The Problem

Cement is an inexpensive and universally available material for well completion and plugging purposes, but it has certain inherent properties that make it a less than ideal material to seal against gas flow. The grade of cement usually employed shrinks as it sets, compromising its ability to form an effective seal against a solid surface such as a steel casing wall. Cement requires many hours to set completely, and during the setting process, it passes through a weak transitional phase during which pressure is not contained and flow channels can form. Cement is attacked by naturally occurring or deliberately sequestered acid gases such as CO2 and H2S. Cement quality can degrade over time due to changes in crystal structure or mechanical stresses from tectonic motion of the earth and certain well operational procedures.