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Abstract
Prior to any CO2 geological storage operation, the caprock’s ability to prevent CO2 leakage must be carefully assessed. This ability is primarily related to the caprock’s pore structure and to the interfacial properties of the caprock and the fluids in place, namely the brine that imbibes the caprock and the CO2 stored in the underlying reservoir. This paper reports an experimental effort to characterize some of these parameters, using as a working example the carbonate-rich caprock and reservoir conditions of the Rousse depleted gas field in the South-West of France, where an estimated 120000 tons of CO2 will be injected during two years. The parameters examined are: (i) the caprock wetting behavior in the presence of CO2, (ii) the caprock intrinsic (single-phase) permeability, (iii) the CO2 breakthrough (or displacement) overpressure, i.e., the minimum pressure difference between CO2 and brine required for CO2 to penetrate and flow through the brine-saturated caprock, and (iv) the caprock effective permeability to CO2 after breakthrough. The latter two parameters are indicative of, respectively, the caprock’s capillary sealing efficiency and CO2 leakage rate once breakthrough has occurred.

The main observations and results are as follows. (i) The water-wet character of the Rousse caprock in the presence of CO2 in storage conditions is confirmed through a series of contact angle measurements on substrates. (ii) Single-phase (brine) permeability coefficients measured in steady-state conditions are extremely small and strongly sensitive to the effective stress, i.e., to the difference between the confining pressure and the pore fluid pressure. They do not exceed 20-25 nanodarcy (10-21 m2) for effective stresses below 4 MPa, and 1 nanodarcy for effective stresses in the range of 10 MPa and above. (iii) Gas breakthrough in two different brine-saturated caprock samples, using either CO2 or N2 as the displacing gas phase, occurs in one sample for a CO2 overpressure in excess of 7.6 MPa, and in the other sample for a N2 overpressure in the interval of 4.5-6 MPa. In the latter sample, the effective permeability to N2 after breakthrough is extremely low, below 1 nanodarcy; some “aging” effects are observed upon subsequent resaturations with brine and CO2 breakthrough experiments, with a CO2 breakthrough overpressure lower than expected and an increasing effective permeability to gas, yet still lower than 2 nanodarcy after two consecutive brine resaturations and CO2 breakthroughs.

Introduction
In a CO2 geological storage operation, a large part of the CO2 injected in the formation buoyantly rises until it reaches a low-permeability barrier acting as a seal. The ability of the caprock to prevent leakage must be assessed prior to planning a geological storage operation in a depleted hydrocarbon reservoir or a deep saline aquifer. In the case of hydrocarbon reservoirs, the existence of a top seal (or caprock) is clearly proven, but with respect to hydrocarbons. In the case of deep saline aquifers, little is known on the confining properties of the overlying low-permeability layers, for simplicity also referred to here as caprock (rather than aquitard or aquiclude).