Regional-Scale CO 2 Storage Capacity Estimation in Mesozoic Aquifers of Poland

Regional-Scale CO2 Storage Capacity Estimation in Mesozoic Aquifers of Poland — The CO2 storage capacity estimated at the regional scale provides a preliminary recognition of as far as the possible uses of CO2 sequestration as an option of emission reduction. There has been presented the estimation of CO2 storage capacity for aquifers of Lower Cretaceous, Lower Jurassic and Lower Triassic of Polish Lowland. The Lower Jurassic deposits exhibit the greatest CO2 storage capacity of all the horizons, storage capacity of the Lower Triassic deposits is lower, the smallest capacity was estimated for the Lower Cretaceous deposits. The use of specific capacity maps for carbon dioxide in aquifers has been suggested in order to estimate the potential on national and basin scale as well as the search for prospective areas for CO2 storage. Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 66 (2011), No. 1, pp. 37-45 Copyright © 2011, IFP Energies nouvelles DOI: 10.2516/ogst/2010039 Deep Saline Aquifers for Geological Storage of CO2 and Energy Stockage géologique du CO2 et de l'énergie en aquifères salins profonds IFP Energies nouvelles International Conference Rencontres Scientifiques d’IFP Energies nouvelles ogst09123_Uliasz 17/03/11 15:55 Page 37


INTRODUCTION
The capture and storage of CO 2 in deep-seated geological formations is considered to be a method of reducing greenhouse gas emissions into the atmosphere.The following structures are recommended for underground CO 2 storage: oil and natural gas fields, unmineable coal seams (accompanied by enhanced coalbed methane recovery) and deep aquifers (Holloway, 2002(Holloway, , 2005;;Holloway and van der Straaten, 1995).To choose the best location for geological CO 2 storage, a number of aspects must be considered: emission source location, geological structure of the area, geothermal and hydrodynamic conditions, existense and properties of reservoir cap rocks, economic and regulatory aspects, etc.One of the crucial factors for the assessment of suitability of the geological formation for CO 2 storage is its storage capacity i.e. the amount of CO 2 that can be safely injected into the geological structure without any environmental damage.
Estimates of CO 2 storage capacity are made on a world, national, basin, local and storage site scale.The world-, national-and basin-scale estimates are carried out to recognize the general possibility of using this method of CO 2 emission reduction in a given region.Detailed and thorough storage capacity estimates are performed on a local and storage site scale.Depending on the assessment scale, different methods are used ranging from general assessment formulas to model and computer simulations on a local and site storage scale.The process of CO 2 storage in aquifers occurs through a number of storage mechanisms, including structural trapping, hydrodynamic, dissolution, mineral trapping and residual gas trapping.When estimating the storage capacity on a large scale (world, national and basin scale), we commonly do not take into account all the mechanisms, limiting ourselves to structural trapping and sometimes to hydrodynamic and dissolution.Local-and storage site-scale estimates are made based on most or all of the storage mechanisms.
This paper presents the estimation of CO 2 storage capacity in Mesozoic aquifers of Polish Lowlands in regional scale.To estimate the storage potential and to search for prospective areas suitable for CO 2 storage, maps of CO 2 storage capacity per unit area were constructed for individual aquifers.Lower Jurassic, Lower Cretaceous and Lower Triassic deposits are the aquifers prospective for underground carbon dioxide storage in Poland (Fig. 1).All of them are widespread over much of the Polish Lowlands (central and northwest Poland), they contain thick permeable formations, show good reservoir properties and are overlain by a good caprock series (Fig. 2) (Tarkowski and Uliasz-Misiak, 2006;Uliasz-Misiak, 2008).

