Modeling of reservoir to surface network coupling : a case study of gas storage management
Authors: Khodri D. , Lehuen P. , Labaune B. , Nabil R. 
Published: December 2007

Abstract
The French aquifer gas storage of Germigny-Sous-Coulombs is located in a very extended reservoir within a relatively low closure anticline. Consequently, early in the withdrawal period, gas rate is reduced due to water encroachment. Moreover, the specificity of the gathering system (two large gathering lines to the terminal, lateral lines, well platforms) makes the withdrawal strategy difficult to manage. Accurate performance forecasts through numerical simulation require to describe precisely both the effective well productivity and the gathering system geometry.
This paper describes an integrated approach to determine the optimum management plan for the gas storage. History matching of the reservoir pressure and saturation is performed with a full field in-house reservoir simulator. The geometry and the physics from the bottomholes to the wellheads and the surface network are incorporated into the reservoir model, as well as relevant operation constraints. The pressure drop on each segment of the network is accurately calculated and well productivities are matched using flow parameters such as turbulence effects between reservoir and well, and rugosity in pipe. Well performance predictions obtained with this coupled approach are compared to the actual behavior of the gas storage. This approach has proven to be efficient in defining a strategy to improve Germigny-sous-Coulombs gas storage performance : first maximizing the working gas volume is successfully achieved in two weeks instead of 2 or 3 months necessary in similar forecasts made previously without the reservoir to surface network coupling. Second, an operational strategy for reducing water production is proposed. As a consequence, a higher reliability is expected for the prediction of the best locations for additional wells with high productivity potential. Finally it has been possible to optimize the additional surface facilities needed for the storage development (pipes, platforms, compressors) thanks to the coupling between reservoir and surface facilities.

Introduction
To develop the best strategy for injecting and withdrawing gas from an aquifer storage, we need precise details on gas production mechanisms and reservoir characteristics such as volume, lithology, rock properties and fluid properties. Reservoir description must be as detailed as possible, to determine operational improvements and provide decision-making assistance during the storage development and production phases. The modeling performed was used to compare different development and operation forecasts at Germigny-sous-Coulombs storage facility, to determine various performance levels, and to optimize the choice of additional equipment.

Germigny-sous-Coulombs storage facility
Germigny-sous-Coulombs is GDF's ninth aquifer storage (Fig. 1). During the geological exploration phase, drilling showed the existence of a closed anticline, a good reservoir in the Wealden formation (lower Cretaceous), a caprock, and a control aquifer in the Barremian formation [Ref. 1]. The Wealden reservoir is interbedded with fine shales and capped by an impermeable layer of clays and marls. The reservoir rock is located at 900 meters below ground level. It is 80 meters thick and has a mean porosity of 18%. Closure is estimated at 35 meters and encloses a surface area of about 100 square kilometers. Interference tests show a transmissivity of 37 Darcy-meters for the storage aquifer and 10 Darcy-meters for the Barremian control aquifer. The reservoir has no faults liable to alter its hydrodynamic properties. The multilevel surface network includes 26 storage well heads, an 12 platforms, connected to the terminal station (Fig. 2). Because of low vertical permeability through the rock layers, Gaz de France has equipped the storage well heads with completions which allow selective injection in the various reservoir layers (Fig. 3). In 1982, inert gas was injected on well CR16 (Fig. 1 & Fig. 9). P. 203^