Most heavy oil resources are found in shallow reservoirs. The challenging conditions of steam injection process to recover heavy oil from these wells impose additional requirements on zonal isolation material properties. At minimum, the ideal cement system must be designed with mechanical strength and flexibility sufficient to withstand the stresses induced by significant thermal loads. However, this ideal system has to acquire these properties at low placement temperatures and keep them at the minimum required level until abandonment and above. Conventional systems traditionally used for steam injection application don't satisfy these minimum properties requirements, which lead to various forms of leaks or even to complete steam breakthrough to the surface.
New specialized cement was developed to reliably maintain zonal isolation in shallow wells under steam injection conditions. This system, designed at the density range of 1,400-1,700 kg/cm3 (11.7–14.2 lbm/gal) is able to develop an excellent combination of mechanical strength and flexibility at the placement temperatures of 15-30 °C. Cement obtained after the initial low temperature curing was then aged at 350°C for 6 months. Every month, a temperature cycle (350°C to ambient temperature to 350°C) was simulated. Mechanical properties (compressive and tensile strengths and Young's modulus) were measured after 1, 3, and 9 months. Numerical simulations of sealant integrity performed with these properties in an annular geometry under shallow well conditions confirm long-term durability of the new system.
The cement's ability to withstand stresses encountered during steam injection was validated in a specific large-scale laboratory setup. A piece of cement sheath was subjected to two consequent temperature cycles from ~ 20 to 180°C with the a ramp-up of 0.6°C/min, equivalent to the worst case scenario for the proposed injector wells in the validation location. New cement system was capable of passing these conditions whereas conventional systems failed to maintain integrity of the well.
Thus, this study confirmed the capability of the new specialized cement system to ensure zonal isolation in shallow wells.