Lower Cretaceous
The Lower Cretaceous reservoir horizons consist mainly of sandstones and sandy and carbonate-sandy deposits of Barremian-Albian age.They are separated by discontinuous series of poorly and non-permeable sediments composed of siltstones and mudstones (Malinowski (ed), 1991).
The elevations of the Lower Cretaceous aquifer vary from +270 metres a.s.l.(in the areas of outcrops or incrops under the Cenozoic cover) to -2 800 metres in the central part of the basin (Mogilno -Łodź Depression).
Total thickness of the Lower Cretaceous formation varies from several metres to over 2 000 m.The largest thickness is usually observed in axial parts of troughs (over 2 000 m), decreasing to several metres towards their marginal parts.Local anomalies were observed in the vicinity of salt diapirs and faults.
Generally, the distribution of cumulative thickness of Lower Cretaceous groundwater horizons corresponds to the total thickness of this formation and varies from 150 to 300 m (Górecki (ed), 2006).
Effective porosity of the Lower Cretaceous sandstones ranges between 3 and 45%, most commonly it is 15-25%.The permeability coefficient determined from drill core rock samples is 11-4 265 mD (geometric mean is 334 mD).
The TDS concentration in Lower Cretaceous groundwater varies from below 2 g/dm 3 in the marginal part (outcrop areas) to 20 g/dm 3 in the central parts of structural units.
Locally, the values TDS can exceed 50 g/dm 3 (Górecki (ed), 2006).Within the Lower Cretaceous section, the Barremian -Middle Albian sandstones (Mogilno Formation) make up a potential reservoir suitable for CO 2 storage.Over most of the area, the Lower Cretaceous formations are overlain by Upper Cretaceous rocks (limestone, chalk).Due to low permeability and large thickness these are a very good seal for the underlying sandstones (Marek and Pajchlowa, 1997).
In the southern and south-western parts of the basin, Lower Jurassic subcrops under the Cenozoic formation (and local outcrops) occur at the elevations of between 0 and +350 m a.s.l.Along the south-eastern and eastern margins, the elevations vary from +200 to -1 800 m.The depth of -3 600 m has been found along the NW-SE-trending axis of the aquifer in the central part of the basin.
Total thickness of the Lower Jurassic formation varies from several metres to over 100 m in the marginal parts of the aquifer, and to 400-1 200 m in the central part of the basin (Górecki (ed), 2006).
The Lower Jurassic fine-and medium-grained sandstones and sands are featured with open porosity values ranging from 15 to 33%.
The Lower Jurassic waters show higher mineralization values as compared to the Lower Cretaceous ones.The TDS content ranges from 0.2 to 174 g/dm 3 .The lowest TDS values (below 2 g/dm 3 ) are recorded from southern and eastern margins of the aquifer, i.e. from the zones of sub-Quaternary and sub-Tertiary subcrops of Lower Jurassic beds (Górecki (ed), 2006).
The best conditions for geologic CO 2 storage are displayed by the Upper Toarcian (Borucice Beds) and Lower Aalenian sandstones (Upper Sławęcin Beds) occurring below the Upper Aalenian clay-mud cap, as well as by the Upper Pliensbachian sandstones sealed by the Lower Toarcian claystone-mudstone series (Marek and Pajchlowa, 1997).

Lower Triassic
The Lower Triassic deposits are facially differentiated.The Lower Buntsandstein lithology is dominated by mud-clay facies, the Middle Buntsandstein is composed of sandstones, whereas the Upper Buntsandstein (Röt) is represented by carbonate facies with claystone and mudstone interbeds (Marek (ed), 1983;Dadlez, 1989).
The top surface of the Lower Triassic formation occurs at the elevations ranging from several metres above sea level in the south-western part to below -4 500 m a.s.l. in the central part of the basin.Total thickness of the Lower Triassic formation varies from several tens of metres to over 1 200-1 600 m (Górecki (ed), 2006).
Effective porosity is 15-30%.Permeability of these deposits is highly variable and ranges from approximately 70 to 140 mD, locally even up to 2 410 mD (Tarkowski and Uliasz-Misiak, 2005).
The TDS concentration in waters from the Lower Triassic aquifer varies from several g/dm 3 in the marginal parts of the basin to over 350 g/dm 3 .Over about 90% of the aquifer's area, TDS values are in excess of 50 g/dm 3 (Górecki (ed), 2006).
The Middle Buntsandstein sandstones (Pomerania Formation and its lithofacies equivalents) behave as reservoir rocks within the Lower Triassic series.These sandstones are sealed at the top by Rhaethian silty and clastic-carbonateevaporitic sediments (Upper Buntsandstein) (Marek and Pajchlowa, 1997).

