Prevent Cement Strength Retrogression Under Ultra High Temperature

Abstract

Abstract The cement strength retrogression is expected for Portland cement when it is exposed to high temperature (>110°C / 230°F) environment. Strength retrogression is defined as a change in the hydration products that can be described as a decline of cement strength at elevated temperatures where decreased strength is observed with increasing time. When strength retrogression occurs, not only is there a reduction in the strength of cement, but also an increase in permeability which compromises zonal isolation and/or pipe support, the operation life span of the wellbore would be seriously influenced. It has been a common practice by adding Silica into the cement to counteract and control cement strength retrogression for many years in oil & gas industry. While as drilling operation becomes more and more challenging and downhole conditions are much more complex than ever before, for example tremendous HPHT and deep-water wells are under exploration which raise the new challenges and technical problems to operators. Facing with such challenges the cement strength is expected to be compromised which may result in failure of zonal isolation and common use of silica to mitigate the retrogression may not be an adequate way to contribute the reliable strength and permeability of cement under such high temperature well conditions (>220°C/430°F) in a long term. For a better zonal isolation and pipe support, it is necessary to come up with a novel solution to address cement strength retrogression in challenging well conditions especially at ultra-high temperatures. This paper would introduce a unique design additive - CemStal (Cement Strength Stabilizer) to help address the cement strength retrogression problem even under ultra high temperature (320°C/610°F) well conditions. The conventional silica was used to compare with CemStal under different temperature conditions and various density of cement slurries to test their ability to inhibit cement strength retrogression. All tests were using almost same formula except the silica and CemStal which is added to help prevent strength retrogression, the densities of cement slurry varied from 1.35SG (low density slurry),1.88SG,1.90SG (regular density slurry) to 2.65SG (high density slurry) and curing temperatures for the test were 220°C/430°F (high temperature), 320°C/610°F (ultra high temperature), the curing intervals of 2, 7, 28, 49 and 98 days’ results were investigated in this study.