Confining Properties of Carbonated Dogger Cap-rocks (Parisian Basin) for CO2 Storage Purpose

The Geocarbone-INTEGRITE project, funded by ANR (Agence Nationale de la Recherche) from 2006 to 2008, has the overall objective of gaining knowledge and technology to predict storage integrity and safety for long term geological CO2 sequestration. The aim of this study (Sect. 1 and 2 of the project) is to evaluate the sealing capacities of the carbonated Dogger cap-rocks located on top of the reservoirs where CO2 could be injected for storage purposes in the Parisian Basin. A petrographic analysis is done on 3 different cores from geological formations of the 'Comblanchien and Dalle Nacree' tight carbonates facies. At the same time, a petrophysical study of these facies is performed. Porosity, pore size distribution and permeability are determined using special devices adapted to low permeability and tight rocks (≤ 10 micro-Darcy). Permeabilities are measured with nitrogen, helium and brine under confining pressure using unsteady-state and steady-state methods. Porosities are low and vary from 2 to 9%, depending on the facies, and single-phase permeabilities vary from 0.3 to 20 micro-Darcy. Using different techniques of porosity and permeability measurement allows evaluation of the uncertainties. The sealing efficiency of a cap-rock towards CO2 is primarily controlled by the threshold capillary pressure at in situ conditions. This threshold pressure is first estimated by mercury injection (HPMI). It is also directly measured on core samples at in situ conditions with Nitrogen (N2) and Carbon dioxide (CO2) (to see the effect of CO2 compared to a neutral gas on the threshold pressure). Two different techniques, described in detail in this article, are used: the dynamic method and the pressure step method. For the CO2/brine system, measured threshold capillary pressures vary from 0.4 bar to 22 bar, depending on the facies. The Pth values, even for the tighter facies (CH106), are probably too low to exclude penetration of CO2 into the cap-rock. This however does not rule out storage of CO2, as long as it can be proved that the permeability of the cap-rock is low enough and its thickness large enough to limit the extent of this penetration even though overpressure is maintained for a long period. (authors)