Method of maintaining the integrity of a seal-forming sheath, in particular a well cementing sheath

A method for maintaining the integrity of a sheath forming a seal, in particular a cementing sheath, positioned around a metal casing for an oil, gas, water, geothermal or analogous well in rock, the sheath being subjected to mechanical and/or thermal stresses during the lifetime of the well which can cause a risk of cracking of the sheath by failure in tension or in shear, or by detachment of the sheath at the casing- sheath and/or sheath-rock interfaces, for example, the method being characterized in that it consists in:

• (a) calculating or estimating pressure and/or temperature variations in the well and/or variations in in-situ stresses, which can occur during the lifetime of the well;
• (b) for a given sheath, evaluating the various stresses which will be applied to that sheath in particular as a function of the variations defined above and taking into account the geometrical characteristics of the well and of the casing, and also the mechanical properties of the rock;
• (c) from the above evaluation of the various stresses, determining the nature of the stress which is likely to cause sheath deterioration in the first instance;
• (d) evaluating the influence of the mechanical and/or physical properties of the sheath, the rock and/or the casing on the above-defined stress;
• (e) selecting a sheath with mechanical and/or physical properties which are likely to attenuate the effects of the above-defined stress;
• and
• (f) positioning the sheath as selected in this way around the well casing.