Chemo-poro-elastoplastic modelling of an oilwell cement paste: Macroscopic shrinkage and stress-strain behaviour

Analysing the response of cement sheath in oil, gas or geothermal wells submitted to mechanical loadings needs an appropriate constitutive law for the cement paste in wells conditions. The knowledge of the actual initial state of stress is a key point which requires a simulation of cement paste behaviour from placement to hardened state. A chemo-poro-elastoplastic model for macroscopic shrinkage and stress-strain behaviour of cement paste at the early age is developed in this paper. A modified Cam-Clay type yield surface and an associate plastic flow rule are used. The hydration is considered as a hardening mechanism of the yield surface, which also evolves with the development of plastic strains. The model shows a good capacity in capturing the basic mechanisms of both macroscopic shrinkage and stress-strain behaviour of an oilwell class G cement paste hydrated under various conditions of temperature and pressure. A strong coupling is found between the macroscopic shrinkage and the stress-strain response.