One of the main objectives of primary cementing is to provide zonal isolation by preventing the percolation of gas through a cemented annulus, which could result in channels in the cement sheath. Gas channeling, once established, has proven to be extremely difficult and costly to repair.
The most current theory used to explain gas migration is the early gelation of the cement slurry, which leads to a decrease of hydrostatic pressure within the cement annulus. Over the years, various models have been developed, most of them revolve around the concepts of static gel strength (SGS), critical static gel strength, (CSGS), and transition time. Unfortunately, these approaches have failed to accurately predict gas migration. One of the main reasons for this failure is that these approaches are based on fluid-mechanics theories, and do not take into account the cement phase changes during hydration (fluid to solid) besides their effects on the SGS. Additionally, most models do not take into account the true mechanisms at the origin of gas percolation: Matrix, chimney, and micro-annulus.
This paper presents the use of a new gas migration model that eliminates these drawbacks, in order to investigate the effect of the cement composition on the cement sheath integrity. This model considers two different stages in the life of the cement sheath (fluid-type and porous-solid type), and is characterized by constitutive laws, which are integrated over the length of the cement column by time to determine if gas migration will occur and what are the mechanisms according to which it would occur. The simulations demonstrate the crucial role that the cement composition has on the state of stresses and pore pressure in the cement sheath, on the opening and closure of micro-annuli and on the vertical displacements of cement sheath during cement early ages. They highlight that an analysis of cement sheath integrity during hydration requires checking a combination of different mechanisms during the life of cement from fluid-type to porous-solid type.