An Efficient Model for Evaluating Gas Field Gathering System Design
Authors: Dempsey J.R. , Patterson J.K. , Coats K.H. , Brill J.P. 
Published: December 2007

This simulation model permits accurate and efficient evaluation of gas field gathering system design. It provides simultaneous integration of three pressure drops - reservoir, gathering-system - associated with gas production. This complete simulation permits more accurate determinations production. This complete simulation permits more accurate determinations of deliverability than are possible with the standard on studies.

Introduction
It has long been recognized that gas well deliverability is a function of three pressure drops; these occur in the reservoir, in the production strings, and in the surface piping and compressor network. Actual gas well deliverability and, consequently, total field deliverability can be computed only when all three pressure drops are considered simultaneously. pressure drops are considered simultaneously. Because each of the pressure drops is associated with a different flow system, three different simulation equations are involved. To obtain meaningful results from compression studies, reservoir studies, or gas gathering system design, one must integrate these three simulation segments in such a manner that the flows and pressures balance at each node in a multiwell gathering system.
The most common approach to gathering system design does not account for interwell interference and its effect on a well's deliverability. At best, the standard approach consists of imposing one or more backpressure curves on a piping network system. So long as all the wells are being produced at constant rates, this approach does not introduce large errors. However, in general, individual well rates do fluctuate for various reasons. Many systems are produced by floating part of the wells (that is, producing at capacity) and choking others, and in the course of a performance prediction many of the wells are floating performance prediction many of the wells are floating on the system in order to meet total contract obligation. When this occurs, the calculated deliverability of each well must be updated according to the transient reservoir pressures, and the appropriate backpressure of each well must be used at all times during the prediction. One shortcoming of the older approach to prediction. One shortcoming of the older approach to design studies is that a steady-state backpressure curve fixes the drainage radius of a well, and when used over long prediction periods it can introduce large errors in the determination of compression location and timing. Further, the standard approach does not -readily permit the evaluation of infill drilling as an alternative for enhancing gas-field deliverability.
A rigorous approach to gathering system design must consider all the reservoir, piping, and compression data together. By subjecting this total-system description to a calculation procedure that integrates the various components, the influence of a modification to any one component is properly taken into account throughout the entire system. Consequently, compression alternatives, variations in line sizes and loops, infill drilling, and combinations of these can be evaluated while the effects of interference with the reservoir are being considered.

Calculation Approach
Since the flow rates and pressures must balance at each node in the system, one can choose either of these as the iterate and compute the remaining variable directly. An approach that considers flow rate as the iterate gives the best results, and the discussion below is based on the formulation.
JPT
P. 1067