During transition of a cement slurry (liquid state) to a set cement sheath (solid state), the system goes through a gel state. During gelation, the cement column can lose the ability to transmit hydrostatic pressure effectively against formation pore pressure. Additives can be used to shorten transition time and thus help the slurry system to quickly achieve static gel strength capable of resisting pressures that otherwise can cause an influx of formation fluids into the gelled system with the potential of compromising barrier dependability prior to hard set.
Achieving gas migration control can involve:
•Transition Time – the time required for the gel strength to go from 100lbf/100ft2 to 500lbf/100ft2; the conventionally acceptable time is 30 minutes or less.
•Transition Time Reducers – extend the time required for the slurry to achieve 100lbf/100ft2 by suppressing onset of gel strength.
•Pre-Set (Slurry) Expansion - provides pressure maintenance in slurry during reduction in the hydrostatic column