the increase of non-uniform in-situ stress will significantly reduce the bearing capacity of slurry sheath;the influence of ratio of major and minor axis and casing eccentricity on the bearing capacity of slurry sheath is less than that of non-uniform in-situ stress,and their increase is not conducive to the bearing capacity of slurry sheath;(4)The calculation results of slurry sheath with complex shape considering both ratio of major and minor axis and casing eccentricity under non-uniform in-situ stress show that the increase of ratio of major and minor axis and casing eccentricity has adverse effects on the bearing capacity of slurry sheath.When the eccentricity of casing is small,the eccentricity of casing toward short axis of elliptical wellbore is beneficial to improve the bearing capacity of slurry sheath.