An Innovative Methodology for Designing Cement-Sheath Integrity Exposed to Steam Stimulation

Abstract

Summary Heavy-oil production is one of the new challenges the oil and gas industry faces today, with trillions of barrels of reserves around the planet. To extract heavy oils, the viscosity has to be reduced to gain mobility and start the oil flowing. Of all the possible techniques, steam stimulation is, today, the most promising. Steam is injected through the well down to the reservoir to warm it to a temperature of up to 250°C, inducing extreme thermal stresses in the well (especially when the temperature gradient through its components is maximum, as it is during initial steam injection or during workover when the well has to be quickly cooled down and heated up again to restart production rapidly). To avoid zonal-isolation failure and steam release at the surface, new rules need to be considered in designing the barriers of a well exposed to steam stimulation, especially in very shallow fields. Where conventional rules may be applied for casing design, pioneering rules should be considered for designing the cement sheath. This paper presents an innovative methodology for designing robust cement sheaths in very shallow conditions. First, stresses in the field are assessed with a rock-mechanics simulation. Then, the thermal gradient in the well components during the heating process is evaluated with a thermal simulation of the well. Both results are entered into Total's dedicated software to determine the mechanical properties the cement should have to withstand the thermal stresses. Different cement systems from service companies are evaluated in a cell, reproducing the field conditions to validate the simulation results. This methodology was applied successfully to the Joslyn field wells, in Canada, where a resilient cement system with low Young's modulus and high tensile strength was selected and pumped. Because the results were positive, the methodology has been extended to other Total fields around the world where steam simulation is being considered.