The sealing efficiency of cap rocks – laboratory tests and an empirical correlation

One of the major concern for the gas sequestration/storage feasibility in natural underground formations is the assessment of the sealing efficiency of the low-permeable sequences overlying potential storage formations.The sealing efficiency is quantified via the threshold pressure and/or residual pressure difference parameters; the experimental laboratory procedures for their evaluation have been widely investigate by the oil and gas industry at least from the fifteens of the last century. In order to evaluate the effect of adopted lab procedures, the present paper collects, categorizes and analyzes experimental data from the technical literature, investigating representative lithologies, via N2 brine fluid system, in reservoir thermodynamic condition. The analyses show an satisfactory coherence and consistency of data for a range of permeability of [10^2-10^5] nDarcy, independently from the adopted lab procedures. Data scattering for higher or lower permeability values seems to be more case sensitive rather than attributed to the adopted procedure or the investigated lithology. Furthermore, the empirical correlation byThomas et al. (1968) was verified on the bases of the collected data. Even if the equation was derived only from standard test measurements on a limited number of lithologies, it turns out to be a reliable predictive approach for the identification, at least, of the order of magnitude of the threshold pressure (or the residual pressure difference) if only absolute permeability values are availlable.