In various formulations, microsilica cement slurries are finding increasing utility in oilfield cementing applications. In the course of these applications, many microsilica slurries are being used in environments which subject the set cement to bottom hole static temperatures in excess of 230deg.F.
While 35% fine silica by weight of cement (BWOC) has been extensively researched and accepted as a high temperature cement stabilizer, very little if any research has been published on the effectiveness of published on the effectiveness of microsilica in this application. The results of this study reveal that microsilica displays promise as a viable alternative stabilizing material, with the added benefit of medium to low slurry density.
Microsilica, also known as silica fume or ferrosilicon dust, is a by-product of the industrial manufacture of ferrosilicon and/or metallic silicon in high temperature electric arc furnaces. The material typically contains 93-96% SiO with trace amounts of other elements. Completely amorphous, the material consists of spherical particles as small as 0.1 microns (See Table 1). As a concrete admixture, the material has found widespread acceptance in the construction industry, and is being integrated into many oilfield cements as well. Due to certain inherent characteristics, the use of microsilica in oilfield cement slurries will usually dictate higher mix water ratios than would be utilized in non-microsilica slurries. The higher water ratios result in lower slurry densities. Since many of the oil and gas wells drilled today are into low pressure or depleted reservoirs, there is an increasing demand for mid to low density filler and/or completion cements.