Gas Field Deliverability Forecasting and Facility Scheduling

Abstract

Abstract Developing optimal production schemes for gas fields is an important functionof the oil and gas industry. The resulting information is used not only forproject optimization, but is also a basis for determining cash flows forcorporate planning and reserves base calculations. Traditionally, deliverability has been considered a reservoir problem and, as aresult, the functions of production forecasting and economic analysis have beenseparated in the generation of a deliverability forecast. Under such a system, optimized production schedules are used to generate revenue forecasts which areconsidered acceptable. This paper presents a model for determining gas field deliverability forecastswhich are economically optimized with respect to facility scheduling andproduction quantities. The model can handle the concurrent production to a gaspurchase contract from several reservoirs, each of which may contain anunlimited number of wells. Within each reservoir, it is assumed that there is auniform, but unique, pressure distribution and that wells perform according tothe performance predicted for an average well within the reservoir. Although other papers(1,2) have 'discussed this subject in somedetail, it is felt that the methods presented herein are a more rigorous anddefinitive approach to the problem, within the constraints of the aboveassumptions. Introduction The concepts of gas deliverability forecasting have been known for many years(1,3). Howe.ver, forecasting has generally been confined to optimizingproduction facilities to maintain gas purchase contract volumes for as long asthe reservoir remained capable of delivering this volume. The general implicitassumption has been that an optimized production schedule (i.e., maintenance ofa contracted volume) would result in an optimal economic return. When studiedwith optimal recovery as an objective, a project might have been abandonedbecause of apparently poor economics or, at best, revised until an economicallyacceptable alternative was found. This procedure of trial and error would berepeated a number of times for a given pool with no real feel for eachproject's long-range effect on over-all company profitability. Within the past several years, comparisons have been made between forecasts offacility requirements and deliverability that have been prepared usingconventional techniques and forecasts that were prepared using techniquesincorporating economic optimization of the facility scheduling. Thesecomparisons have indicated the advantages of incorporating economic analyseswith deliverability forecasts in order to maximize the project return oninvestment, to conserve resources not advantageous to produce at this time andto attain the realistic objective of forecasting, which is to maximizecorporate profit. In essence, the concept of economic optimization removes the obligatoryconstraint on a gas vendor to maintain contracted volumes for the maximumperiod of time without consideration of the incremental economics of past andfuture capital investment. In an economically optimized system, the contractedvolume acts as a constraint only in that it is the maximum volume that may beremoved from the system. When substantial capital investment becomes necessaryto maintain contracted volumes (i.e., facility schedule forecast), theeconomically optimal values may be less than contracted volumes.