Effect of cement sheath condition on casing mechanical integrity in shale gas horizontal wells

The borehole trajectory of extended-reach shale gas wells is complex,and the casing damage problem caused by cement sheath defects often occurs during the fracturing development process.In view of the characteristics of high pump pressure and large displacement in the fracturing development of shale gas wells,using the finite element analysis method,the paper optimizes the model mesh division,and establishes a casing-cement sheath-stratigraphic combination model for the calculation of casing mechanical integrity in complex borehole trajectories of shale gas wells,and finally explores the effects of angle,thickness,and length losses and composite defects of cement sheath on the mechanical integrity of casing in local flexure zones of the slanting and horizontal sections.The results show that the circumferential stress of the casing decreases with the increase of the elastic modulus of cement sheath and is less affected by the Poisson's ratio.The angle loss of cement sheath causes stress concentration in the casing,which is reduced when the missing angle exceeds 90°.With the increasing loss of cement sheath thickness,the stress on the casing increases.The effects of cement sheath thickness,casing size and pumping pressure on casing stress are similar under different missing angles in both inclined and horizontal sections,and the trend is that the casing stress increases and then decreases as the missing angle increases.The missing critical part of cement sheath will produce a sudden change in stress,and this location is also the most prone to casing loss.The maximum value of casing stress increases and then decreases with the increase of angle loss under the same thickness loss,and the maximum value of casing stress increases with the increase of thickness loss under the same angle loss.With the increase of angle loss,the equivalent force of the casing along the pipe length direction corresponding to the missing part of cement sheath first increases and then decreases.The thicker the cement sheath is,the smaller the casing equivalent force will be.Under the same thickness,the thinner the remaining thickness of cement sheath is,the greater the casing equivalent force will be,and the stress level reaches the maximum when the whole cement sheath is missing.