Abstract
An experimental study was carried out to investigate the addition of specific additives to cement in order to eliminate the micro-fractures and micro-annuli that cause gas migration.
The experiments performed monitored the change in the cement slurry pressure during the setting of the cement. During the setting period of the cement, two time cycles of cement expansion and contraction were observed. This is due to the individual contributions of each part of the cement mixture. To obtain the optimum tightness of the cement, final optimum concentrations of the additives were obtained experimentally, where the cyclic pressure behavior of the cement was optimized for the best final cement results.
By utilizing the correct amount of Anchorage Clay, XC-Polymer, Ironite Scavenger, Ultrafine cement and Synthetic Rubber powder in a class G mixture at a given temperature and confining pressure, an impermeable cement mixture can be obtained. The correct amount of Synthetic Rubber used for cyclic pressure reduction is a function of cement setting temperature and pressure as well as the elastic properties of the rubber.
By using laboratory testing at different pressure and temperature with different rubber concentrations and elastic properties, it is estimated that the entire annulus can have an impermeable cement from surface to total depth. The difference in temperature and pressure with depth dictates the concentration and elastic properties of the rubber as the required expansion and contraction changes with depth.
