Abstract

The producing wells and the surface pipeline network comprise a dynamic system. Flow rate changes in the individual wells affect the flow in the pipeline network and the fluid distribution among the gathering stations and, vice versa. Many times, the surface network is constructed responding to the necessity of merely connecting the wells to the separators or gathering stations without accounting for the impact that dramatically increased or decreased well flow rates may have. Nevertheless, in the life span of a reservoir, the hydrocarbon production may be substantially altered owing to several processes that may take place, such as fracturing, or gas/water injection. Thus, it is imperative that the surface pipeline network be effectively designed to ensure an unobstructed production regardless of the well flow rates. It is readily acknowledged that a computational tool is needed to perform a hydraulic analysis of the pipeline network for any set of flowing conditions.

An iterative algorithm is developed for the quantitative analysis of any surface pipeline network configuration. Pipe gas flow under isothermal conditions is studied. The algorithm uses the minimal information that is commonly available for a surface pipeline network, namely the well mass flow rates, the temperature and molecular weight for the well pipes, the chokes pressures, the gathering stations pressures and the geometric characteristics of the pipes, and does not require the knowledge of the location of the split nodes. It calculates the mass flow rate, temperature and molecular weight in each pipe of the network as well as the nodal pressures. It also derives the flow direction in each pipe identifying the location of the split nodes. The algorithm incorporates an approximation scheme used for large and complex network configurations. The scheme is validated through mass balance and pressure drop tests. The algorithm is a generic tool of network analysis as it can be modified to account for liquid or two-phase flow in the pipes. Application of the iterative algorithm indicates a notable accuracy (the absolute error in calculating mass flow rates and nodal pressures is less than 1%).

Introduction-Definitions

In most oil and gas production facilities, the flow from several wells is combined in a common pipeline network feeding one or more separators or gathering stations. When the individual flow rates are controlled by critical flow through a choke, there is little interaction among the wells. However, when the flow is sub-critical, the downstream pressure can influence the performance of the wells and the flow through the entire piping network may have to be treated as a system1. Thus, it is imperative to know the mass load and pressure in each pipe of the network to make sure the structural integrity of the pipe is not jeopardized. Also, for a surface network with several gathering stations, it is important to predict the amount of effluent each station accepts under different well feed conditions.

The producing reservoir and the surface pipeline network form a dynamic system. Changes in the well flow rates affect the flow in the pipeline network and the fluid distribution among the gathering stations and, vice versa, blockage of one or more pipe segments of the surface network may impede the flow coming out of one or more wells. Often, the surface network is constructed based on the necessity of simply connecting the wells to the separators or gathering stations without taking into account the impact of dramatically different well flow rates from the initial ones. However, in the life span of a reservoir, the hydrocarbon production may be substantially altered. Thus, the surface pipeline network should be properly designed to minimize the effect of such severe fluctuations. Diverse flow rates induce various flow regimes in the network pipes imposing different strains on the pipe metal, even modifying the normal flow path from an individual well to the network exit. Hence, it is readily acknowledged that a computational tool is needed that perform a hydraulic analysis of the pipeline network for any set of flowing conditions.

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