European Geologist Journal 46

Environmental issues of gas exploitation platforms in the North Adriatic offshore Croatia


by Ratko Vasiljević

Ratko Vasiljević1

ECOINA ltd, Avenija Savezne Republike Njemačke 10, 10020 Zagreb, Croatia



Intensive exploration for natural gas offshore Croatia (in the North Adriatic) started in the early 1990s, while oil exploration began in 1998. The annual production of natural gas amounts to approximately one standard billion cubic metres per year, small on a global scale, but during the last twenty years useful experience has been gathered in environmental protection issues related to exploration and exploitation. The North Adriatic is a small semi-closed basin and is sensitive to pollution. Activities of exploration and exploitation, especially drilling activities, were subordinated to environmental protection. Exploitation platforms have been found to contribute to biodiversity and even act as sea turtle protection areas. Production has declined significantly in recent years and it is necessary to plan decommission activities, whose costs could be reduced by leaving some repurposed infrastructure in place. This paper summarizes experiences in environmental protection in North Adriatic during last twenty years, and uses them as a baseline for proposals for the further management of exploitation platforms.


The Adriatic Sea is located between the Italian and Balkan Peninsula. The Adriatic is the north basin of the Mediterranean and enters deep into the Central European mainland. Exploration of the Adriatic offshore region has lasted over 40 years and natural gas exploitation started in 1998.

The gas in the gas fields is of biogenic nature, occurring in the shallow Plio Quaternary sands and sandstones of the Po Depression mixed with terrestrial organic matter. According to geochemical facies, the gas in the reservoirs of the exploitation fields in North Adriatic ​​is generated by bacterial decomposition, fermentation and reduction of carbon dioxide or acetate, under the influence of methanogenic bacteria during the Quaternary period. In bacterial-derived gas, the dominant component is methane and the content of higher hydrocarbon homologs is less than 1% and belongs to the class of dry gases (Barić, 2006). The formation of bacterial methane occurs in non-marine and marine environments after sulphate reduction has been completed.

Approximately 20 platforms are active; the maximum annual production of gas was achieved between 2007 and 2010 and it reached a total production of about 1.8 billion cubic meters of gas. Another significant production peak, significantly lower than maximum, was in 2012, reaching approximately 1.1 billion cubic meters of gas. Since then, production has continuously decreased. In a do-nothing case, gas field production should last until 2040 (ECOINA, 2013). 

Considering continuous depletion in production of hydrocarbons in Croatia, it is necessary to continue with new exploration activities. The Adriatic Sea represents a most interesting target for further oil and gas exploration. Additional reservoirs could well exist in the Northern Adriatic offshore area. Besides the main Quaternary reservoirs with biogenic gas, new targets should be deeper pre-Tertiary carbonates in the northern and southern parts of the Adriatic Sea.

Considering the fact that the Adriatic Sea is a relatively small semi-closed basin, these activities could have a significant environmental influence. This paper summarizes some of key environmental issues recognised in fifteen years of experience in estimation of environmental impact assessment from the exploration and exploitation of gas in the North Adriatic (ECOINA, 2009, 2013).

During impact assessment, some of the most relevant cases were consulted (Patin, 1979, 1998), along with calculation of suspended particles disposed into the sea and spreading of possible oil contaminants. Results were confirmed by monitoring results (PMF/IRB, 2012).

For high-quality resolution of environmental issues, a multidisciplinary approach is necessary, which includes not only geology but also biology, technology, chemistry, process engineering, and other fields.

Gas exploitation activities can be roughly divided into three phases: commission and drilling, exploitation and decommission. 

Commission and drilling include the position of the drilling rig, drilling using drilling fluids, and accompanying management of used fluid and drilling material. After drilling, the exploitation platform is installed and connected with the connection pipes for other platforms or facilities on land. 

In the phase of commission and drilling, the main environmental issues are: noise from drilling that can affect sea organisms, especially mammals; risk of sea pollution from drilling fluids that can affect sea quality and consequently ecosystems; and covering the sea bottom with drilling detritus and placement of pipelines, which can affect benthos (the community of organisms living in, on, or near the seabed).

