European Geologist Journal 51

Geotechnical challenges at Adriatic offshore gas platforms Subsidence due to exploitation at the Izabela Exploitation Field

 

by Ratko Vasiljević*

* ECOINA Ltd, SR Njemačke 10,  10 020 Zagreb, Croatia.

Contact: rvcro@yahoo.co.uk

Abstract

The “Izabela” Exploitation Field is in the central part of the northern Adriatic. It consists of two gas exploitation platforms anchored directly onto the seabed. The seabed of the exploitation field is an alluvial plain where sandy, dusty and clay sediments were deposited during the Quaternary. The dominant reservoir rocks are Pliocene and Quaternary sands and loosely bound sandstones. Gas-bearing deposits are shallowly located (between 500–1000 m depth). One of the main challenges was the question of seabed subsidence due to gas exploration and its cross-boundary influence on the Italian coastline. The current research aims to present the response to this challenge at gas field Izabela, focusing on a subsidence model of the seabed as a consequence of the pressure drop in the reservoir due to exploitation.

Cite as: Vasiljević, Ratko. (2021). Geotechnical challenges at Adriatic offshore gas platforms: Subsidence due to exploitation at the Izabela Exploitation Field. European Geologist, 51. http://doi.org/10.5281/zenodo.4954495

Introduction

The Izabela exploitation field is located within the North Adriatic exploration area. Its southern edge borders the North Adriatic exploitation field, where offshore research 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 Pliocene-Quaternary sands and sandstones.

The northern Adriatic offshore area can be considered a submarine continuation of the Po river basin on the continental shelf, where about 7,000 meters of sand and clay deposits were deposited during the Pliocene. Most of these sediments were formed by the erosion of the Alps and the Apennines. This material is brought to the northern Adriatic along the Po River and several smaller streams (Adige, Brenta and Reno). Currently, the Po River brings most of the sediment, about 20 million tonnes per year (Colantoni et al., 1979). Coarse-grained material is deposited closer to the shore, while the finer fraction is transported further. The location of the gas field is an alluvial plain where sandy, dusty clay sediments were deposited during the Quaternary mixed with terrestrial organic matter, with prevailing remains of terrestrial plants.

The gas in the gas fields is of biogenic nature, occurring in the shallow Pliocene-Quaternary sands and sandstones mixed with terrestrial organic matter, rich with cellulose and lignin. This kind of material is a precursor for kerogen type III (also known as gas prone) in late phases of diagenesis. 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 (Vasiljević, 2018).

Considering the relatively small surface of the Adriatic Basin and possible transboundary influence of the subsidence of sea bottom, it was necessary to estimate any possible negative influence on the Italian coastline. This paper presents a theoretical approach to estimation of subsidence in the environmental impact assessment from exploration and exploitation of gas at the Izabela gas field (ECOINA, 2007).

The expert committee of the competent authority approved the proposed methodology (MZOPUG, 2007) and it was used in further Environmental Impact Assessment studies for the gas exploitation fields IKA, IDA, Andreina and Ravenna (ECOINA, 2008), for new production platforms at the fields Ivana, Irina IKA (Oikon, 2011), and results of modelling were also accepted for the new development exploitation field Irena at the Izabela gas field (ECOINA, 2016).

Gas bearing properties and production forecast

The Izabela exploitation field, located within the North Adriatic exploration area, just north of the exploitation field North Adriatic. The Izabela gas field is placed approximately 60 km northwestern of Pula, 21 km north of the Ivana gas field and 40 km east of Italy (Figure 1). The depth of the sea at the location of the Izabela gas field is 38 metres. The Izabela gas field was discovered by the Izabela 1 exploration well, drilled through clays of Santerno Formation. The well was located at the southwestern edge of the structure and drilled hydrocarbon-containing layers in the Carola Formation (Figure 2).


Figure 1: Exploitation fields in the northern Adriatic (Režić, 2016).


The stratigraphic column drilled at the Izabela exploitation field consists of Middle and Upper Pliocene and Upper Pleistocene deposits. A detailed division of the stratigraphic column was made according to the zoning performed by the General Italian Oil Company AGIP (Azienda Generale Italiana Petroli), subsidary of the Italian National Hydrocarbons Authority – ENI (Ente Nazionale Idrocarburi) (Figure 2).  According to this zoning, the oldest formation in the northern Adriatic is the Santerno Formation (Middle Pliocene to Middle Pleistocene). The Santerno Formation is followed by the Garibaldi Formation (Middle Pliocene to Lower Pleistocene). Concordant on the Garibaldi Formation, there is the Carola Formation (Middle Pleistocene). The youngest member is the Ravenna Formation (Upper Pleistocene). The Santerno, Carola and Ravenna formations were developed on the Izabela exploitation field. Gas-bearing deposits in the Izabela exploitation field are in Pleistocene sands and loosely bound sandstones of the Ravenna and Carola formations at depths of 300 to 900 m.


