An innovative experiment is being carried out by ULTimateCO2 in the underground rock laboratory of Mont Terri in Switzerland to understand the processes affecting well integrity.
The ULTimateCO2 Mont Terri experiment: general principles and various steps
The geological storage of CO2 in deep aquifers consists in injecting CO2 into permeable geological formations that initially contain brines in the rock pores. The geological formations chosen for storage must have an overlying impermeable caprock to prevent the injected CO2 from escaping upwards (under the pressure and temperature conditions typically envisaged for storage, the CO2 is in gas form, which is less dense than water and thus rises under the effect of gravity). The caprock also prevents any vertical migration of brines (and dissolved CO2) due to the increased pressure in the aquifer relative to hydrostatic pressure.
However, if the seal of the caprock is broken, there is a risk that the fluids could migrate upwards towards shallower aquifers containing groundwater resources. Existing wells intersecting the storage formation thus represent potential leakage pathways (and abandoned wells are relatively common in sedimentary basins suitable for CO2 storage). To avoid leakage, the wells must have a good seal at their intersection with the caprock. Well integrity depends on the capacity of a well to prevent fluids communicating between two aquifers, e.g. a storage aquifer and an overlying groundwater aquifer. This problem is being studied by the ULTimateCO2 experiment in the underground rock laboratory of Mont Terri in Switzerland since the end of 2012.
In the experiment, the Opalinus Clay is considered as a representative caprock and the general objective is to study well integrity according to various conditions (temperature, pressure and geochemical environment).
A well typically consists of a casing that is separated from the surrounding geological formation by a cement layer. For our experiment, a well section 2.30 m long was drilled, equipped with steel casing and then cemented. The materials used (steel and cement) and the geometry of the various parts (drilling diameter, casing, thickness of the concrete ring) are the same as those used in oil exploration. The well is divided into a lower section and an upper section (empty volumes filled with synthetic water of a composition similar to that of pore water found in the Opalinus Clay). Fluids are then injected or extracted from these two sections to allow:
- 1) pressure tests to estimate when the two sections are hydraulically connected (sign of well integrity);
- 2) fluid sampling to characterize the evolution of water chemistry as a function of the reactions between clay, cement and steel.
The well system was installed in October 2012 and gradually filled with water until February 2013. Since then, experiments have been conducted to characterize both i) the evolution well integrity (connection status between the two sections) and ii) the geochemical interaction processes occurring at the clay / cement / steel interfaces under various conditions, namely temperature, pressure and geochemical environments (initial geochemical environment first, then an acidic environment with water containing dissolved CO2).
2013 focused on observation of the well in its initial geochemical environment (i.e. without CO2) with gradual change of temperature of the system. Imposed pressure tests allowing circulation of pore water from the lower to the upper section (hydraulic aspect) and sporadic fluid sampling (geochemical aspect) were carried out:
- at initial temperature (17°C) during March 2013;
- at a higher temperature (50°C in the lower section) from May to August 2013;
- at a lower temperature (30°C in the lower section) from September to December 2013.
2014 is devoted to studying the system in a different geochemical environment: firstly that of initial synthetic water containing dissolved CO2 at constant temperature (30°C in the lower section). Pressure tests similar to those of 2013 will be carried out, as well as sporadic fluid sampling. More specifically, the lower section will be filled with water containing dissolved CO2 that will then be circulated towards the upper section. For this, the water initially contained in the lower section will become saturated in CO2 by injection of a calibrated quantity of gaseous CO2 (but which dissolves directly), to prevent the remaining CO2 being in a gas phase. This operation began in mid- February. To better understand the fluid migration from the moment the well is brought into contact with CO2, a full mixture of tracers is injected simultaneously with the CO2 (argon, xenon, helium, deuterium, bromine and PFCs). The content of the upper section is continuously monitored in order to detect the arrival of the various tracers injected into the lower section. This test will run until at least late 2014. When completed, a final step consists in over-coring the well and extracting samples for laboratory analysis of mineralogical changes observed at the clay / cement / steel interfaces.
Dernière mise à jour le 27.01.2016