Reservoir quality and petrophysical properties of Cambrian sandstones and their changes during the experimental modelling of CO2 storage in the Baltic Basin

The objectives of this study were (1) to review current recommendations on storage reservoirs and classify their quality using experimental data of sandstones of the Deimena Formation of Cambrian Series 3, (2) to determine how the possible CO2 geological storage (CGS) in the Deimena Formation sandstones affects their properties and reservoir quality and (3) to apply the proposed classification to the storage reservoirs and their changes during CGS in the Baltic Basin. The new classification of the reservoir quality of rocks for CGS in terms of gas permeability and porosity was proposed for the sandstones of the Deimena Formation covered by Lower Ordovician clayey and carbonate cap rocks in the Baltic sedimentary basin. Based on permeability the sandstones were divided into four groups showing their practical usability for CGS (‘very appropriate’, ‘appropriate’, ‘cautionary’ and ‘not appropriate’). According to porosity, eight reservoir quality classes were distinguished within these groups. The petrophysical, geochemical and mineralogical parameters of the sandstones from the onshore South Kandava and offshore E6 structures in Latvia and the E7 structure in Lithuania were studied before and after the CO2 injection-like alteration experiment. The greatest changes in the composition and properties were determined in the carbonate-cemented sandstones from the uppermost part of the South Kandava onshore structure. Partial dissolution of pore-filling carbonate cement (ankerite and calcite) and displacement of clay cement blocking pores caused significant increase in the effective porosity of the samples, drastic increase in their permeability and decrease in grain and bulk density, P- and S-wave velocity, and weight of the dry samples. As a result of these alterations, carbonate-cemented sandstones of initially ‘very low’ reservoir quality (class VIII), ‘not appropriate’ for CGS, acquired an ‘appropriate’ for CGS ‘moderate’ quality (class IV) or ‘very appropriate’ ‘high-2’ reservoir quality (class II). The permeability of the clay-cemented sandstones of ‘very low’ reservoir quality class VIII from the lower part of the E7 reservoir was not improved. Only minor changes during the alteration experiment in the offshore pure quartz sandstones from the E6 and E7 structures caused slight variations in their properties. The initial reservoir quality of these sandstones (‘high-1’ and ‘good’, classes I and III, respectively, in the E6 structure, and ‘cautionary-2’, class VI in the E7 structure) was mainly preserved. The reservoir sandstones of the Deimena Formation in the South Kandava structure had an average porosity of 21%, identical to the porosity of rocks in the E6 structure, but twice higher average permeability, 300 and 150 mD, respectively. The estimated good reservoir quality of these sandstones was assessed as ‘appropriate’ for CGS. The reservoir quality of the sandstones from the E7 offshore structure, estimated as ‘cautionary-2’ (average porosity 12% and permeability 40 mD), was lowest among the studied structures and was assessed as ‘cautionary’ for CGS. Petrophysical alteration of sandstones induced by laboratory-simulated CGS was studied for the first time in the Baltic Basin. The obtained results are important for understanding the physical processes that may occur during CO2 storage in the Baltic onshore and offshore structures