Gas Storage Field Development Optimization
Authors: Duane John W. 
Published: December 2007

Optimization of gas storage field development requires that many combinations of flow system variables, together with their related costs, be examined in a systematic fashion until the minimum development cost method is obtained for any potential field deliverability requirement. Graphical methods of optimizing gas storage field development are presented. It is also shown graphically that, for any given peak-day rate design, there exists a definite upper feasible limit to the number of wells.

Introduction

The development of a field for gas storage usage involves considerations of multiple factors, many of which are mutually dependent. Some of the main variables which must be considered in storage field development are

potential cyclic capacity and peak-day rate market requirements, number of wells, compressor-inlet line pressure (and related compression), overpressuring, average well open flow (and back-pressure slope), casing sizes and gathering system design.

Each of these variables affects the field deliverability performance and each also affects the total project development cost. The term "storage field design requirement" is defined (for the purposes of this article) as a given combination of peak-day rate and cyclic capacity. The peak-day rate equals the daily gas rate which will be required to be produced from this field under the most austere condition of system demand (usually maximum required rate imposed late in the withdrawal season). The cyclic capacity equals the amount of gas that can be sold at which time the peak-day rate can just be met. The storage field design requirement is, thus, the maximum output requirement to be placed upon the field. The term "field design" will be used here. Optimization of storage field design involves obtaining the optimum method of total field development and determining the economic limits of field development. The optimum method of field development is defined here as the method (or methods) which results in the maximum desired field usage (i.e., the maximum desired peak-day rate and maximum desired cyclic capacity) obtained with the minimum total cost of development. The economic limits of field development are defined by the potential range of field operations (i.e., peak-day rate and/or cyclic capacity) which clearly are not economically feasible due to high cost of development, and/or those ranges of operation which are not attainable, regardless of cost.

Optimization of storage field design requires that many combinations of the previously mentioned flow system variables, together with their related costs, be studied and examined in a systematic fashion until the minimum development cost method is obtained for any potential field design condition.

Field Design Optimization Graphs

As an aid in determining the optimum storage field design, a new graphical technique has been devised. This analytical tool has been called a field design optimization graph. The optimization graphs enable one to rapidly scan the entire spectrum of potential field operations and read total development costs (wells, compression and gathering system, if included) directly for any point in the design field (i.e., corresponding to any peak-day rate and cyclic capacity combination). The optimization graphs enable one to study storage field design analytically and on a more quantitative basis. They help to provide an insight into the effect of key variables in field design (such as the number of wells, line pressure, open flow, overpressuring and casing sizes) on the ultimate field performance and on the corresponding total development costs. The main type of field design optimization graph used to optimize field development consists of a graph showing the peak-day rate effect on field design and costs (Fig. 1). The graph basically consists of a plot on the vertical axis of the number of wells required to meet a given total field peak-day rate vs the gas inventory level (with respect to original) plotted on the horizontal axis, at which point the peak-day rate can just be met by that number of wells. Each curve on the graph corresponds to a specific peak-day rate, and rates from q = 400 to q = 2,000 MMscf/D are covered by the family of curves on this graph.

JPT

P. 323ˆ