An experimental study was carried out to investigate the performance of new cement additives in controlling or eliminating gas migration through cemented wellbore annulus. The shrinkage behavior of the proposed cement slurries was obtained as a function of time and applied pressure at constant bottom hole temperature. Ultra-sonic measurements were correlated with the strength and bulk density of the cement. These measurements were used to screen out cement slurries for further testing in a cement simulation unit This unit consists of a 7″ casing pipe placed inside a 10″ controlled temperature water bath. The experimental unit was 6 ft long and was used to simulate borehole pressure and temperature conditions. The borehole was modeled by drilling a 2″ diameter hole through 2.5 ft long cylindrical core samples.
A large number of additives was tested to evaluate their effects on the cement volume fluctuations which are the primary causes of gas migration. A thorough procedure was followed to evaluate the effectiveness of all tested additives in controlling or eliminating the creation of micro-fractures during cement setting under borehole pressure and temperature conditions. The results obtained from this extensive experimental study show that the use of the proposed additives decrease significantly the cement volume fluctuations during cement setting.
The new cement additives proposed in this study reduce the contraction-expansion mechanism effects during cement slurry setting, thus minimizing and controlling the creation of micro-fracture.