The second phase is relatively stable, as during the exploitation of gas the only emission into the environment is the discharge of processed formation water into the environment.  Due to the prohibition of fishery around platforms and pipelines, the impact on ecosystem is positive since these installations represent shelter for living organisms, especially for endangered sea turtles.

In third phase, decommission, the influence is temporarily and limited to working activities, but given its relatively high costs it may be worth considering the possibility of leaving infrastructure in place to act as artificial reefs.

Calculation of the influence of suspended particles from drilling and exploited formation water discharged into the sea

The main targets for the environmental assessment were particles disposed of into the sea during the drilling process and unprocessed formation water during the process of exploitation. Drilling activities were planned at platforms Ravenna A; Andreina; Ida D; Ika C; Ika SW A and Ika SWB. These activities can have a wide range of influence by spreading through the sea, and some organisms such as benthos are unable to avoid them. Noise can affect sea organisms too, such as fish and mammals, but they are mobile and can avoid these zones during drilling.

Drilling was performed using water based bentonite mud with environment friendly additives. Since the reservoirs contain methane only, polluted by primarily hydrocarbons was not to be expected. Sediments are mostly contributed by the River Po, and this process continues to take place in recent times, so the drilled material is literally the same as that on the sea bottom. For this reason, the solution of direct disposal of drilled material into the sea was chosen.

Formation water is exploited in conjunction with gas, and due to its geochemical properties, dissolved hydrocarbons were not expected. However, before final disposal into the sea formation water was processed gravitationally in a caisson to extract hydrocarbons from lubricant residues from the platform installation stage.

The spread and sedimentation of suspended matter in the sea is primarily dependent on sea currents. Sea currents are created under the influence of winds, differences in pressure, temperature and different salinity. They can be horizontal and vertical. There are currents that appear at the bottom and are caused by moving the water from the warmer into the colder areas, and those that appear when the surface of the sea becomes colder and the cold water convects toward the bottom. The velocity of the current changes from area to area, but also depends on the time period. The average current velocity in the Adriatic is about 0.5 knots (0.26 m/s) but reaches up to 4 knots (2.06 m/s).

Salinity also affects the speed of particle deposition, as it increases
buoyancy. The average salinity in the Adriatic is 38.3 g/ml. In the northern part, salinity is lower than in the central and southern parts due to the influence of the River Po. Seawater density is affected by the sea temperature. The average sea temperature is 11 ºC. During the winter, the sea is coolest and the surface temperature is about 7 ºC. In the spring the surface temperature rises to 18 ºC and in the summer to about 22-25 ºC, with temperatures up to 27 ºC in the northern part.

The North Advent block is influenced by two currents. The southern part of the field is influenced by the inlet of the eastern Adriatic that transports saline water in the Adriatic. This current transfers the water in the northwest direction and changes the direction of the field to the west. The northern part of the field is dominated by the cyclonal currents of the northern Adriatic, which forms the northern Adriatic current, characterised by high density sea water.

During the development of the dynamic model of spreading and sedimentation of suspended particles in seawater for the winter period a 2D numeric model was used. Input parameters for suspended particles are:

  • Maximum amount of contributed material from one platform is 180 m3
  • Sea depth ranges between 40 to 70 m
  • Disposed material consists of solid particles insoluble in water, non-volatile and not affected by biological or chemical degradation
  • Median particle diameter is 200 µm
  • Minimum particle diameter is 50 µm
  • Average density of particles is 2500 kg/m3
  • The water temperature is 11 ºC
  • Dynamic viscosity
    {\displaystyle \mu }m08 × 10-3 Pas
  • Kinematic viscosity ν 1.05 × 10-6 m2s-1
  • Salinity (38 g/l)
  • Seawater density 1022 kg/m3


Suspended particles constantly obey Stokes formula and the mechanisms of spreading particles are advection and dispersion only. For calculation of the influence of the formation water disposed into the sea, the chosen density of hydrocarbons was 850 kg/m3. Average oil density values in standard conditions are between 800 and 900 kg/m3. All other parameters are the same as for suspended particles. 

