Environmental safety in the implementation of carbon dioxide geological storage technologies in the Donbass

. The Donbass has the largest potential in Europe for the geological storage of carbon dioxide (CO 2 ), which needs to be implemented on a large scale to mitigate the effects of global climate change. The environmental risks of CO 2 leaks in the processes of capturing, transporting and geological storage of CO 2 at the enterprises of the energy and industrial sectors of the economy of the eastern regions of Ukraine are analyzed. Geographic information systems have been created in these areas with layers of geological structures suitable for long-term storage of supercritical CO 2 . The impact of CO 2 leaks from geological repositories on the environment is estimated. In the proposed CO 2 storage areas, some CO 2 leakage scenarios were analyzed due to the filtering of CO 2 fluids through porous rock layers, through abandoned wells and tectonic faults of the Donbas geological structures. The potential effects of CO 2 leakage on groundwater quality in the region are also assessed.

The introduction of CCS technologies in the energy and other industries will temporarily mitigate the effects of climate change before developing climate-friendly energy sources. But when using technologies CCS, there are risks of CO2 leakage, which can have an adverse effect on humans and the environment.
The territory of Donbass has the most significant potential [5] in Europe ( Fig. 1) for geological storage of CO2 in order to mitigate the effects of climate change [6]. This potential for CO2 accumulation is estimated at 45.7 to 428.3 billion tonnes, which causes interest in the possibility of using it for the needs of the whole of Europe, but it should take into account the great environmental risks of geological storage of CO2 for the territory of Donbass.