Figure 2: Schematic representation of the lithostratigraphic units of the Pliocene-Quaternary deposits of the northern Adriatic according to AGIP (Modified after ECOINA Ltd, 2007).


Prediction of production from the Izabela field was given in the Plan of Development (EDINA, 2006). The field was planned to be developed by one satellite platform with one well. It was assumed that the well would be vertical and would be completed with 2 7/8″ tubing. The field was supposed to be put in production in 2010, but it started production in 2020 (INA, 2020). The production profile is presented in Table 1.


  Production per year 106 × Sm3/year
Year Izabela South Izabela North Total
1st 204 137 341
2nd 204 137 341
3rd 214 191 405
4th 167 150 317
5th 134 121 255
6 th 109 96 205
7 th 89 80 169
8 th 72 67 139
9 th 60 56 116
10 th 49 47 96
11 th 42 38 80
12 th 36 32 68
13 th 31 26 57
14 th 27 21 48
15 th 23 18 41
16 th 20 16 36
17 th 18 14 32
18 th 14 12 26
19 th 11 8 19
20th 10 3 13
  1534 1270 2804

During 20 years of production, 280 × 106 Sm3 of gas is expected to be produced yearly (INA d.d., 2020). Field pressure is expected to fall from the initial pressure of approximately 45.6 bar to approximately 8.5 bar. Although simulation was extended beyond the planned production time, due to the relatively fast extension of pressure drop through the aquifer, there is practically no reservoir pressure increase after production stops. This is due to still relatively high aquifer transmissibility, and since the largest subsidence is expected to occur in the last year of production when the reservoir pressure is lowest, possible pressure buildup after production does not seem important for maximum subsidence evaluation (EDINA, 2006).

Subsidence modelling

In order to simulate the possible transboundary impact by subsidence of the seabed due to the exploitation of natural gas under pressure on the Italian part of the Adriatic continental shelf, a subsidence model was developed.

To create the model, the numerical simulator Eclipse was used to simulate the pressure drop in the reservoir, with the following parameters and settings:

  • Cell size 100 × 100 m
  • Grid dimensions 60 × 90 × 31 m
  • 167,400 cells, of which 51,937 are active
  • Water pressure mode simulated by analytical aquifer. Aquifer surface is 3 times larger than the reservoir. Larger aquifers have not yet been observed in currently producing gas fields in the Croatian offshore Adriatic area so assumption of aquifer size seems justified.
  • The total (external) dimensions of the model are 6 km × 9.1 km
  • 2 platforms – 6 wells
  • Sand backfill – Skin factor = 10
  • qgmin = 10,000 m3/day
  • Depression Dp = 20 bar
  • Max WGR = 100 cm3/m3
  • Period of production: 20 years
  • Simulated period: Seventy years after start of production

A nucleus of strain, semi-analytical model was used in order to evaluate the subsidence induced by the gas production from Izabela field. The hypothesis of this theory is that the reservoir can be assumed as contained in an infinite half-space with linear porous-elastic behaviour and that the whole medium, inside and outside the reservoir itself, is characterised by two mechanical parameters, uniaxial compaction coefficient cm and Poisson’s ratio (Geertsma & van Opstal, 1973).

From the nucleus of strain concept, it follows that the subsidence at position x, y above reservoir – i.e., the displacement perpendicular to the free surface, due to nucleus of strain of small but finite volume () positioned at xn, hn, cn, under the influence of a pore pressure reduction Dpn amounts to:

Information relevant to equation (1), mainly reservoir geometry data and pressure evolution maps, have been directly obtained from a dynamic model carried out with Eclipse software. Each cell into which the reservoir in the dynamic model is discretised represents a single nucleus of strain and contributes to the surface subsidence as a function of its volume, its depletion and location with respect to the observation point.

The uniaxial compaction coefficient (cm) used in the model has been determined using published correlations (Geertsma, 1973; Geertsma & Opstal, 1973) for unconsolidated sandstones (Figure 1) and amounts to 1.41 × 10-4 bar-1. Poisson’s ratio (n) has been assumed equal to 0.3.


Figure 3: Uniaxial compaction coefficient (Cm) vs. porosity, for sandstone reservoirs (Geertsma, 1973).


As a matter of technical resolution – reliability of the modelling techniques and accuracy of geodetic measurement – we believe that a 2 cm contour from the centre of the subsidence bowl is the minimum reliable contour we can estimate.

Disposition of the observed points in the model is given in Figure 3, where the model is shown in relation to the exploitation field Izabela (the western border represents the border of the continental shelf) and in Table 2. Figure 4 shows the dynamics of subsidence of isolated points on the seabed as a function of time.


Figure 4: Size and location of the initial numerical model of the Izabela gas field within the production concession (ECOINA, 2007).


Figure 5: Disposition of observation points in the model (ECOINA, 2007).


Table 2: Coordinates of observation points in the model (shown in Figure 3).

  x Y
Observation point A 5350346 4986291
Observation point B 5353627 4985081
Observation point C 5353826 4981856
Observation point D 5352534 4983670
Observation point E 5353925 4980949
Observation point F 5355019 4978429
Observation point G 5356212 4979336
Observation point H 5352832 4982461

 


Figure 6: Seabed subsidence dynamics (ECOINA, 2007).