STORAGE CAPACITY ESTIMATION METHODOLOGY
2.1 Estimation of Regional-Scale Storage Capacity in Mesozoic Aquifers of the Polish Lowlands Estimation of regional-scale volumetric storage capacity was carried out for the Lower Cretaceous, Lower Jurassic and Lower Triassic horizons.The CO 2 storage capacity was estimated using a simplified formula, taking into account the following pieces of information: average porosities, average proportion of permeable beds, average CO 2 density in reservoir conditions, and structural and thickness maps of these horizons.The mechanisms such as dissolution and mineral trapping, affecting the storage capacity, were neglected during the assessment.The volume of rocks of individual reservoirs was estimated through a superposition of structural and thickness maps.It was calculated for deposits occurring at the depth (-1 000)-(-3 000) m.The pore volume of rocks was calculated taking into account the average porosity and proportion of permeable beds in individual aquifers.
To estimate the effective capacity of CO 2 storage in aquifers, a formula created based on the methodology proposed by Bachu et al. (2007)  The calculations were based on average proportions of aquifer in the total thickness of individual reservoirs (60% for the Lower Cretaceous and Lower Jurassic and 35% for the Lower Triassic), derived from literature sources (Górecki (ed), 2006).The percentage contribution of permeable beds to the Lower Cretaceous section are generally at the level of 30-90%.In the Lower Jurassic lithological column, permeable beds account for 40 to 80% of the total thickness.In the Lower Triassic lithological column, the percentages vary from 25 to 50% (Górecki (ed), 2006).
Average porosity of the aquifers was calculated (at the confidence level of 95%) from laboratory measurements made on rock samples collected from deep boreholes.
For the Lower Cretaceous deposits, data from 90 boreholes were analysed.The porosity varied from 10 to 34%, with the average of 20.5% and standard deviation of 6.4.For the Lower Jurassic deposits, data from 140 boreholes were analysed.The porosity varied from 10 to 46%, with the average of 17.3% and standard deviation of 6.7.For the Lower Triassic deposits, data from 140 boreholes were analysed.The porosity varied from 5 to 25%, with the average of 9.7% and standard deviation of 6.9 (Uliasz-Misiak, 2008).
For individual Lower Cretaceous, Lower Jurassic and Lower Triassic aquifers, constant values of CO 2 density in reservoir conditions were assumed: 730 kg/m 3 , 770 kg/m 3 and 710 kg/m 3 , respectively.The average values of carbon dioxide density were calculated on the basis of reservoir temperatures and pressures measured in wells located in the Mesozoic aquifers.
The effectiveness of CO 2 storage estimated on a national, basin and regional scale is understood as this part of the pore volume of the aquifer, which can be used for underground CO 2 storage.This coefficient calculated for the aquifers has a few components that reflect physical barriers preventing from filling the whole pore space.The effective storage capacity coefficient corresponds to this part of the basin's area, thickness and porosity, which is feasible to be used for CO 2 storage.It also refers to this part of the area and thickness that will be directly in contact with the CO 2 (DOE, 2008).Its value ranges from 1 to 8% (May et al., 2005;IPCC, 2006).In the present study, CO 2 storage capacity was estimated for the lowest (1%), medium (2%) and highest (4%) effective storage capacity coefficients.The choice of storage effectiveness coefficients was based on literature data, where in most publications it is assumed that the smallest value of this coefficient is 1% and the maximum is ca.5-6% (van der Meer, 1995;May et al., 2005;Doughty, Pruess, 2004;IPCC, 2006).

