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.