Many difficulties attributed to poor cement bonding (ie. annular gas migration and poor zonal isolation) have been encountered In Southern Saskatchewan.
The most important criteria for obtaining a good cement job are: proper casing centralizalion. efficient mud removal, and constant cement slurry properties. Variations in rheology, flow regime and fluid loss have been attempted to obtain a good quality cement job.
Over the past year, Canadian Fracmaster has been successful in using a lhixolropic cement blend with both expanding properties and ultra-low fluid loss. This paper will cover the properties and applications of this blend, as well as the advantages to operators over more conventional blends. Several case studies will be highlighted to aid in illustration.
The objectives of any primary cement job are many; but one of the most important is to restrict fluid and gas movement between formations. This goal of isolating oil, gas, and water producing zones is also the most difficult. Many factors come into play in providing this good bonding. These (actors are included in proper drilling practices as well as proper cement slurry design.
This paper will only touch upon some of these factors with the main emphasis being on the cement slurry design.
In southeastern Saskatchewan (here are three formations of interest. The top formation is the Midale, the middle is Ihe Frobisher-Alida, and the lower is the Tilston. All three zones are Paleozoic in age and members of the Madison group.
All three formations are composed of fossiliferous-fragmental to algal, oolilic and pisolitic limestones and dololnitcs. The lower section or the Midale is composed or evaporatic dolomite and anhydrite with the Frobisher-Alida and the Tilston being composed of the same.
The Frobisher-Alida also has some sandstone lenses present in the middle beds.
These formations are naturally fractured making it difficult to obtain a good cement bond.
Bonding refers to both cement to pipe and cement to formation bonding. Shear bonding mechanically supports the pipe in the hole, and is determined by measuring the forces required the initiate pipe movement in a cement sheath.
Hydraulic bonding also blocks the migration of fluids or gas in a cemented annulus.
Points to consider concerning pipe/cement bonding:
Changes in the internal pressure or the casing will cause a corresponding change in hydraulic and shear bond strength For example, if the casing string is closed in while the cement is setling, the heal of hydration causes a pressure buildup inside the casing. As the casing cools it contracts, perhaps causing a microannulus to be formed.
Bond strengths will increase with pipe surface roughness.
A more effective bond is provided by water wet pipe.
Points to consider concerning cement/formation bonding:
A good hydraulic bond depends upon contact between the cement and the formation.
A thick drilling mud layer greatly reduces hydraulic bonding.
Failure to remove drilling mud can be more detrimental to formation bond than pipe bond
A more effective bond is obtained in a dry formation, free of filler cake.