Storage Capacity per Unit Area in Aquifers of the Polish Lowlands
It is proposed to express the potential of CO 2 storage in aquifers as a specific storage coefficient understood as the amount of CO 2 storage capacity expressed in mass units relative to the unit area.The CO 2 storage capacity per unit area is understood in a similar way as the groundwater resources coefficient is.The latter is the amount of groundwater resources expressed relative to the unit area of their occurrence.It usually refers to disposable and renewable groundwater resources and is expressed in volume units per time units and area units (km 2 ) (Dowgiałło et al. (eds), 2002).
The storage capacity per unit area enables estimating the storage potential within the area analysed.The greater its value, the more prospective is the area in terms of the occurrence of geological structures suitable for CO 2 storage.The magnitude of this coefficient is dependent mainly on the thickness of the deposits and the content of permeable rocks within them.This coefficient can be used in a preliminary estimation of the storage potential in a given region or basin, as well as to select the zones intended for further exploration in terms of storage possibility.
The CO 2 storage capacities per unit area for individual aquifers are presented in maps which can be the basis for exploration for geological structures suitable for geological CO 2 storage.The maps of CO 2 storage capacity per unit area were constructed by transformation of digital models of aquifers, as thickness and structural maps, and by taking into account constant parameters (porosity, proportion of waterbearing strata).The same constants (porosity, CO 2 density) as those taken for the calculations of storage capacity, were used for this purpose.The effective storage capacity coefficient was assumed to be 1%.CO 2 storage capacity per unit area in Mesozoic aquifers will be performed in deposits at a depth of (-1 000)-(-3 000) m, part of area of aquifers was eliminated.

Storage Capacity in Aquifers
Storage capacity in the Lower Cretaceous, Lower Jurassic and Lower Triassic reservoirs has been estimated for areas where the the top of the deposits occurs at depths greater than -1 000 m. Below this depth, CO 2 will be in a supercritical phase.Low, best and high effective CO 2 storage capacities were estimated in selected aquifers for the effective storage capacity coefficients of 1%, 2% and 4%, respectively.
The Triassic deposits occupy the greatest area (147500 km 2 ).The Lower Jurassic deposits occupy half of the area (74 000 km 2 ).The smallest area is occupied by the Lower Cretaceous deposits (24 700 km 2 ) (Tab. 1).The pore volume of water-bearing rocks depends on the area of occurrence, on the proportion of permeable rocks and on their porosity.Despite a considerable difference between the areas of occurrence of the Lower Jurassic and Lower Triassic deposits, both the horizons have similar pore volumes (3 300 km 3 and 3 200 km 3 , respectively).It is due to both a lower proportion of permeable rocks and porosity of the Triassic deposits.The pore volume of the Lower Cretaceous rocks shows the lowest values (about 500 km 3 ) despite good reservoir parameters.
The Lower Jurassic deposits exhibit the greatest CO 2 storage capacity of all the horizons.It varies from 25 700 Mt to 103 000 Mt of CO 2 .Storage capacity of the Lower Triassic deposits is lower, ranging from 22 800 Mt to 91 000 Mt of CO 2 .The smallest capacity was estimated for the Lower Cretaceous deposits -from 3 700 Mt to 15 000 Mt of CO 2 (Tab.2).
Total storage capacities of CO 2 in the Mesoszoic aquifers of Polish Lowlands are as follows: 52 200 Mt (for the lowest storage capacity), 104 500 Mt (best) and 209 000 Mt (highest).CO 2 storage capacity per unit area in the Lower Cretaceous deposits (Fig. 3).The highest values (over 0.3 t/m 2 ) are observed in the central part of the basin.Over most of the area, the values are lower, ranging between 0.1 and 0.2 t/m 2 .The most prospective area for underground CO 2 storage in the Lower Cretaceous deposits is the central part of the basin where the depth to the top of the formation is 1 000-2 000 m.This is also the area of the greatest thickness of the Lower Cretaceous succession.The eastern and partly western parts of the basin cannot be considered areas suitable for underground CO 2 storage due to the lack of Lower Cretaceous deposits.
CO 2 storage capacity per unit area in the Lower Jurassic deposits (Fig. 4).This coefficient attains the highest values in the central part of the basin, locally exceeding 1.3 t/m 2 CO 2 .Lower values (0.2-0.4 t/m 2 ) are observed over the remaining area.The zero value area in the central part of the basin is due to a small depth to the top of Jurassic deposits.
The most prospective area for CO 2 storage in the Lower Jurassic deposits is the central part of the basin where the area of maximum values corresponds to the area of the greatest thickness of the succession (over 800 m).
CO 2 storage capacity per unit area in the Lower Triassic deposits (Fig. 5).The highest values, exceeding 0.3 t/m 2 , are observed in the central part of the basin.Over the remaining area, the values vary from 0 to 0.1 t/m 2 CO 2 .
The area of the greatest storage capacity values in the Lower Triassic deposits coincides with the greatest thickness of the succession attaining 1 500 m.The most prospective area is the central part of the basin.

