Well Bore Integrity Workshop Houston, TX, USA, 4-5 April 2005

Publication Overview

The integrity of well bores, their long-term ability to retain CO₂, has been identified as a significant potential risk for the long-term security of geological storage facilities. A workshop was held in April 2005 to bring together over 50 experts from both industrial operators and from research organisations. Industrial operations are part of CO₂ enhanced oil recovery (EOR) projects or acid gas waste disposal projects. Current research includes laboratory investigations that attempt to simulate long-term geochemical and mechanical processes that may affect well completion materials – mainly cement; field studies of well completions that have been exposed to CO₂ during industrial projects as described above, and modelling studies, both of local reactions and upscaled simulations of leakage across basins.

Publication Summary

• Ensuring well integrity over long timescales has not been attempted before and represents a new challenge to the oil and gas industries.
• It will not be possible to promise a leak-free well, but rather we should emphasise that we can build wells employing state-of-the-art technologies which will reduce risks.
• Portland-based cements will react with CO₂, leading to cement degradation. The main reactions involve carbonation of the major cement components – Portlandite and calcium silicate hydrates which are converted to carbonate minerals such as aragonite, calcite and vaterite.
• Degradation results in a loss of density and strength and an increase in porosity.
• Laboratory experiments of these reactions are able to simulate those observed in wells that have been exposed to CO₂ in EOR injection and production wells. However, the degree of reaction (i.e. the rate of reaction) may not necessarily be comparable between laboratory and field.  This may be due to the need to speed up laboratory experiments, often by increasing temperatures, to reproduce longer timescales.
• One, two and three dimensional models are now being developed to simulate processes observed both in the laboratory and in the field, at the small-scale of specific leakage mechanisms within a well and also over the larger scale examining broad leakage on the basin-scale.
• However, we are unable to use these models in a predictive sense due to a lack of detailed knowledge on specific issues, discussed below in the key research needs.
• New cements have been developed and deployed that reduce the amount of alteration caused by acid attack. These cements either reduce the proportion of Portland-based cement in the mix, add inhibitors or use completely new calcium phosphate-based cements that do not contain any reactive portlandite.