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The project for UGS Chiren expansion (PCI 6.20.2) envisages capacity increase in stages of the only gas storage on the territory of Bulgaria in order to achieve larger gas volumes stored, increased gas reservoir pressure and higher daily average withdrawal and injection rates. The project provides for increase of the working gas volume up to 1 bcm and increase of the injection and withdrawal rates up to 8-10 mcm/d.

UGS Chiren has been the only gas storage on the territory of Bulgaria for 40 years. It is a key instrument for the functioning of the gas market in Bulgaria, covering seasonal fluctuations in natural gas consumption in the country by securing the necessary flexibility caused by the differences between the supplies and consumption, and ensure emergency reserve. UGS Chiren is a crucial instrument guaranteeing the security of gas supplies.

The project for its expansion aims on one hand at creating conditions to guarantee the security of supplies to Bulgarian users and the users in the countries from the region, and on the other - UGS Chiren development as commercial gas storage in an interconnected regional and Europe-wide market, as UGS Chiren is an integral part of the regional gas system, consisting of interconnections, LNG terminals, storage facilities.

The planned development of gas projects in the region is a prerequisite for market development, diversification and enhanced market integration, and is directly related to expansion of the only gas storage in Bulgaria. In the medium term, Chiren UGS is emerging as a commercial storage facility, playing an important role in developing competition on the regional gas market and securing additional flexibility of gas transmission networks at a regional level, with considerable contribution to congestion management and seasonal optimization of the use of gas transmission systems. 


  • In 2009 the elaboration of Technological design for UGS Chiren operation was assigned. According to the design, completed in 2011, there are possibilities to expand the capacity. The Technological design recommends carrying out additional geological and geophysical studies aimed at specifying the parameters of expansion.

  • On 19.03.2015 following the launched and completed procedure on selection of a contractor, a contract was signed and Chiren Reservoir geomechanical simulation was successfully completed.  The study is one of the principal geological and geophysical studies, required to be conducted before engaging in the actual storage expansion works. The study was completed in November, 2015. The main conclusion of the report is that currently according to the information available on the geological structure, Chiren UGS is reliably tight and fit for increase in the formation pressure up to 150 bar.

  • On 2.09.2015 following launched and conducted procedure on selection of a Contractor, a contract was signed for conduct of a study of subject-matter „Surface gas analysis on Chiren structure area”.   The surface gas analysis is one of the main methods of verifying the tightness of the underground gas storages, whose purpose is to check for the presence of gas accumulations close to the surface within the borders of spread of the gas reservoir and to determine the surface zones of hydrocarbon anomalies, if any. The study was completed in line with the concluded contract. The results of the analyses show that, at the present stage of geochemical survey, no hydrocarbons have been found within Chiren gas storage, other than of biogenic and surface origin.

  • In 2015 Bulgartransgaz EAD prepared a project proposal Conduct of 3D field seismic surveys as part of the project for UGS expansion 6.20.2, proposed to be financed under the Connecting Europe Facility – CEF - Energy-2015-1, at a total estimated value of EUR 7 800 000 and a grant amount of 50% of the project value - EUR 3 900 000.

  • On 23.10.2015 Bulgartransgaz EAD signed officially Grant agreement № INEA/CEF/ENER/M2015/1029442 for Conduct of 3D field seismic surveys in the area of Chiren structure, part of project 6.20.2 UGS Chiren expansion, covering implementation of the activities: Activity 1: Conduct of 3D field seismic surveys in the area of Chiren structure and Activity 2: Quality control in conduct of 3D field seismic surveys and processing of data acquired.

    The aim of 3D studies is maximum structural and lithological featuring of the gas reservoir and obtaining information about the occurrence of open fracture systems for localization of areas for drilling new high-performance exploitation wells, necessary to increase the maximum flow rate of Chiren UGS, as well as of observation wells controlling the processes of gas migration in the peripheries of the geological structure.
  • On 16.11.2015 a procedure for selection of a contractor was announced named “Quality control in conducting 3D field seismic surveys on the area of Chiren structure and during subsequent processing of the data acquired from seismic surveys”, which resulted in selection of a contractor of the specified activity. On 14.07.2016 a contract was signed with the selected contractor - MinGeoUniverse - NIS - MGU EOOD (Bulgaria).

  • In the period September 2015 - September 2017, two public procurements were conducted (due to appeal of the first one) with subject “Conduct of 3D field seismic surveys on the area of ​​Chiren structure”. The second procedure resulted in selection of a contractor and on 25.05.2018 a contract was concluded with Geofizyka Toruń S.A. (Poland).

  •  In the period 2018 - 2019, activities were carried out to ensure access rights to the properties (activity that is not part of the funded Action № 6.20.2-0021-BG-SM-15), located on the territory of the studies (a total area of the studies - 198 sq. km or nearly 23 000 separate properties). The activity of securing consent from the owners and users of properties was being executed actively throughout 2019.

  • In the period January 2020 - August 2020, 3D seismic field surveys were carried out, which, in addition to obtaining 3D seismic data, also included execution of borehole seismic (VSP) in two wells.

  • Since September 2020, work continued with processing of the acquired data and will be completed with seismic data analysis and interpretation.



  • The results from the completed set of studies/preparatory activities will serve as a base in determining the best option for Chiren UGS expansion and for the next steps related to PCI implementation, namely, design and construction of ground and underground facilities.



  • At the beginning of 2014 a Preliminary market test was carried out aimed at informing all interested parties of Bulgartransgaz EAD intentions to expand the storage capacity and the same were invited to state their intentions to book long-term capacity. The work on preparation of various types of documents to organize the next stage of the study is ongoing.

  • On 26.06.2014 a notification about the project for carrying out 3D field seismic surveys in the area of Chiren structure was submitted to RIEW Vratsa. Subsequently, the case has been transferred to the MEW, as two protected areas fall within the boundaries of the area of the studies of UGS Chiren. "Bozhiya most-Ponora" code BG0000594 and PA "Bozhite Mostove" BG0000487. After investigation by the MEW, the competent authority issued Opinion No НСЗП-273 of August 21, 2014, stating that no EIA is required and the project can be implemented in compliance with the measures set out in the Opinion.

  • On 07.01.2015 Notification Ref. No НСЗП-4/07.01.2015 about the project for carrying out ground gas analysis on the Chiren structure area was submitted to MEW. After investigation by the MEW, the competent authority issued Opinion No НСЗП-4/23.01.2015 stating that no EIA is required and the project can be implemented in compliance with the measures set out in the Opinion.

  • On 03.02.2021 on Bulgartransgaz EAD website was published a notice of investment intention Design and construction of new above-ground facilities - a compressor station with all its auxiliary technical facilities to ensure reliable and continuous operation in gas injection and withdrawal mode, as well as a new gas metering station (GMS), with regard to expansion of the capacity of Chiren underground gas storage (UGS) and their connection with the existing ones. The investment proposal has been notified to the MoEW and is being considered.



Bulgartransgaz EAD

1336 Sofia, post box 3, residential area Lyulin 2

66 Pancho Vladigerov Blvd.

Head office: (02) 939 63 00

Fax: (02) 925 00 63

e-mail: info@bulgartransgaz.bg