Historically the cement slurries used on primary cementing jobs performed across massive salt and evaporite sequences have always had some technical limitations. These limitations resulted in poor initial cement compressive strength, which lead to casing collapse due to well encroachment from the highly plastic formations, and also to poor zonal isolation because of bad cement/formation bonding.
This paper discusses the design and field applications of a new salt-rich cement slurry with controlled properties for cementing and isolating these difficult formations. Apart from precise thickening times, these slurries also exhibit excellent fluid loss control and very low rheological parameters. This latter property allows convenient turbulent flow placement in even relatively large annulus configurations. A laboratory study was made in order to determine the optimum salinity of the starting slurry so that any salt dissolution downhole will not have any detrimental effects on the slurry properties.
Long term studies are also shown m order to demonstrate that these slurries provide excellent protection from aggressive brines which are often associated with the salt/evaporite formation. Normal cement systems do notwithstand this degradation caused by brine contact, especially that from magnesium. This extra strength protection reduces the risk of long term casing corrosion and collapse.
Field case studies from North America and the Middle East are described and compared to the prior art.
During the last few years there have been arguments put forward in favor of using both low (less than 5% BWOW) and high (greater than 18% BWOW) NaCl slurries for cementing across massive salt formations. The low NaCl percentage group claim that with fresh water or 3% NaCl slurries there are less problems related to casing collapse and poor cement/formation bonding than when using conventional high salinity slurries1,2. This has been shown by actual case studies of wells cemented in the Williston Basin of North Dakota and Montana, where two years after cementing there was still no reported case of casing collapse. With an earlier cement system an average of 20% of the wells had experienced casing collapse. They also claim that the present day salt saturated slurries have long thickening times, and very often result in surfacemixing problems.
The high salinity group maintain that in order to reduce the possibilities of salt dissolution downhole and thereby change the slurry properties, it is best to start with a near saturated slurry on surface3,4. Another argument in favor of this latter concept was put forward by Beach5 when he showed that long term exposure of a set fresh water cement slurry to a saturated salt solution resulted in cracking and fissures. He concluded that the only way to halt cement deterioration short of destruction is to balance ion concentrations of the cement slurries with that of the formations. Slagle and Smith6 also showed how bonding improved between cement and salt formations when the cement slurry was saturated in salt.