Concentration of hydrocarbons

Concentrations of hydrocarbons in the formation water, before gravitational processing, were analysed by an ISO 17025 accredited laboratory; values ​​were between 2.04 mg/l (Ivana A) and 8.8 mg/l (Ika A).

Due to large differences in sea density and the density of hydrocarbons (the sea is about 20 % denser than hydrocarbons), immediately after the discharge from the stratum the dominant motion mechanism is in movement towards the surface. After reaching the surface, hydrocarbons move under the influence of surface currents. As the model input, the maximum measured value of 8.8 mg/l of total oils is used, before gravitational separation, in order to simulate the worst-case scenario.

Gravity separation takes place in the caisson at atmospheric pressure so that the level of water in the stratum corresponds to the sea level, according to the law of connected vessels, and for this reason, only the immersed part was taken for the volume (F).

Table 1: Characteristics of the caisson for discharge locations (ECOINA, 2013).


Depth of sea (m)

Water column in caisson (m)

Caisson diameter (m)

Outlet surface of the caisson (m2)

Volume of the submerged part of caisson (m3)

Ivana A






Ika A







For the flow calculation, the maximum values of the level of water to be modelled in the worst possible case were used. Flow parameters are given in Table 2.

Table 2: Flow parameters (ECOINA, 2013).


Maximum flow of formation water (m3/day)

Retention time in caisson (s)

Flow (m3/s)

Flow velocity at the outlet (m/s)

Ivana A





Ika A


1 693




Sea currents

For modelling, mean vector velocities of surface currents were taken as:

  • Platform Ivana A: 0.08 m/s NW 315°
  • Platform Ika A: 0.2 m/s W 270

The concentration limit of 2 mg/l was taken as the limiting condition of the model, which in most cases represents the minimum limit of reliable detection by spectroscopy according to DIN 38409: 1981 H18 used by the authorised laboratory.

For the mathematical model the Bouyant jet model was selected, which is incorporated in the software Disper 2.0. This model is developed for immersed outlets of circular cross-sections and is satisfactory for uniform flow conditions without bottom impact; thus, it is applicable to the location.

Results and discussion

For the given conditions, the following numerical simulation results for dispersion of suspended matter into the sea were obtained:

  • The plume of suspended material on each platform is pre-stretched with an approximate length of 4 km and approximate width of 0.5 km (Figure 1).
  • 50 % of the material precipitates within an area with a length of 200 m and width of 80 m.

Figure 1: Sketch of the precipitated material from the suspension deposited into the sea from the platforms Ravenna A; Andreina; Ida D; Ika C; Ika SW A and Ika SW B (ECOINA, 2009).

According to simulation results, concentrations near the surface above discharge site will be lower than the boundary of reliable detection by the spectroscopy method (DIN 38409: 1981 H 18). Thus, on the surface of the sea near the platform Ivana A (Figure 2) and near the platform Ika A (Figure 3), detectable concentrations of hydrocarbons in the sea water were not expected.

The input concentration in the model for all platforms was the maximum measured concentration of total oil on the Ika A platform before entering the caisson. The minimum water retention time in the caisson during the maximum inflow of the formation water is 770 seconds (about 13 minutes) on Ivana A, which is sufficient to extract most of the hydrocarbons gravitationally.

For modelling, a conservative approach was taken; only transport by the sea currents was included in the model, i.e. the influence of the waves – which further affect the dispersion – was not taken into account.

Figure 2: Model of dispersion of hydrocarbons from the unprocessed formation water discharged at Ivana A platform (ECOINA, 2013).

Figure 3: Model of dispersion of hydrocarbons from the unprocessed formation water discharged at Ika A platform (ECOINA, 2013).

