Reduction of the porosity in hardened oilwell cement slurries by the usage of fine-grained additives

One of the major problem in cementing technology of casing string columns, especially on natural gas deposits, are gas migrations [exhalations] from annular space of the borehole, which are difficult to eliminate. This paper presents the results of fresh slurries properties and microstructure of hardened oilwell cement slurries changes, caused by addition of fine-grained additives. In the studies, two cements and four fine-grained additives were used: two microcements, mix of microcement and silica fume and only silica fume. Prepared slurries were cured in similar conditions as in the borehole. Fine-grained additives cause significant reduction of filtration, which is particularly relevant in the case of horizontal drilling, e.g. during exploration of shale gas. Microadditives with high fineness cause significant reduction of pore sizes and surface area of pores which is decreasing of hardened slurry permeability. The introduction of microadditives also increases the strength and bond of cement matrix with steel by about 25% - 30%.