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Abstract

As gas storage becomes increasingly important in managing the nation's gas supplies, there is a need to develop more gas storage reservoirs and to manage them more efficiently. Using computer reservoir simulation to predict gas storage reservoir performance, we present specific procedures for optimization of gas storage reservoir performance for two different problems. The first is maximizing working gas volume and peak rates for a particular configuration of reservoir, well and surface facilities. We present a simple procedure to determine the maximum performance with a minimal number of simulation runs. The second problem is minimizing the cost to satisfy a specific production and injection schedule, which is derived from the working gas volume and peak rate requirements. We demonstrate a procedure to determine the optimum combination of cushion gas volume, compression horsepower and number and locations of wells. The use of these procedures is illustrated through application to gas reservoir data.

Introduction

With the unbundling of the natural gas industry as a result of FERC Order 636, the role of gas storage in managing the nation's gas supplies has increased in importance. There is a need to develop more gas storage reservoirs and to manage them more efficiently. In screening reservoirs to determine potential gas storage reservoir candidates, it is often desirable to determine the maximum storage capacity for specific reservoirs. In designing the conversion of producing fields to storage or the upgrading of existing storage fields, it is desirable to determine the optimum combination of wells, cushion gas and compression facilities which minimizes investment. Survey of the petroleum literature found little discussion of simulation-based methodologies for achieving these two desired outcomes.

Duane presented a graphical technique for optimizing gas storage field design. This method allowed the engineer to minimize the total field development cost for a desired peak-day rate and cyclic capacity (working gas capacity). To use the method, the engineer would prepare a series of field design optimization graphs for different compressor intake pressures. Each graph consists of a series of curves corresponding to different peak-day rates. Each curve, in turn, shows the number of wells required to deliver the given peak-day rate as a function of the gas inventory level. Thus, the tradeoff between compression horsepower costs, well costs, and cushion gas costs could be examined to determine the optimum design in terms of minimizing the total field development cost. Duane's method implicitly assumes that boundary-dominated flow will prevail throughout the reservoir.

Henderson, Dempsey, and Tyler presented a case history of storage field design optimization using a single-phase, two dimensional numerical model of the reservoir . They varied well placement and well schedules in their study in order to reduce the number of wells necessary to meet the desired demand schedule. They used a trial and error method, and stated that their results were preliminary. They found that wells in the poorest portion of the field should be used to meet demand at the beginning of the withdrawal period. Additional wells were added over time in order to meet the demand schedule. The wells in the best part of the field were held in reserve to meet the peak-day requirements, which occurred at the end of the withdrawal season.

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