Long term reactivity of sandstone reservoir rocks at Montmiral

At Montmiral, supercritical CO2 has accumulated in Triassic sandstones below 2400m depth (Czernichowski-Lauriol et al., 2002; Pearce et al., 2004). Based upon the tectonic setting and the results of petrographic studies, this accumulation occurred in post-Pyrenean times about 15 million years ago. The reservoir is sealed by Domerian and Callovian clay and marl. The Montmiral CO2 field (98% CO2) was discovered during gas and oil exploration in the 1960's and is currently used for industrial CO2 production. The availability of core material and access to reservoir fluids through the

CO2 producing well have allowed determination of the geochemical processes occurring as a result of CO2 emplacement. This has been achieved through diagenetic studies, fluid inclusion analyses, isotopic analyses, numerical modelling, and has also been supported by a detailed reconstruction of reservoir evolution (Gaus et al., 2004).

Observed dissolution and precipitation reactions that can be attributed to interactions with injected CO2 are, in diagenetic order: K-feldspar dissolution; extensive anhydrite cement removal; infilling of secondary porosity in K-feldspars by kaolinite (Figure 7); barite precipitation; mica splaying in open porosity; dolomite/ankerite development as a major pore-filling cement in some horizons; and the corrosion of ankerite. All these processes created an additional secondary porosity of about 2.4%.

Figure 7. Observed chemical reactivity at Montmiral: large secondary pore in corroded K-feldspar that is part of a coarse lithic grain, which is infilled by fine grained moderately developed kaolinite books (Q = quartz, F = feldspar, D = dolomite, k = kaolinite, Fe = Fe oxyhydroxides, S = siderite, M = mica) (Riding and Rochelle, 2005).

Figure 7. Observed chemical reactivity at Montmiral: large secondary pore in corroded K-feldspar that is part of a coarse lithic grain, which is infilled by fine grained moderately developed kaolinite books (Q = quartz, F = feldspar, D = dolomite, k = kaolinite, Fe = Fe oxyhydroxides, S = siderite, M = mica) (Riding and Rochelle, 2005).

Batch and flow-through geochemical modelling using PHREEQC was applied to reproduce the observed effects, to identify their driving parameters and to assess their impact in terms of potential mineral trapping and porosity changes. It has proved unnecessary to consider reaction kinetics; in the case of Montmiral, elevated temperatures and long contact times between the CO2 and the rock allowed an assumption of local thermodynamic equilibrium to be used.

The modelling reproduced most of the observed changes that can be attributed to CO2 ingress. However, in the batch modelling the extent to which these reactions occurred was not reproduced. The modelled reactions do not have a large impact on the mineralogy of the reservoir and an insignificant porosity increase was predicted (0.02 %). The observed reactivity implies that the sediment has been flushed intensively with CO2-rich pore waters (open system) and that a flow regime in the reservoir must have been in place at a certain point in the reservoir's geological history. 1D reactive-transport modelling showed that an increase of secondary porosity of 2.4% requires that the pore water of the reservoir must be renewed 6000 times with CO2-rich brine. This means that if we consider flow velocities between 0.1 and 100 meters per year, this would require between 100 and 100,000 years of flushing.

Mineral trapping can therefore be insignificant when considering certain closed systems. However, the presence of a flow regime can enhance mineral trapping, and a mineral trapping capacity of 8 kg CO2 per m3 of reservoir rock was modelled. In the case of Montmiral, the predicted trapping phase is dolomite. However, the precipitation of tiny amounts of dolomite is very difficult to confirm by observations. Dawsonite (another potential secondary carbonate mineral) was neither observed nor modelled at Montmiral.

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