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.
Gas leakage through cemented wellbore annuli or cement plugs is a major problem of concern in many oil wells in the world. There are over 3,000 oil wells in the United States which have exhibited gas leakage problems and are abandoned because of government regulations. Enormous amount of money is spent on squeeze cementing to redeem gas leakage without producing a permanent solution. The danger posed by this problem is not only expensive to control temporarily, but also a menace to the environment. Since 1960 attention has been given to the problem of gas channeling through micro-annulus and microfractures to enhance cementing by reducing gas leakage at the surface or between reservoir formations.
The three main reasons for gas channeling through a cemented annulus are (i) the mud cake that remains between cement and the permeable formations provides a weak zone for the passage of the water and gas, resulting to failures in cement jobs, (ii) the inability of cement to hold the high fluid pressure at the period of its initial set which may cause water accumulation, resulting to micro-fracture within the cement body, and (iii) the cement's inability to maintain overbalance pressure on the gas bearing formation, because when the slurry is static (cement is not pumpable) it begins to develop a static gel. Static gel strength provides high resistance to the cement movement and prevents the hydrostatic effect of the cement slurry on the formations.
Previous investigators, Carter and Slagle and Keller et al. showed that gas migrates as a result of the physical properties of cement and drilling fluids, and also because of other physical properties such as pipe movement, well parameters and casing centralization. Some other investigators related the gas migration through the micro-fractures and micro-annulus to the cement chemical behavior, whereas others related this to the cement shrinkage and its behavior.