Fig. 1.
Potential areas for CO2 accumulation in Europe [5] Geological structures of Donbass [7] have been identified previously, suitable for longterm storage of supercritical CO2, which is transported through the existing gas transmission system with some upgrades, and then pumped into the Paleozoic sediments of Donbass [8] in several promising areas (Fig. 2) that are not contain territories of large settlements, operating coal mines and areas of tectonic disturbances in the form of salt rods.  Fig. 2 the following territories are identified, which are determined on the basis of studies of archival documentation on the geological structure of Donbass [8]: 1 -Carboniferous sediment zone; 2 -zones of Permian salt deposits; 3 -Devon salt stocks; 4 -eight promising areas for the geological storage of CO2 (N1-N8). The purpose of this study is to determine the likely scenarios for CO2 leakage from prospective geological storage sites and assess the risk of CO2 leakage through abandoned wells and tectonic faults, as well as to assess the impact of CO2 leaks on the groundwater quality of the region using the example of the largest geological site for CO2 storage in the north Donbass.
Possible CO2 leaks in the processes of its capture and transportation will be of an emergency nature, therefore they should be easily identified and repaired. CO2 leaks from underground storage are difficult to predict and difficult to detect. Therefore, the territories in which promising areas of geological storage of CO2 will be located have risks of a negative impact on humans and the environment.
In the process of CO2 injection and storage, three scenarios of CO2 leaks from its geological storage sites can be implemented mainly [3]: -leakage due to insufficient tightness of the rock-tire; -leakage of abandoned and existing wells, trunks, drifts; -leakage through existing faults and cracks in mountain formations.
Other scenarios of CO2 leaks are possible in the form of combinations of these three main scenarios, and the negative impact on humans and the environment will only increase.  Fig. 3 shows a simplified diagram of these three scenarios of CO2 leaks from geological repositories, where the following details are indicated: 1 -the surface layer of soil with a thickness of about 50-100 m; 2 -groundwater; 3 -various layers of geological structures, including a high-density tire formation, located deeper than 800 m; 4 -aquifers or sedimentary rocks that are able to accumulate supercritical CO2; 5 -rocks of deeper geological layers, located deeper than CO2 storage areas; 6 -well for injection of supercritical CO2 into its storage area; 7 -zone of distribution and storage of fluid CO2 under the impermeable rock-cap. Positions 8, 9 and 10 in Fig. 3 correspond to the implementation of the first scenario of CO2 leakage due to the lack of tightness of the rock-tire. Estimates of CO2 leakage volumes in this scenario are carried out by numerical simulation of thermo-hydrodynamic and mechanical-chemical processes [9], taking into account the real parameters and composition of rocks located above the CO2 storage area. These processes (8) proceed very slowly (it may take 100 or 1000 years until the product of the processes reaches the groundwater or surface), but it is extremely dangerous for humans and the environment (supercritical CO2, both liquid and two-phase, is a very effective solvent of rocks and products of such chemical reactions may be toxic). And getting into the groundwater (9) such toxic products can significantly impair their quality. Then these products and gaseous CO2 reach the surface (10) and can adversely affect humans and the environment.
The second scenario of CO2 leakage can be realized in the area of developed and existing oil and gas fields. There are many such deposits in the Donbass, some of which continue to be exploited [10], and some are conserved or abandoned, although hydrocarbons still remain there. In order to increase oil and gas recovery from seams, you can use CO2 injections as part of projects on the implementation of CCS technologies in the Donbass.
Positions 11 and 12 in Fig. 3 correspond to this scenario of CO2 leakage from geological storages, while in the process of raising CO2 to the surface through abandoned and operating wells (11), it can be dissolved in groundwater (12), which will affect their quality.
Since the abandoned and operating wells are located near or on the territory of oil and gas fields, it is possible to estimate the risks of CO2 leakage in this way by comparing the areas of fields and areas of prospective geological storage of CO2. The estimate of the probability of CO2 leakage through the wells of existing fields is defined in Table 1, where areas of promising areas of CO2 geological storage and areas of oil and gas deposits are taken into account, it is considered that there are a large number of abandoned and operating wells in the fields, and the shafts and drifts of coal mines were previously excluded from areas of promising areas of CO2 geological storage. Positions 13, 14 and 15 in Fig. 3 conditionally reflect the third scenario of CO2 leakage through existing faults and cracks in mountain formations (13), where gaseous CO2 rises along tectonic structures of a given area to groundwater (14), affecting their quality, and going to the surface (15), affecting vegetation, animals and humans. To assess the likelihood of the implementation of the third scenario of CO2 leakage from promising areas of CO2 geological storage in Donbass, you can combine the location of deep and regional faults [10] with geological sites (Fig. 5), which show the following deep geological faults: 1 -South-Kharkiv fault; 2 -Krasnoretsky fault; 3 -Krivoy-Rog-Pavlovsky fault; 4 -Yurievsky fault and 5 -Golubovsky-Mikhailovsky fault, as well as other reliable and suspected regional faults in promising areas of CO2 geological storage. If we assume that the zone of CO2 leakage through geological faults is on average 1 km wide, then an estimate of the probability of such a CO2 leakage can be presented in Table 2. The probability of leakage of CO2, % 7,0 10,0 4,9 0,3 7,9 7,5 0,3 1,5 As an example, assessing the impact of CO2 leaks on groundwater quality is compatible (Fig. 6) the contour of the promising area of geological storage of CO2 N8 (1) with a hydrogeological map of Ukraine (first from the surface of aquifers) [11] showing aquifers: in the Eocene, Oligocene and Miocene (2); in the fracture zone of the Marl-Cretaceous stratum of the Upper Cretaceous (3); in the upper and modern estuary sediments (4); in the deposits of the Jurassic (5); in Triassic sediments. Also in Fig. 6 shows the bed of the Seversky Donets River (7) and its tributaries (8). At the same time, 46.7% of the horizon 2, 43.0% of horizon 3, and 40.9% of horizon 4 fall within the territory of section N8, which indicates a significant impact on all aquifers of possible CO2 leaks in section N8. Fig. 6. The first aquifers from the surface in the territory of a promising area of storage of CO2 N8 Similar estimates for the first aquifers in the territory of other promising areas of the geological storage of CO2 (N1-N7) can be made on the basis of data from this map [11].
A study of the scenarios and risks of CO2 leakage during its geological storage in the Donbass area will allow for measures to be taken to comply with environmental safety measures when implementing carbon dioxide capture and storage technologies to mitigate the effects of global climate change.