Influence of Weighting Materials on the Properties of Oil-Well Cement

The integrity of oil and gas wells is largely dependent on the cement job. Maintaining the properties of the cement layer throughout the life of a well is a difficult task, particularly in high-temperature and -pressure conditions such as those in deep wells. Cementing deep wells require slurries with high densities. Heavyweight cement systems are those designed with weighting materials. These materials have a higher specific gravity in comparison to cement. The purpose of this work is to investigate the influence of weighting materials on the properties of Class G oil-well cement and to make necessary recommendations for their use. The rheology, fluid loss, gas migration, and dynamic elastic properties of three cement slurries containing different weighting materials, namely, hematite, barite, and ilmenite, were studied. The results indicate that cement slurry designed with barite exhibits the best rheological behavior that would provide a perfect solution for deep wells where cement placement is a concern. The barite slurry had the lowest plastic viscosity. The plastic viscosity of the hematite and ilmenite-weighted systems was higher by 11.5 and 12.4%, respectively. The barite-based slurry also had the highest yield point of 84.3 lb(f)/100 ft(2), whereas the yield points of hematite and barite cement were 37.9 and 29.5 lb(f)/100 ft(2), respectively. Furthermore, the gel strengths of barite cement were the highest, with 10 s and 10 min gel strengths of 11.5 and 39.5 lb(f)/100 ft(2), respectively. Ilmenite had the most positive impact on fluid loss control, which would be appropriate in high permeable formations. It had a fluid loss of 66 mL/30 min, lower than those of the hematite (80 mL/30 min) and barite (82 mL/30 min) systems. Furthermore, the best dynamic elastic properties were exhibited by the ilmenite system, with the smallest Young's modulus (27.3 GPa) and the highest Poisson ratio (0.252). This would make the ilmenite to be very useful in developing heavyweight cement composites that could withstand severe external loads imposed on the casing and cement. The hematite cement was the most impermeable to gas migration, with a gas volume of 127.8 cm(3), whereas the volume measured in the barite and ilmenite systems were 20.9 and 78% higher, respectively. This makes the hematite to be very useful in deep gas wells where gas migration control is important.