The estimation of CO2 storage potential of a gas-bearing shale succession at the early stage of reservoir characterization: a case study from the Baltic Basin (Poland)

Abstract

Estimation of the CO₂ storage potential of gas-bearing shales in the Lower Paleozoic Baltic Basin is at an early stage of reservoir exploration and production, based on data from one vertical exploration borehole, supplemented with some information from adjacent boreholes. The borehole section examined is 120 m long and comprises three intervals enriched with organic matter separated by organic-poor intervals. In our approach, the storage capacity is represented by: (1) sorption potential of organic matter, (2) open pore space and (3) potential fracture space. The potential for adsorbed CO₂ was determined from Langmuir isotherm parameters taken from laboratory measurements and recalculated from CH₄ adsorption curves. The pore space capacity was estimated in two ways: by utilizing results of laboratory measurements of dynamic capacity for pores >100 nm and using results of helium porosimetry, the first of these being considered as the most relevant. Due to the low permeability of the shale matrix we have adopted the standard assumption that the CO₂ is able to reach effectively only 10% of the theoretical total sorption and pore volume. For hydraulic fracture space, the theoretical maximum opening of vertical fractures in the direction of minimum horizontal stress was considered, decreased by the expected portion of fracturing fluid flowback and by partial fracture closure by burial compaction. The effectiveness of three CO₂ storage categories for the individual organic-rich and organic-poor shale units shows an obvious positive correlation of TOC content with the storage efficiency by sorption and within pore space, and a negative correlation with the storage efficiency in hydraulic fractures. It was estimated that sorption, over the maximum storage interval (120 m thick), is responsible for ~76% of total storage capacity, pore space accounts for 13% (for the most relevant porosity model) while the contribution of fractures is about 11%. In the minimum storage interval (35 m thick, including the best quality shales) the estimated proportions of sorption, pore space and fractures in the total storage capacity are 84, 10 and 6% respectively. Finally, the result for the best quality storage interval (35 m thick) was compared with the Marcellus Shale of similar thickness (average ~38 m) and with other options of CO₂ storage in Poland. The most organic-rich units in the area studied have a CO₂ storage capacity efficiency (i.e. storage capacity per volume unit of shale) only slightly less than average for the Marcellus Shale, because sorption capacity – the dominant component – is comparable in both cases. However, the open pore space capacity in the Marcellus Shale appears to be far higher, even if the potential fracture space calculated for the borehole studied is taken into consideration, probably because the free gas content in the Marcellus Shale is far higher than in the Baltic Basin. CO₂ storage in depleted shale gas wells is not a competitive solution compared to storage in saline aquifer structures or in larger hydrocarbon fields.