For the heavy oil fields of the Orinoco oil belt in Venezuela, a new cementing concept was successfully applied to maintain zonal isolation during long term exposure to temperatures up to 1,202°F (650 °C) in a process called in-situ combustion. These unconventional wells are often associated with weak and unconsolidated formations complicating proper cement placement and the resulting cement sheath must withstand extreme stresses due to the temperature and pressure cycles during the in-situ combustion process.
During a comprehensive lab study API cement based slurries were engineered with high temperature stable aluminosilicate fibers. The corresponding cement specimens were cured and then exposed in a furnace with temperature cycles up to 1,202°F (650 °C) simulating the anticipated wellbore changes. Mechanical properties and permeabilities of these cementing systems were used in a computerized cement-sheath model to evaluate potential failures from stresses during the in-situ combustion process.
The cementing systems containing 50% of the aluminosilicate fiber were suitable to withstand thermal degradation without any visual cracks. The computerized cement-sheath simulations indicated that stresses induced by prompt pressure and temperature changes during the heat cycles are not causing failures for the lead cement sheath which was critical to provide zonal isolation above the combustion zone. The biggest improvement of this thermal shock resistant cementing system towards the corresponding cementing systems not containing the aluminosilicate fibers was the significantly reduced Young's modulus by around -20%, while the tensile strength increased by at least +60% resulting in a desired resilient cement sheath. The actual cement jobs in the field were successfully executed as planned without any losses or incidents. So far, no well integrity issues have been observed since the well was cemented in March 2012 with the following combustion process.
The thermal shock resistant cementing system, based on API cement, has advantageous towards refractory cements (such as high alumina cements) due to economics, ready availability, but in particular because it performs reliably by adjusting the slurry performance with common chemical admixtures and being flexible in design simplifying operations while contributing to a high-quality job.
