Converting the Pecorade oil field into an underground gas storage
Authors: Coffin Philippe , Lebas Genevieve 
Published: February 2016

Abstract
The need for additionnal Underground Gas Storage (UGS) in Europe and in France is increasing. TOTAL has therefore undertaken feasibility studies to convert the Pécorade depleted oil field, situated in South West France, into an UGS.
The Pécorade field offers some good characteristics to become an UGS, but is also deep (2500 m) and countains hydrogen sulphide.This paper describes the different challenges faced by the project:the sizing of the working volume (volume of gas which can be stored and cycled each year), which required the acquisition and processing of a new 3D seismic program, and the construction of specific geological and reservoir modelsthe safety and environmental issues such as caprock integrity and sour gas productionthe conversion of existing oil&gas well producers into gas injectors and producersthe processing of cycled gas (including sour gas treatment)the cost of the project compared to more conventional UGSs.
The main benefits expected from the project are:the development of a sizeable working volume (in the order of one billion cubic meter) which would secure additional gas storage for the French and/or Spanish marketan improved oil recovery (the annual cycling of the gas would allow for 20% to 30% of additional oil to be produced).
These studies also confirmed that the development of a sizeable UGS is long-term, difficult and complex project.
The decision to launch the project is mainly dependent on the results of the future pre-project studies, regulations and market conditions.

Introduction
While Europe's gas consumption is increasing, domestic gas production is declining, and gas imports are on the rise. Therefore Europe is eager to secure additional gas imports, cater for seasonal gas demand and prevent supply shortages. One of the means to achieve these targets is the development of new Underground Gas Storage (UGS), and it is estimated that Europe UGS capacity must at least double in the next 25 years (1).
France has not been endowed with a proliferation of oil and gas bearing reservoirs and as a result the vast majority of existing UGS belong to the aquifer type. However, the development of this type of UGS is now facing more stringent environmental regulations; therefore TOTAL has undertaken studies to convert one existing depleted oil field into an UGS.
This paper describes the feasibility studies performed and the particular challenges overcome to evaluate the project.
The Pécorade field is situated in South West France, 150 km from Bordeaux, and has produced oil since 1978 (Fig. 1).
The field was selected as a possible UGS after a screening study of the oil and gas fields situated in the area. Pécorade presents the following advantages:it is situated nearby an existing gas networkit has a proven gas capacity as it contained an initial gas-capit is depleted, with reservoir pressure having declined from an initial pressure of 26.5 MPa to today's 10 MPa,it has a proven cap rock integrity.
However the reservoir is deep by UGS standards (2500 m) and contains hydrogen sulphide; these are major disadvantages and drive the development cost of UGS up. Therefore a feasibility study was launched and completed to assess the possibility of economically converting this oil field into an UGS. The study had to face several challenges.

Size Up the Working Volume
In the year 2000, when it was decided to launch the feasibility study, the Pécorade field had only been covered by 2D seismic lines acquired in 1970's and 1980's. The poor quality of these data did not allow for an accurate evaluation of the structural volume as well as good management of integrity questions.