Case Studies of Expanding Cement to Improve Wellbore Sealing

Abstract

Abstract Good cement bond at the casing-cement and cement-formation interfaces is essential for effective zonal isolation. Poor bonding can lead to underground fluids and gases to enter the annulus and create sustained casing pressure (SCP), jeopardising the working envelope of the well and limiting its production. One of the causes of a poor cement-formation bonding is attributed to a cement shrinkage. Cement systems that expand after setting can help improve primary cementing job results by sealing microannulus. The enhanced bonding is the result of enhanced shear bond and adhesion of the cement against the pipe and formation. Cement expansion is achieved by addition of the expanding additives into cement system. The mechanism of expansion is based on set cement volume growth over initial volume post setting. This is driven either by gas bubbles created during chemical reaction or by crystal growth within set cement matrix. Careful optimization of the cement slurry designs with an addition of the expansion additives to conventional and complex blend systems allowed greatly improving the cement bond evaluation log results without compromising other mechanical properties of cement. This paper outlines the successful application of expanding cement to seal different sizes of wellbore; the study evaluates the effect of the expansion by comparing the cement evaluation log from numerous cementing jobs. Examples included in the comparison are cemented production strings (casings and liners) with different types of cement systems used across 9 5/8-in. production casings and 7-in. and 4 1/2-in. production liners.