ABSTRACT

A typical approach taken to modeling gas field production gathering networks is to input "fixed" flow rates at a particular point in time to size pipelines and/or identify restrictions to flow. A numerical reservoir simulation model is often used to forecast the production rates for each well in the field. The flowing conditions in the surface gathering network can vary significantly over the life of reservoir due to the field development strategy being pursued, pressure depletion of the reservoir, and the volumes of the gas and liquid that are produced. Until recently, the "best" means available to account for changes in flow rate potential or pressure losses in the reservoir and surface gathering network has either been to use a "simplistic" description of the reservoir inflow performance coupled to a "simplistic" surface network or, to run the reservoir model and pipeline model separately, manually exchanging pressures and flow rates data in a "trial-and-error" fashion until the models converge. This paper summarizes the recent "movement" by reservoir simulation software vendors to allow their models to couple to 3rd party pipeline and reservoir simulation models. The Sexsmith Gas Condensate Field located in Alberta, Canada is used to illustrate how these "integrated" models can more closely approximate the production characteristics of the reservoir and act as a focal point for planning and management of the surface gathering network and the transportation pipelines located downstream. The paper also offers an opinion of where further effort should be focused to make these modeling tools more useful to the end user.

INTRODUCTION

The design of a multiphase flow gathering network requires an estimate of flow potential from each well. Rarely is the aerial extent of the reservoir so well defined at the outset that all potential well locations can be identified. The flow potential of the fluid phases produced up the wellbore into the gathering network vary between wells depending upon the characteristics of the reservoir, and pressure depletion or "drive" mechanism of the reservoir. Accordingly, the efficient transport of reservoir fluids through the gathering network is usually difficult to predict over the producing life of the wells. The forecast of production rates from wells using a numerical reservoir simulation model is likewise dependent on backpressures caused by pressure losses in the wellbore and surface gathering network. The historical approach taken by the reservoir engineer has been to prepare flowing bottomhole pressure versus flow rate (VFP) "look-up" tables to approximate the backpressure caused by the wellbore and surface network. Each table is unique with respect to tubular dimensions, lengths, etc. When dealing with multiphase flow many permutations (e.g. different water-gas ratios, condensate-gas ratios) are often required to construct a VFP table over potential range of flowing conditions. Such an approach is suitable for single well, single flow line gathering networks however, such configurations are not typical of most pipeline networks.

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