Combined application of mercury intrusion and nitrogen gas adsorption techniques in the characterization of the porous seal rock system
Keywords:
seal rocks, specific surface area, pore size distribution, mercury intrusion porosimetry, nitrogen gas adsorptionAbstract
The analysis of pore space is crucial for a profound understanding of the transport of fluids in seal rocks. The permeability and capillarity are important properties which define the sealing efficiency of those rocks; nowadays these values are widely used by a variety of companies in the evaluation of rock seals, especially in petroleum exploration or in the storage of carbon dioxide. The main objective of this study is to obtain parameters that describe the porous microstructure of the seal rocks which can be correlated with empirical models that predict the intrinsic permeability. Seal rock has a broad pore size distribution ranging from micro- to macropores. The analysis of its full spectrum pore space, therefore, becomes difficult. To overcome these limitations, in this study, mercury intrusion porosimetry (MIP) and nitrogen gas adsorption (N2 GA) were applied in combination as complementary experimental techniques. The first one is ideal to obtain the porosity values and the size distribution of meso- and macropores, while the second, associated with the Brunauer, Emmett and Teller Theory (BET), permits the determination of the specific surface area, and, if in connection with a theoretical model based on Kelvin equation, leads to the size distribution of micro- and mesopores. The study was conducted on nine samples of seal rocks supplied by Cenpes/Petrobras originating from different depths within Brazilian geological formations. The combination of the MIP and N2GA curves shows that the majority of the samples are similar, presenting anisotropic porous structures with polymodal pore size distribution (PSD) curves and porosity values ranging from 0.33% to 10.45%. The values of the specific surface area (So) measured by N2GA were higher compared to those calculated by MIP. This is reasonable since the majority of the analyzed samples had a mean pore size between 20 to 1000 Å.
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