Characterising and evaluation of the seal capacity based on core and cutting analysis of the Nini-4 and Nini-4a wells. Project Greensand - WP4 final report

The purpose of the EUDP funded Project Greensand is to examine if the Paleocene sand in the depleted Danish North Sea oil- and gas fields can be utilized for safe long-term storage of CO₂ (Carbon Storage). The project tests the Nini West reservoir and seal from experiments. The seal is tested on samples from below the reservoir and from the overlying mudstones, to demonstrate the viability of the caprock properties and thus to contribute to the overall evaluation of the storage site.

The mudstone that seals the Nini West oil field are proven caprocks in the Danish North Sea and elsewhere in the North Sea. The most important property controlling seal efficiency of a mudrock is the pore throat size because it controls permeability and capillary entry pressure (Katsube and Williamson, 1994; Bruno et al. 2014). In addition, the mineralogy of the seal is investigated as the minerals may react with CO2 and potentially alter the properties of the seal. As an example in a CO₂ sequestration context, calcareous shales may not unconditionally be an ideal seal due to mineral reactions taking place between CO₂ dissolved in the aqueous pore fluid and reactive minerals like carbonates (i.e. on the EU Weyburn Monitoring Project see Riding and Rochelle, 2005). Further Krushin (1997) observed that organic shales offered a lower seal quality due to matrix shrinking during subsidence and following hydrocarbon generation, which gave rise to larger entry pressure pore throats.

Here we report on the work package 4 (WP 4). The purpose of this WP was to address the seal capacity on core and cuttings material by analysing the transmissivity, mineralogy, permeability, pore size distribution and geomechanical properties. An accompanying report (Holmslykke et al. 2021) address the reactivity of the seal from experimental data. In addition to this a 3D geomechanical study commissioned to Baker reports om the mechanical properties. The seal complex and general geology for the Nini West storage complex has been outlined in the Greensand report WP4 TRL 3 by Schovsbo (2020) and by the Greensand consortium report to DNV for fulfilment of the Statement of Feasibility. All reports form the basis for the final Greensand project reporting on the seal capacity.