CONCLUSIONS
The estimation of CO 2 storage capacity in regional aquifers and the maps of CO 2 storage capacity per unit area provide information helpful in selecting geologic formations and searching for geological structures which are most suitable for geological CO 2 storage.
Of all the analysed regional aquifers of Polish Lowlands, the Lower Jurassic deposits show the greatest CO 2 storage capacity.The Lower Triassic deposits show lower capacity, and the Lower Cretaceous deposits -the lowest one.
The highest values of CO 2 storage capacity per unit area for all the analysed aquifers are observed in central Poland.The values are variable, however the highest ones are observed in the Lower Jurassic deposits (over 1.3 t/m 2 ).The Lower Cretaceous deposits show lower values (over 0.5 t/m 2 ), whereas the Lower Triassic deposits -the lowest ones (over 0.3 t/m 2 ).
The aim of the CO 2 storage capacity assessment conducted in the present research was to gain a preliminary estimation  of the CO 2 storage potential as a means of reducing its emission in Poland.Further research on estimating storage capacity should be conducted on a local scale, for selected tectonic structures located in the aquifers of Lower Cretaceous, Lower Jurassic and Lower Triassic.
Figure 2 Extents of Lower Cretaceous, Lower Jurassic and Lower Triassic deposits in the Polish Lowlands against the Mesozoic tectonic units of Poland (modified after Poz • aryski, 1974).
Figure3CO 2 storage capacity per unit area in the Lower Cretaceous deposits.
Figure5CO 2 storage capacity per unit area in the Lower Triassic deposits.
Poland distinguishes itself in Europe by its differentiated geological structure since the fragments of three large European geological units run across its territory.They are the East European Platform, the West European Variscan Platform, and the Alpine Carpathian Foldbelt.Due to complexity of the geological structure, they show variable suitability for CO 2 geological storage.The East European Platform (NE Poland) and Sudetes Mts.(SW Poland) are not adequate areas to look for suitable locations for CO 2 storage due to the occurrence of shallowseated crystalline and metamorphic rocks and small thickness of the sedimentary rock cover.The usefulness of the Carpathian Mts.(S Poland) for underground CO 2 storage is limited because Carpathian sedimentary rocks, in spite of their significant thickness, are strongly tectonized, revealing the lack of sealing sedimentary caprock.Good storage conditions can be found in the Polish Lowlands, where sedimentary rocks are featured with large thickness and good reservoir properties.
B Uliasz-Misiak / Regional-Scale CO 2 Storage Capacity Estimation in Mesozoic Aquifers of Poland

TABLE 1 The
CO 2 storage capacity per unit area in the Lower Jurassic deposits.