The objective of plug and abandonment of wells can be described as "restoring the cap rock". In that respect, the long-term integrity of the plugging material is crucial. I.e. it is important that the plugging material can resist downhole chemicals and otherwise withstand downhole conditions. In this paper, we have performed ageing tests with cement samples at relevant downhole conditions to determine the long-term integrity of well cement as plugging material. Portland cement samples with and without silica flour as additive have been separately exposed to crude oil, brine and H2S (in brine) at 100 °C and 500 bar for 1, 3, 6 and 12 months. The long-term integrity of the samples was determined by measuring changes in weight, volume, mechanical strength and permeability, as well as physical appearance.

It is seen from the results that the addition of a pozzolan such as silica has a significant impact on the long-term integrity of Portland cement, especially in a corrosive environment such as H2S. All the samples were affected by most of the different chemical environments, but the cement samples without silica were considerably more affected than the samples with silica as additive. Furthermore, the exposure to H2S in brine resulted in the formation of an unexpected white deposit, which precipitated both inside and outside the samples.

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