In Figure 4, it can be seen that the most significant subsidence is expected in the area of the middle of the deposit, in the amount of 10.4 cm, 12 years after the planned start of production. Since production started in 2020, the most significant subsidence can be expected in 2032. The reservoir is protected by ridges to the southwest and northeast that prevent the spread of subsidence in the vicinity of the reservoir, while to the northwest and southeast it is open for subsidence, which at a distance of 9 km west will be a maximum of 2 cm at the lowest reservoir pressure.

The model gave the following numerical results:

  • Maximum settlement: 10.4 cm 12 years after the start of production.
  • There is no subsidence at the edges of the model, the largest radius of the subsidence area is 10 km, and subsidence of the reservoir can in no way affect the Croatian or Italian coast.

According to Geertsma (1973) and the Dutch Institute of Applied Technologies, subsidence of less than 10 cm has practically no impact on the seabed.

Figure 5 displays the maximum subsidence of the seabed, 12 years after the start of production.


Figure 7: Simulated seabed subsidence 12 years after start of production (ECOINA, 2007).


Conclusion

The present study has the aim to estimate subsidence caused by production from the gas field Izabela. It was estimated that gas production would be 2.804 × 106 Sm3 in 20 years of production, during which reservoir pressure would decline from 45.6 bar to approximately 8.5 bar (19% of initial pressure). The grid of the numerical model incorporates the aquifer in order to evaluate propagation of the pressure disturbance and resulting subsidence.

The nucleus of strain semi-analytical model was used in order to evaluate the subsidence induced by gas production from Izabela field. Each active cell of the numerical model represents a single nucleus of strain and contributes to the surface subsidence as a function of its volume, depletion and location with respect to the observation point. The uniaxial compaction coefficient (cm) used in the model has been determined using published correlations for unconsolidated sandstones and amounts to 1.41 × 10-4 bar-1. Poisson’s ratio (n) has been assumed equal to 0.3.

Results of the subsidence modelling indicate that the largest subsidence will occur at the middle of the production period and will amount to -10.4 cm. Considering that subsidence of less than 10 cm has practically no impact on the seabed, even the largest value of subsidence is still well below any significant value. The extent of the -2 cm subsidence contour never exceeds a radius of 5.5 km from the centre of the reservoir, so it is safe to assume that gas production from the Izabela field will not have any impact on Croatian or Italian coastlines.


References

Barić, G. 2006. Naftna geokemija (Petroleum Geochemistry), INA Industrija nafte d.d. Zagreb.

Colantoni P., Gallignani P. Lenaz R. 1979. Late Pleistocene and Holocene evolution of the North Adriatic continental shelf. Marine Geology, 33. 41-50.

ECOINA 2007. Eksploatacija plina iz eksploatacijskog polja “Izabela”, Studija utjecaja na okoliš (Gas exploitation from the “Izabela” exploitation field, Environmental Impact Assessment).  ECOINA, Zagreb.

ECOINA 2008. Studija o utjecaju na okoliš eksploatacije plina iz eksploatacijskog polja ‘Sjeverni Jadran’ (Environmental impact assessment on the gas exploitation from the North Adriatic exploitation field), ECOINA, Zagreb., http://stariweb.istra-istria.hr/fileadmin/dokumenti/novosti/SUO_PUO/SUOSjeverniJadran.pdf (Consulted: April 7 2020).

ECOINA 2016. Eksploatacija prirodnog plina iz plinskog polja Irena na području Eksploatacijskog polja ugljikovodika Izabela Elaborat o utjecaju zahvata na okoliš (Screening on the Environmental Impact or the Exploitation of natural gas from the Irena gas field in the area of the Izabela exploitation field). https://mingor.gov.hr/UserDocsImages/ARHIVA%20DOKUMENATA/ARHIVA%20—%20OPUO/2017/elaborat_zastite_okolisa_744.pdf (Consulted: April 7 2020).

EDINA 2006. Gas field Izabela, Plan of Development (In ECOINA 2007.).

Geertsma, J (1973): Land Subsidence Above Compacting Oil and Gas Reservoirs, JPT, June Journal of Petroleum Technology, 25(6): 734–744.

Geertsma, J. and van Opstal, G. 1973. A Numerical Technique for Predicting Subsidence above Compacting Reservoirs, Based on the Nucleus of Strain Concept, Verhandelingen Koninklijik Nederlandsch Geologisch Mijnbowkundig Genootschap, 28, 63–78.

INA, d.d. 2020. Production on North-Adriatic field Izabela launched, July 4 2020.https://www.ina.hr/en/announcement/zapocinje-proizvodnja-sa-sjevernojadranskog-polja-izabela/ (Consulted: April 7 2021).

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This article has been published in European Geologist Journal 51 – Geotechnics – Building sustainable foundations

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