The results of the modelling showed that at a distance of approximately 10 m from the discharge the concentration of pollutant will drop to 0.8–1.5 % of the initial concentration (dilution would be between 60 and 130 times), while at a distance of 300 m concentration will fall to 0.2–0.3 % of the initial concentration (dilution will be greater than 300 times).At a distance of 200 m, elevated total oil content may occur, mostly lower hydrocarbons, in concentrations higher than 50 μg/l.  According to research, at dozens of locations in the world, dilution is up to 200 times at a distance of 10 m from the discharge site (Somerville, 1987), a drop in concentration of 0.5% of the initial concentrations of pollutants. At a distance of 100 m, dilution is up to 1000 times (concentration drop to 0.1% of initial concentration of pollutant). At a distance of up to 200 m, the high content of total oils (almost exclusively lower hydrocarbons) may occur briefly in concentrations above 50 μg/l (Somerville et al., 1987).

As the model input, the maximum measured value of 8.8 mg/l of total oils is used before gravitational separation. The limit values for disposal ​​prescribed by the Barcelona Convention are 40 mg/l, so even without the full functionality of the water treatment equipment the oils fully meet the regulatory requirements. Apart from the Barcelona Convention, hydrocarbon concentrations also meet the Marpol convention (although it does not apply to this issue, but only to drainage water from the machinery spaces). 

The results of the modelling give a slightly lower dilution than Somerville et al. (1987), but as mentioned earlier, the model did not take into account the influence of waves that additionally affect dispersion, i.e. the model represents a conservative estimation.

Mineral oils (PAHs) at concentrations up to 1 mg/l may stimulate or inhibit growth in phytoplankton at higher PAHs concentrations (up to 100 mg/l) (Abbriano et al., 2011). The modelling results have shown that even near the discharge mineral oil concentrations will be below the limit of concentration that could adversely affect phytoplankton. Like phytoplankton, many zooplankton species are susceptible to increased mineral oil concentrations; zooplankton mortality increases with longer exposure to higher concentration of mineral oils (Abbriano et al., 2011). Increased concentrations of mineral oils mainly have a stimulating effect on increasing the number of bacterial plankton. However, due to the low concentration, no increase in abundance is expected, which means no major effect on other members of the microbial feed network.

Potential negative impacts of hydrocarbon exploitation on marine fauna have not been established with the results of research up to now (PMF/IRB, 2012). Namely, based on the results of this long-term monitoring, which covered the review and analysis of the processing of the platform, the toxicity/genotoxicity analysis of the seawater near the platform of Ivana A did not identify an ecotoxicological effect that would represent risk exposure to the organic pollutants, nor did it reveal the presence of pre-mutagenic and/or mutagenic xenobiotics (PMF/IRB, ­2012).

Issues with decommissioning

A problem that has emerged, not only in the North Adriatic but also in the rest of the world, is the issue of decommissioning.

The International Maritime Organization (IMO) recommends removing the platform when the sea depth does not exceed 100 m and, theoretically, it is quite simple.

  1. Cleaning of all armature and gas pipeline from residual gas by inert gas (Nitrogen).
  2. Insulation of residual gas and formation water in reservoir by plugging the boreholes. All productive levels in boreholes will be sealed off through cement plugs. The cement plugs shall be preferably set through tubing, using coiled tubing techniques. All casing strings will be cut at the appropriate depth at the bottom of the sea and plugged by cement cap.
  3. The pipes from the sea bottom will be cut off the platform and removed.
  4. Platforms will be removed by mechanical decomposition (after cutting off all platform equipment) and transported to the land.
  5. The construction material of the platforms and pipeline is steel and can be reused as a secondary raw material.

But what about expenses? In spite of the fact that these expenses are calculated, in practice it is never enough. According to information on the website of the Society of Petroleum Engineers, “Global spending on oil and gas decommissioning is expected to be $13 billion per year by 2040.” These costs are sure to grow: In the North Sea, the companies said that in the North Sea alone more than 400 fields are expected to cease production by 2026 at an estimated cost of $56 billion. Globally, more than 700 fields are expected to require decommissioning” (SPE website). These statements are general and each area has its own economy, but it is undeniable that the decommissioning prices are high. In this paper I would like to offer an alternative for solving this problem.

