Thèses soutenues en 2007

Stephane LAFORTUNE

Noble gas, N₂ and O₂ isotopes for surveying injected CO₂ migrations inside geological storage reservoirs, and for predicting leakage

sous la co-direction de Pierre AGRINIER et de Manuel MOREIRA (Laboratoire de Géochimie et Cosmochimie) : soutenue le 30 novembre 2007 (http://www.ipgp.jussieu.fr/~lafortun/these/)

To attenuate the impact of anthropogenic carbon dioxide emissions, CO₂ can be stored in natural reservoirs (aquifers, oil- and gasfields).

fig.1 : Theoritical model for storage sites in saline aquifers. In this model, the lower aquifer is used to store CO₂ and the upper aquifer is used as an observation formation in which dissolved gas concentrations are monitored to control the absence of gas leaks (of tracers or CO₂).

The total amount of stored CO₂ and its mean residence time in reservoirs depend on the result of the competition between the three major phenomena governing the storage: (1) migrations of CO₂ in the storage reservoirs (controlled by hydrodynamic fluxes); (2) kinetics of mineralogical reactions which can durably fix CO₂ in a stable (solid) form; (3) leakage of CO₂ out of the reservoirs through the top, bottom or lateral barriers.


The study of the trapping efficiency of geological formations is fundamental in order to evaluate the pertinence and safety (in terms of human and environmental risks) of a reservoir. The reservoirs must have a good trapping structure with reduced leakage rates. Assessment of these leakage rates (thanks to predictive modelling or a set of adapted observation wells) will make it possible to determine any likely impact on the environment (fig.1).

fig.2 : a. Niskin water sampler for volcanic lakes (Lac Pavin-Massif Central, France) - b. Production wells on Weyburn oilfield (near Regina-Saskatchewan, Canada)

On present storage sites, CO₂ is injected in a no-pure phase and the injected gas contains air elements. N₂ and Ar (or other noble gases contained in the injected gas) would be used as tracers or precursors, the monitoring and concentration evolution of which would warn of possible future CO₂ leakage in reservoir surrounding formations. This stems from the possibility that these gases migrate faster than CO₂ because they are smaller, lighter and relatively inert. O₂ which is a highly reactive compound in reducing environments could be a useful tool for marking leaks associated with the set of injection wells (upstream leaks).

We will try to validate our monitoring method on natural (like volcanic lakes - fig.2a) or anthropogenic CO₂ storage sites. A scientific collaboration with the "IEA GHG Weyburn CO₂ Monitoring & Storage Project" (a CO₂ injection project in an EnCana oilfield realized to enhance the oil recovery - website) will permit to access to the Weyburn oilfield (fig.2b) and to sample firstly the injected gas and the storage formation (fluids and rocks), and in the future the aquifers just above and below the storage.

Our theoretical and analytical results will try to highlight the relevance of using the compounds as warning signals for CO₂ leakage and as tools for studying the trapping efficiency of a reservoir before massive injection of CO₂.