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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)

Wójcicki Adam, Jarosiński Marek, Roman Michał Grzegorz

Geological Quarterly

2021

Vol. 65, No. 1

art. no. 3

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 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.

Projektowanie wtórnego zabiegu udostępniania złóż typu niekonwencjonalnego z uwzględnieniem geomechanicznego modelu Ziemi

Słota-Valim M.

Nafta-Gaz

2014

R. 70, nr 9

563--573

Złoża typu niekonwencjonalnego wymagają nowego podejścia na każdym poziomie pracy z obiektem złożowym, począwszy od fazy rozpoznania formacji złożowej, przez zaprojektowanie otworu wiertniczego i procesu wiercenia, po uzbrojenie odwiertu, w tym zabiegi udostępnienia złoża. Te ostatnie stanowią technologiczny proces hydraulicznego szczelinowania, mający na celu wygenerowanie sieci połączonych ze sobą szczelin, umożliwiających przepływ dotychczas uwięzionych węglowodorów. Na każdym z powyższych etapów istotne jest rozpoznanie własności mechanicznych formacji gazo- lub roponośnej. Znajomość parametrów geomechanicznych umożliwia między innymi zaprojektowanie właściwego kierunku horyzontalnego odcinka otworu, zabiegów udostępnienia złoża, a także uniknięcie wielu problemów technicznych podczas procesu wiercenia, co bezpośrednio przekłada się na jego czas i koszty. W pracy zaprezentowano model geomechaniczny dla obiektu syntetycznego, pozwalający na analizę wielu procesów zachodzących na skutek eksploatacji węglowodorów. Szczególną uwagę poświęcono zmianom w rozkładzie naprężeń będących efektem postępującej produkcji, a także ich konsekwencjom przy projektowaniu wtórnych zabiegów udostępniania złóż niekonwencjonalnych.

Unconventional reservoirs require a new approach at every level of their operation, starting from the exploration of the reservoir formation, through the design of the borehole and the drilling process and ending with the development of the reservoir including treatments like hydraulic fracturing. At each of these stages it is important to identify the mechanical properties of hydrocarbon bearing formations. With the knowledge of the geomechanical parameters, among others, it is possible to design the proper direction of the horizontal section of the borehole, reservoir development treatments and to avoid many technical problems during the drilling process, which directly translates into time and cost of drilling. The paper presents a geomechanical model of a synthetic object allowing for the analysis of many processes occurring as a result of exploitation of hydrocarbons. Particular attention was paid to changes in stress distribution resulting from ongoing production as well as their consequences for the design of secondary development treatments of unconventional formations.