| Abstract | Geološko uskladištenje ugljičnog dioksida duboko u podzemlju predstavlja jednu od mjera nužnih za sniženje
emisija ovog stakleničkog plina u atmosferu. Koncept kaptiranja i geološkog skladištenja ugljika uključuje izdvajanje
CO2 iz dimnih plinova velikih stacionarnih izvora (npr. termoelektrana, cementara, rafinerija i dr.), siguran transport
plina cjevovodima ili brodovima od izvora do mjesta uskladištenja, te njegovo utiskivanje u podzemlje.
Ugljični dioksid se u svrhu trajnog uskladištenja može utiskivati u iscrpljena naftna i plinska ležišta, u slojeve
ugljena, te u duboke slane vodonosnike, - duboko smještene porozne i propusne stijene čije su pore ispunjene vodom
povišene mineralizacije, za koje se procjenjuju najveći kapaciteti uskladištenja CO2. Također, dio ugljičnog dioksida
ostat će trajno uskladišten u podzemlju i pri utiskivanju u naftna ležišta s ciljem povećanja iscrpka.
Pouzdana procjena kapaciteta uskladištenja duboko u podzemlju predstavlja kritičan parametar za uspješno
izvođenje operacija geološkog skladištenja ugljičnog dioksida. U tu svrhu razvijene su različite metode s obzirom na
izvore podataka i veličinu područja istraživanja. Procjene na regionalnoj razini uglavnom se temelje na prosječnim
vrijednostima svojstava, dok je za lokalne procjene potrebno detaljno poznavanje geološke građe, mineralnog sastava
stijena, uvjeta tlaka i temperature, te kemijskog sastava slojne vode. Predlaže se nova metoda koja uzima u obzir
prostorne varijacije debljine propusnih naslaga, njihove poroznosti i dubine zalijeganja, odnosno uvjeta tlaka i
temperature o kojima ovisi vrijednost gustoće utisnutog ugljičnog dioksida. Postupak je testiran na primjeru
gornjomiocenskih pješčenjaka u zapadnom dijelu Savske depresije. Predlaže se da se pojedini regionalni duboki slani
vodonosnici stratigrafski definiraju prema litostratigrafskim jedinicama razine člana. Tako prvi duboki slani
vodonosnik izdvojen u zapadnom dijelu Savske depresije odgovara donjopontskom članu - pješčenjacima Poljana
formacije Kloštar Ivanić. Kao regionalni izolator odabran je kontinuirani slijed lapora u neposrednoj krovini
pješčenjaka Poljana.
Prostorna distribucija parametara na kojima se temelji procjena specifičnog kapaciteta uskladištenja ugljičnog
dioksida prikazana je kartama izolinija, a korišteni su podatci iz dubokih istraživačkih bušotina. Prema karti
specifičnog kapaciteta uskladištenja CO2, najveće su vrijednosti procijenjene u središnjem i jugoistočnom dijelu
istraživanog područja, odnosno u dijelovima gdje duboki slani vodonosnik ima najveće debljine i najveće dubine
zalijeganja. No, upravo su to područja gdje regionalni izolator ima najmanje debljine, pa se posebna pažnja treba
posvetiti detaljnoj karakterizaciji pokrovnih stijena. Najvažniji parametar pri procjenama specifičnog kapaciteta
uskladištenja ugljičnog dioksida za jedinice koje karakterizira značajna promjenjivost debljine, predstavlja efektivna
debljina jedinice, dok promjenjivost poroznosti, koju je najteže prostorno definirati, ima manji utjecaj.
Predloženi postupak može naći primjenu u prvoj fazi istraživanja s ciljem definiranja dubokih slanih
vodonosnika u stijenama s međuzrnskom poroznosti i njihovog potencijala za geološko skladištenje CO2. Pri tome
procjena specifičnog kapaciteta uskladištenja CO2 omogućuje usmjeravanje istraživanja od regionalnih k lokalnima
koja bi u kasnijoj fazi trebalo provesti u onim područjima vodonosnika u kojima je indiciran veći specifični kapacitet |
| Abstract (english) | The reduction of carbon dioxide emissions is a prerequisite for stabilization of its
concentration in the atmosphere. After PACALA & SOCOLOW (2004), it can be obtained
only by implementation of various measures, including carbon capture and geological storage.
Concept of carbon capture and storage includes the separation of CO2 from flue gas and its
capture at large stationary sources (thermal power plants, cement mills, refineries, natural gas
processing plants), safe transport by pipelines or ships to the storage sites and its injection
deep into underground.
Geological storage can be performed under different geological conditions in
sedimentary basins: in depleted oil/gas reservoirs, in oil/gas reservoirs in production
(Enhanced Oil/Gas Recovery), in unmineable coal seams and in deep saline aquifers
(BACHU, 2000; 2003). Additionally, CO2 can be injected in caverns in salt formations, in
fissures (fractures) within basalts and in shales rich in organic matter.
