SR09_FGS07.qxd
25.05.2007
14:43 Uhr
Seite 173
The presence of a fracture thus canalizes the reactions along the fracture (calcite, dolomite), with an effect on the close vicinity of the fracture for all the mineralogy. The impact in terms of caprock integrity is predictable, with the dissolution of all the minerals inside the fracture, and an increase of 25% of the porosity in the vicinity of the fracture. However, the transformation of an observed mean mineralogical (and porosity) variation into permeability changes is not simple: these relations will have to be designed carefully, according to the local pore structure and its evolution. Finally, it is essential to bear in mind that the important point is not the impact of the reactions on the mineralogy in itself, but the evolution of the CO2 migration rate according to the scenarios and the degradation of the caprock. Simulations are being carried out to quantify the fluxes of CO2 on a medium scale (~10 m) according to several scenarios. Capillary breakthrough If CO2 leakage can occur subsequent to the pressure build up and temperature decrease resulting from the CO2 injection, via pre-existing or hydraulic or thermal newly formed fractures of the caprock, it may also occur by capillary breakthrough of the CO2 phase. In aquifers, there is no proven capillary barrier of the caprock with respect to CO2, while in
hydrocarbon reservoirs the initial capillary barrier is indeed proven, but with respect to hydrocarbons. As illustrated in the following image, this capillary barrier is an interfacial effect. In fact, caprocks are fine (usually clayey, but sometimes evaporitic) porous media imbibed with water (brine), most often at hydrostatic pressure. Breakthrough of CO2 occurs, i.e., water is displaced by CO2, when the radius of curvature of the water-CO2 menisci (see Figure 6) reaches a characteristic pore radius R characteristic of the caprock structure. This corresponds to an excess pressure in the CO2 phase (as compared to the water or hydrostatic pressure Pw) given by the Laplace law: (1) where γw,CO2 is the interfacial tension between the water and CO2 phases, and θ the contact angle (in water) of the rock substrate/water/CO2 system. The caprock's capillary-sealing efficiency with respect to CO2 is quantified by this excess pressure, which is nothing more than the capillary entry pressure Pce of CO2 into the water-filled caprock; this pressure can itself be easily converted into a maximum height of stored CO2, i.e., into a storage capacity. From Equation (1) it is clearly apparent that a good capillary barrier is provided by a large enough water-CO2 interfacial tension and a
Figure 6: Schematic representation of the interfacial phenomena involved in capillary retention of CO2 (stored in the reservoir-rock) by the water-imbibed caprock.
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