Holistic Design of Cement Systems to Survive CO₂ Environment

Cement systems that can survive in the CO₂ environment are needed in various applications. Examples of these applications include: 1) producers: CO₂ in the reservoir is produced along with the hydrocarbons, 2) injectors: CO₂ is injected for sequestration and/or enhanced oil recovery, and 3) producers and injectors: produced CO₂ can subsequently be injected for the purpose of enhanced oil recovery/sequestration.

However, the carbonation of Portland cement is a well-documented, thermodynamically favorable process. When CO₂ or carbonic acid comes in contact with Portland cement, it initially reacts with it to form water-insoluble calcium carbonate. Longer term, the presence of water dissolved with CO₂ (or carbonic acid), if allowed to contact the cement sheath, can dissolve the calcium carbonate to bi-carbonate, which then could be displaced if a flow channel were to be present or formed during the life of the well. This can threaten long-term effective zonal isolation.

A dual level solution is required to effectively address this challenge. As a first level, the potential for CO₂ to enter the cemented annulus and contact the cement sheath is minimized, by placing the cement slurry in the entire annulus, reducing the permeability and endowing the set sheath with the properties necessary to withstand the well events. The second level involves reducing the amount of material in the set cement sheath that is reactive to CO₂. This holistic approach has worked well in practice. Both the physical and chemical integrity of the cement sheath is addressed using this approach.

Following the design logic described above, a cement system with improved resistance to CO₂ environments was created by 1) designing a reduced-permeability cement sheath to withstand well operations with low cement hydration volume shrinkage and 2) optimizing the cement slurry formulation so that its hydration products have a lower amount of materials that are reactive to CO₂. This cement system was then tested in the laboratory under expected in situ conditions and optimized for different well situations before placement in the field.

The cement systems have been successfully placed in anumber of wells and these wells are all operating as required with no loss of zonal isolation reported. The design approach, laboratory test procedure and results from laboratory and field are presented and discussed in this paper.