Full Field Modelling of a Gas Storage and Power Plant Coupling
Authors: Le Gall J.M. , Nabil R. , Sonier F. , Voigt H.D. 
Published: December 2007

Abstract
When a depleted gas field converted into a gas storage is used to feed a power plant, the gas-mixing phenomenon in the reservoir leads to gas calorific value fluctuations with time. However, since the power plant is supplying a constant power, it requires a constant "energy rate" from the storage. In order to design the storage, the modelling has to meet this particular constraint, while having a correct handling of the well to surface connections. To demonstrate the feasibility of such a project, Gaz de France has developed a new feature in its in-house multi-gas/water reservoir simulator, which already includes a reservoir to surface network coupling. In various scenarios of power plant needs, we manage to minimize the number of infill wells and to minimize the yearly injected gas quantity. Finally, the technical and economical feasibility of dedicating a gas storage to a power plant has been proven. This numerical development has been applied to the actual power plant development project at the German depleted field of Behringen. This correct and rigorous way of handling production controls allows a quick study of many design options. Therefore, the new concept of power plant gas storage coupling will be applied to several other sites.

Introduction
Most of the gas storages in operation in the world are converted depleted gas fields. Sometimes, the original gas in place differs significantly from the gas to be stored. The BTU value (the British Thermal Units value of a gas represents the quantity of energy a volumetric unit of gas can produce) of the original gas, for example, can be out of the H Gas range (the H gas range contains gases whose BTU values lie between 10.8 kWh/m3 and 12.3 kWh/m3). Therefore, when the first H Gas quantities injected into the storage are withdrawn, the BTU values discrepancy can lead to many operational problems. Large BTU value fluctuations are not tolerated by gas customers. Power plants, however, can quite easily adapt their facility in order to properly work with variable gas quality as soon as the power they produce remains constant.
The issue of gas mixing has long been addressed by gas storage operators in order to optimize the way storages are operated.
A full field modelling study presented in this paper examines the feasibility of the power plant/gas storage coupling concept. Standard gas storage performances are appraised by their working gas volume and peak rates. As a consequence, regular reservoir simulator can properly handle their modelling. On the other hand, storages feeding a power plant need to be modelled with a gas rate adjustment based on gas quality variations.
Such a gas rate/gas quality coupling cannot be handled manually. This would be approximative and needlessly time-consuming. This statement has led us to a special reservoir model development. This paper quickly sums up the already existing well-control feature of the model to clearly explain the context in which this new coupling feature has been developed. The principles of the coupling are presented.
This is applied to the Behringen field project, whose coupling with a power plant is currently envisaged. The field is already well known due to data collected during 20 years of production.
The main economical benefits of this full field study are presented : the gas rate injection through the storage life have been minimized and the number of infill wells is also optimized. The study has led us to go further in developing an energy storage on the Behringen field.

Fundamental well control
An extensive description of fundamental well control can be found in Ref. 4.
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