Abstract
Permanent plugging and abandonment (P&A) of oil- and gas wells requires proper sealing between the formation and the casing as well as proper sealing inside the casing. The cement sheath in the annulus is intended to function as an “impermeable” barrier. Typically, shrinkage of cement sheath takes place when the cement sets and a microannulus (MA) may be formed. In addition, cyclic pressure and temperature variations may result in cracks and debonding of the cement sheath. This paper investigates the possibility of improved cement sealing imposed by permanent deformation of the casing, thus providing a mechanical compression force to the cement and thus closing the MA when performing P&A.
Two experimental setups were designed in this context. The first setup termed casing/cement plug test, where the casing is contracted by an external pressure and simultaneous measurement of the flow rate through the setup. The second setup is termed casing/cement annular test where the casing is internally pressurized while the gas flow rate is measured.
Nonlinear finite element analyses were carried out to simulate the two test setups. The numerical results showed an acceptable agreement with the observations in the lab. The second setup was not tested in the lab, but simulated using the FEM code. The numerical analyses indicated that the same concept of casing permanent deformation is also relevant for the annular test. It is shown that the micro-annulus formed due to cycles of pressurization/depressurization as a result of inelastic deformations in the cement can be repaired by inducing permanent deformation in the casing to some extent. Finally, we concluded that permanent casing deformation could play a positive role in favour of closing the micro-annuli in P&A operations.