The Republic of Croatia has signed the Kyoto protocol in 1999 and ratified it in 2007,
thus taking obligation for reducing greenhouse gasses for 5% in the period from 2008 to 2012,
with respect to the initial year 1990. Since almost 25% of CO2 emissions in Croatia come
from large stationary sources, CO2 capture and storage technology offers a possibility for
significant emission reduction.
Preliminary screening revealed that favourable natural conditions for CO2 geological
storage in the Republic of Croatia exist in oil and gas reservoirs as well as in deep saline
aquifers (EU GEOCAPACITY, 2009). It is reasonable to assume that under the circumstances
of constant increase in oil and gas prices and development of new methods for enhanced
oil/gas recovery, partially depleted hydrocarbon reservoirs in Croatia will stay in production
in the near future and will not be available for CO2 storage. Hence, deep saline aquifers
(porous and permeable layers situated at a depth greater than 800 m, saturated with highly
mineralized water) remain the only possible storage objects.
Reliable capacity estimate represents the most important task in the first phase of
assessing the certain area for CO2 geological storage. This requires standardized methodology,
similar to the methodology developed for estimation of mineral resources. The most
frequently cited methodology for estimation of carbon storage capacity in deep saline aquifers
is given in studies of Task Force for Review and Identification of Standards for CO2 Storage
Capacity Estimation of Carbon Sequestration Leadership Forum (CSLF) from 2005 and 2007.
For theoretical storage capacity estimations on a regional scale the authors of CSLF studies
suggest calculation of capacity by static trapping, representing the mass of CO2 that can be
stored within the pore space of each structural and/or stratigraphic trap present within the
regional aquifer, assuming that geometries of all traps are known, which is often not the case.
Somewhat different approach has been proposed by National Energy Technology
Laboratory of Fossil Energy Bureau of US Department of Energy (US DOE, 2007). It takes
into account pore space of the entire aquifer and thus does not require data on number, nor
size of the traps. Capacity is calculated as a product of deep saline aquifer surface, its average
effective thickness, average porosity, average density of CO2 under conditions of pressure and
temperature anticipated for a regional aquifer and storage efficiency factor.
A new method for regional capacity estimate is proposed, taking into consideration
spatial variations of thickness, porosity and depth of permeable layers. The proposed method
is tested on the example of Upper Miocene sandstones corresponding to the Poljana
Sandstones of the Kloštar Ivanić Formation in the western part of Sava depression. These
sandstones were deposited in turbiditic and deltaic facies that caused the specific morphology
of the sandstone bodies. Namely, they are characterised by considerable variations of
thickness and porosity. On the other hand, the structure of sedimentation basin (Sava
depression) causes pronounced depth variability. Spatial variations of parameters were
investigated through the construction of maps based on the data obtained from exploration
wells. These data primarily include spontaneous potential and specific resistivity log
measurements which were used to define depth and thickness of sandstone layers and logs
that were used to define porosity values – density, neutron and sonic logs. Apart from that, in
order to determine CO2 density in p, T conditions of a deep saline aquifer, values of pressure
and temperature in the aquifer are required.
The boundaries of the regional aquifer in Poljana Sandstones are defined with respect
to aquifer depth, with the southern boundary being set simply a boundary of estimation area,
mostly due to data availability. Based on the interpretation of electric logs, marl directly
overlying Poljana Sandstones was chosen as a regional isolator, since it can be identified on
all the available electric logs and it is situated at suitable depth. Further investigation is
needed to prove its ability to act as a barrier to CO2 migration, including laboratory capillary
pressure measurements as well as measurements of relative permeability for brine and CO2.
On the basis of spatial variations of effective thickness, porosity and CO2 density, the
specific storage capacity is calculated andmapped. For that purpose, the area of deep saline
aquifer was divided in 178 blocks – quadric prisms and the specific capacity calculated for
each of the prisms using the values read from the centre of each block. The largest specific
storage capacity was estimated in the central and south-eastern part of the investigated area,
where the deep saline aquifer is of largest thickness and is situated at the greatest depth.
However, the thickness of the regional isolator is reduced in this area, so the special attention
should be dedicated to its detailed characterization. The most important parameter in
estimates of specific storage capacity for units characterized by significant variability of
thickness is the effective thickness that can be relatively reliably defined, while the spatial
variability of porosity, which is far more difficult to define, has lesser impact on estimated
values of specific storage capacity.
The proposed method can be used in the early phase of exploration with aim of
defining deep saline aquifers in rocks with intergranular porosity and estimating their
potential for CO2 geological storage. Mapping of specific storage capacity should enable the
authorities to determine the regions for which exploration allowances might be issued and the
results of this second phase would eventually lead to planning and construction of the
subsurface CO2 storage facilities. |
