Petrophysical assessment of a carbonate-rich caprock for CO2 geological storage purposes
Résumé
We present an experimental study of the retention/flow properties of the caprock of the Rousse depleted gas field, where up to 120000 tons of CO 2 are to be injected over a 2-year period. One of the caprock samples examined comes from the carbonated breccias overlying the reservoir, a deep Jurassic horst, whereas the other samples come from the 2000 m-thick Upper Cretaceous marl (carbonated turbidites) located above these breccias and the reservoir. The single-phase (brine) intrinsic permeabilities of the marly samples turn out to be extremely low and stress-sensitive. They vary from 20-25 nanoDarcy for the most permeable sample at low stress, to below 1 nanoDarcy at effective stresses in the range of 10 MPa and above. These latter values are lower by at least one order of magnitude than the published values for the caprocks of the Weyburn field and Utsira aquifer. However, the permeability of the brecciated sample is considerably higher, and decreases from ca. 0.1 to 0.003 milliDarcy when the effective stress increases to 20 MPa, which possibly reflects some of the reservoir petrophysical behavior. Gas breakthrough (BT) experiments are undertaken on two of the marly samples fully saturated with brine, using either CO2 or nitrogen as the gas phase. One sample is able to resist CO2 excess pressures as large as 7.6 MPa, whereas in the other more permeable sample a relative pressure of 4.5-6 MPa is enough to induce the BT of nitrogen, a gas with interfacial properties more favourable than CO2 to capillary sealing. Nitrogen flow following BT is extremely limited, with an effective permeability below 1 nanoDarcy. Upon subsequent sample resaturation with brine and then BT with CO2, some alteration of the sealing behaviour is observed, manifested by a decreased BT pressure and an increased gas effective permeability.