Method for the determination of residual carbon dioxide saturation using reactive ester tracers

The mechanisms for storage of CO2 in rock formations include structural/stratigraphic, mineral, solubility and residual trapping. Residual trapping is very important in terms of both containment security and storage capacity. However, to date, the contribution from residual trapping (i.e. immobilisation of supercritical fluid via capillarity in pore spaces) is still relatively difficult to quantify accurately. Using a laboratory-based testing program, this study demonstrates the feasibility of using reactive ester tracers (i.e. triacetin, propylene glycol diacetate and tripropionin), which partition between a mobile water phase and a stationary supercritical CO2 phase, to quantify the residual CO2 saturation, Sgr, of a rock formation. The proposed single-well test involves injecting these tracers into the subsurface, followed by CO2 saturated water, where the ester tracers slowly hydrolyse to form products with differing partition coefficients. After a suitable period of time, allowing for partial hydrolysis, water containing the tracer mixture is produced from the subsurface and analysed using gas chromatography mass spectrometry (GCMS). A numerical simulator of the tracer behaviour in a reservoir is used to explain the differential breakthrough of these tracer compounds during water production to estimate Sgr. Computer modelling suggests that the use of esters tracers to determine CO2 residual saturation is a potentially robust method. The supercritical CO2/water partition coefficients directly dictate the amount of time that each tracer spends in the CO2 and water phases. As such for modelling of tracer behaviour and estimating Sgr, knowing the tracer partition coefficient is essential; in this paper, the first laboratory study to determine the partition coefficients of these reactive ester tracers is described.

► Environmentally safe and cheap “food-grade” esters have been identified as potential tracers.
► These hydrolyse in the subsurface to form daughter compounds with different characteristics.
► In a single-well “push–pull” test, these can be used to estimate CO2 residual saturation.
► Modelling shows that these give more accurate predictions compared to inert gas tracers.