Well construction in carbonate rock intervals is accompanied by presence of cavernous fractured zones with low reservoir pressure and is complicated by drill mud and cement slurry losses. Loss of circulation is a typical problem for the most of the Tatarstan fields, so, best practices have been established during the decades. Two-stage cementing technique and light-weight cement slurries are used to provide top of cement up to the wellhead. Light-weight cementing slurry is prepared by adding agents of much lower density compared to cement or those agents that increase water-cement ratio. The major drawbacks of light-weight slurries are low set cement strength and technical difficulties of cement mixing.
The paper summarizes light-weight slurries development for production casing cementing at Tatneft assets. Light-weight cement slurries can be prepared based on standard cement using several techniques. These include increasing the volume of liquid phase in a slurry by adding, for example, water retaining agents; mixing of standard cement with lower-density fluids, such as oil and oil products; adding of solid agents with lower density than that of cement; cement slurry aerating. Tatneft was the first to use clays and gel powders as lightweight agents. The Company still uses light-weight cement slurries based on oil-well Portland cement and bentonite powder which are mixed in the dry state by multiple repacking at cement Contractor’s storage depot. The key contributor to density reduction is high water cement ratio, which adversely affects the set cement strength. Previously, different agents were used as light porous fillers for cement slurries, including perlite, expanded clay, foamed vermiculite, pumice, lignin. However, because of low triaxial compressive strength of porous fillers, they had limited application area. For this reason, other types of materials have come into common use, such as aluminosilicate, glass, ceramic and foam-glass microbeads.
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