Wellbore cement degradation in contact zone with formation rock

Abstract

In the work the risk of CO2 migration in deep wells, caused by integrity loss on cement–rock interface and thow wellbore integrity correlated with the formation rock lithology were determined. 19 composed samples of rock and wellbore cement were exposed to CO2-saturated brine, in the autoclave reactor, under the formation conditions (50 °C and 10 MPa). Mineralogical and textural changes in the cement–rock interface in the case of selected rocks (sandstones, shale, limestones, dolomites and anhydrites) were characterised. The performed examination indicates that both cement and formation rocks react with CO2 saturated brine under the experimental conditions. The cement alteration is characterised by carbonation process in the outer rim, but it is enhanced on the interface with formation rock. It was stated that the performance of the cement–rock interface is essentially dependent on the rock lithology, including mineral composition and rock structure. Some minerals are very easily dissolving, e.g., anhydrite, gypsum, calcite and feldspars, what is contributing to an increase in the porosity and permeability in cement–rock contact zone. Primary dissolution of certain minerals in the first stages of the experiment results in the secondary precipitation after the last stage of reaction contributing to a secondary reduction of pore space.