In the North Adriatic area of ​​exploitation fields of hydrocarbons the depth of the sea is between 35 and 70 ms, i.e. according to bathymetry it belongs to the epipelagic zone.
The area of ​​the project belongs to the open sea of ​​the northern Adriatic and the entire water column represents a habitat suitable for tiny blue fish (Figure 4). Direct observation also showed the presence of larger pelagic organisms: tuna, dolphins, marine turtles (Figure 5).

Figure 4: Photograph of marine life around platform (Oikon, 2011).

Figure 5: Marine turtle observed around platform (Oikon, 2011).

The research conducted on platform Ivana A (PMF/IRB, 2012) included diving activities by the legs of platforms. According to visual detection, 27 species of fish were recorded in the vicinity of the platform. Some of fish species found shelter and food while others came in search of food, and the platforms were incorporated into the environment as artificial reefs, similar to the natural ones that are common along the Croatian part of the Adriatic coast. Due to the risk of strong currents in the North Adriatic, diving activities were allowed by the platform legs only; HSE procedures strictly prohibit any activities in the water outside the platform itself. The research concludes that the platform actually has a positive impact due to the fact that it increases the biodiversity at the site.

This area is an important habitat for sea turtles (Lazar et al., 2004), one of the most common random catches in various types of fishing gear in all of the world’s seas. The incidental catches in the north-eastern Adriatic Sea amount to 2,135–4,334 catches per year. Considering the fact that fishery is forbidden around the platforms and pipelines, these areas represent good shelter for sea turtles. After exploitation is completed, some parts of the infrastructure (pipelines, platforms) will probably be removed. The dynamics of removal will be set by project documentation for this phase. But in case of high expenses it is recommended to leave some parts of the infrastructure to act as a shelter for living organisms. This role was confirmed during the study of the seabed near the gas pipeline, when the presence of sea turtles was recorded. If the installations are allowed to remain they can contribute in a positive way as biodiversity spots and shelters for endangered sea organisms.  

Issues with the processing of formation water

For the two platforms studied, formation water was processed. However, the logical question that emerges is why apply formation water processing if it already satisfies the requirements of the Barcelona Convention?

The first answer is easy – since the Adriatic Basin is semi-closed, it is necessary to minimise all the risks that can appear, especially if you can apply low-cost technology. 

The second answer requires some experience with stakeholders that consider these projects as something that can “endanger the environment”. Of course, any project represents some kind of risk to the environment, and in spite of monitoring results, it would be irresponsible to claim there is no risk. Only approaches that ensure the minimisation of environmental risks and transparent presentation toward stakeholders in the public, fishery, mariculture, tourism, etc., can ensure trust and enable the smoother performance of similar projects in the future.


The Adriatic Sea is a semi-closed basin, and although exploitation activities have not been reported to have a significant environmental impact, it is necessary to confirm this through research. Favourable circumstances are the facts that the reservoir is shallow and gas is biogenic without higher hydrocarbons. This has enabled the application of relatively low-cost technologies: water-based drilling fluid and gravity separation for processing of formation water.

Decommission activities are an important part of each exploitation activity. But the real question is whether companies are generally ready to perform it. Some infrastructure will be probably removed, even reused as secondary raw material. But what about costs – is it possible to decrease them? A possible answer offered in this paper is leaving parts of the infrastructure in place, primarily exploitation platforms, repurposing them as artificial reefs that can have a positive impact on biodiversity. Of course, before this kind of decision, appropriate survey activities should be done for each location.

The risk of releasing unprocessed formation water appears to be low as the concentration of hydrocarbons is low, processing is carried out to minimise such risks and ensure that the trust of stakeholders is deserved. The numerical simulation and laboratory testing investigated confirmed that exploitation of biogenic gas from Plio-Quaternary sediments in the North Adriatic offshore is an environmentally low-risk activity. 

Oil and gas production in Croatia is in continuous decline, and in order to maintain a certain level of production, it is necessary to perform new exploration activities. New targets – deeper formations – require different technology than that used presently, which may represent a bigger environmental risk. So this project is a good representation that with good planning of exploration and exploitation activities along with the application of know-how and a multidisciplinary approach, environmental risks can be minimised.


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This article has been published in European Geologist Journal 46 – Oil and gas exploration in Europe: innovation and new technologies

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