In various formulations, micro-silica cement slurries are finding increasing utility in oilfield cementing applications. In the course of these applications, many micro-silica slurries are being used in environments which subject the set cement to bottom hole static temperatures in excess of 230 °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 micro-silica in this application. The results of this study reveal that micro-silica displays promise as a viable alternative stabilizing material, with the added benefit of medium to low slurry density.
Micro-silica, 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% SiO2 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 micro-silica 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. Many of these wells are also searching out deeper production, and higher temperatures are often encountered. Although there are many suitable materials used to lower the density of oilfield cements, Eilers1, et al, noted that cement extending materials such as Poz "A" may not yield desirable high temperature compounds. Thus, the challenge set forth is one of developing a strong, low density system which is also stable at high temperature. Given the particle size, large wettable surface area and the purity (SiO2) of micro-silica, it would seem theoretically possible to use micro-silica in combination with, or in place of silica flour, in formulating thermally stable low density cement systems. In an effort to verify the legitimacy of the theory, this paper will investigate the high temperature compressive strength development of various micro-silica slurries.
List of Table # 1 (Available In Full Paper)
Extensive research by many investigators has documented the use of 35% (BWOC) fine silica to prevent strength retrogression of Portland Cement at high temperatures1,2. It was therefore decided that as a control, one of the slurries tested must be a "conventional" Class "H" slurry with 35% (BWOC) silica flour. This slurry was tested under the same set of conditions as the other slurries, and then its performance used as a "yardstick" by which to measure the others. The other four slurries all contain 35% (BWOC) silica ranging from 26% silica flour and 9% micro-silica to 35% micro-silica.