Cementation of In-situ combustion (ISC) wells is a challenging affair as wells are often associated with weak and unconsolidated formation, nevertheless cement rise upto surface is desired to prevent casing failure. Furthermore the cement sheath is also required to withstand extreme stresses due to high temperature cycling experienced during in-situ combustion process.
In heavy oil fields of western India, due to the problem of inadequate placement time and flash setting when in contact with portland cement, the portland cement-silica blends were used for cementation of ISC wells instead of alumina cement blends. But this resulted in insufficient cement rise due to losses during cementation and the set cement failed to contain the strength and permeability in course of ISC process causing charging of sub-surface shallower layers.
For mitigating these problems, non-alumina based thermally stable lightweight lead slurry and a ductile high temperature resistance tail slurry have been developed and implemented for cementation of ISC wells, the details of which alongwith successful case histories are presented in this paper.
In the formulated thermally stable slurries there was neither reduction in strength nor increase in permeability even after thermal cycling upto 650°C. The lightweight slurry composition (S.G.1.60) reduced the hydrostatic head to effect cement rise upto surface. Since these formulations are devoid of alumina cement, all the associated field problems were totally eliminated. Superior cement bondage in comparison to earlier results confirmed the successful field validation.
This approach and the development is a unique solution of the problems in cementation of ISC wells, making it suitable for use as an effective alternative to earlier practices. Field implementation of this development has successfully arrested charging of shallower zones which was endangering the ISC process in fields of western India.