Abstract
In cementing applications, the release of gas is used to prevent shrinkage of the set cement in an annulus. With a significant decrease of volume of the wellbore, long term annular isolation may result in microfractures of the cement system; therefore, a failure of the cement bond (Go Boncan and Dillenbeck 2003). Currently, there is not a recommended procedure to quantitatively measure cement additives that facilitate gas expansion. This paper describes a methodical approach to quantify gas expansion within cement slurries.
Traditionally, a known rudimentary technique in measuring gas expansion in cement slurries is to use a glass beaker and to record the observations. Under static and atmospheric conditions, a graduated cylinder has been used to quantitatively measure the displacement of gas release within the annulus at ambient conditions. The new method developed in this paper will demonstrate that gas was being expelled into the atmosphere when the cement slurry containing gas generating additives was tested under ambient conditions. The procedure developed here will demonstrate that gas was entrained within the cement matrix.
An initial assessment of varying particle sizes of coated aluminum granules in a caustic solution showed that a consistent volume of gas was released. This working hypothesis was substantiated by measuring the volume of gas released within a closed system using a data logger. To ensure validation of this procedure, in-situ gas producing cement slurries were measured utilizing this method. With this experimental setup, quantifying the measurement of released gas from the cement matrix confirmed that variation of particle size does not affect performance.
Gas volume displacement measurement using an inverted graduated cylinder was set up to quantitatively determine the volume of the released gas within the cement matrix with the utilization of a stir plate. The system utilized polytetrafluorethylene tubing to transport the released gas that was produced from the matrix to the inverted graduated cylinder that was filled with deionized water. The released gas volume was assessed by displacement.
The enclosed dynamic apparatus with an inverted graduated cylinder was fitted with a paddle. This experimental setup enabled a homogenous blend and prevented void formation as the matrix was continuously agitated. This enclosed dynamic apparatus prevented the release of gas to be consumed by the atmosphere and gas entrainment. With the utilization of this procedure, the functionality of the gas expansion additive was fully attained without hindering performance and the gas release profile was controlled. This technique exhibited reproducibility and the theoretical gas release volumes were as calculated.
After placement, the cement systems must preserve their integrity and provide zonal isolation during the life of the well. It has been possible to accommodate a wide range of conditions through the development of cementing additives that modify different Portland cements for their individual well requirements. During the hardening phase of a standard proportional cement to water content, the cement system becomes solid with low permeability. As a result of the cement matrix, the release of gas cannot migrate at a quantifiable rate with the partially water saturated pores. Low density cement systems with high water to cement ratios can exhibit high permeabilities. Therefore, it is possible for gas to flow and eventually reach the surface though at low rates. Additives are uniquely manufactured for gas expansion to prevent annulus shrinkage and high permeability; thereby preserving the integrity of the well isolation.
