Summary

This case-history paper presents the results of installing frac-pack [one-step tip screenout (TSO)1 fracture stimulation and gravelpack operations]2–5 completions in gas-storage wells in the West Clear Lake field of Harris County, Texas. With the frac-pack technique, operators have fracture-stimulated six wells (new drills and workovers) in the past 3 years as part of an overall facility upgrade. Gas is injected daily into the Frio formation (5,500 ft) of the field and withdrawn at high rates during peak demand periods. A strong waterdrive can "water-out" production at the end of peak-season gas withdrawal. The water recedes when withdrawal is ended and injection restores gas volume and pressure in the reservoir. The paper presents frac-pack installation techniques, performance comparison of conventional methods and frac packs, and completion-technique selection to achieve high production efficiency.

New wells that were completed with frac packs in 1994 have yielded rates up to 100 MMcf/D each with approximately 500 psi drawdown pressure. In 1995, a well capable of producing at 25 MMcf/D with a gravel pack was recompleted with a frac pack, which increased its rate capacity to 49 MMcf/D. Because of the performance of the first three wells, three more wells were completed with frac packs in 1997. The frac-pack completion has proven superior to other sand-control methods in providing high rate deliverability in the West Clear Lake field. Frac-pack completions have, in general, provided a two-fold increase in the daily production rate.

Frac-pack completions and recompletions can be applied in gas-storage wells where high production rates and sand-control measures are necessary for the life of the well.

This paper provides guidelines and results associated with frac packing by

  • Demonstrating the importance of selecting completion techniques that will provide high-efficiency production from gas-storage wells.

  • Presenting case histories of frac-pack-stimulated gas-storage well performance over a relatively long time period.

  • Comparing well performance of conventionally completed and frac-pack-completed gas-storage wells producing at high rates.

  • Providing insight into the operational aspects of frac-pack completions.

Introduction

For decades, gas-storage wells have been used all over the world to help maintain a balance between supply and demand. However, the demand curve for these wells has become much more volatile in recent years.6 Because of today's more aggressive futurestrading market, marketing groups and operators in the petroleum industry must make daily adjustments to market supply-and demand planning, storage, and delivery of gas. Contracts are traded on a daily price basis, and fuel sources are chosen according to their daily cost per BTU. By responding quickly to market conditions, gas companies can lock in higher profit margins on reserves when market changes present opportunities. When gas price movements trigger large-scale contract price swings in either direction, the ability to store or deliver the required volume of gas can lock in margins of as much as U.S. $2 per thousand ft3.

The ideal gas-storage well has the injectivity and deliverability to fill storage with lower-cost gas and deplete storage at higher price levels. If the storage facility maintains this deliverability over time, gas marketing groups can better predict facility gas-storage capacity and deliverability to maximize the potential of gas contracts.

Frac packs are not a new completion method for high-permeability, unconsolidated, waterdrive formations. Treatments for this application in Europe, described by the term "Frac-Pac," precede the work done at West Clear Lake by 3 decades. Dickman and Petzhold5 discussed the necessity of using highly permeable unconsolidated reservoir rock to meet the demands of storage capacity. Their findings showed that other sand-control completion methods, such as gravel packs and openhole completions with a type of consolidated sand prepacked screen, were subject to plugging and erosion failure in gas-storage well service. The frac-pack method provided better results for long-term sand control and increased storage-well flow capability.

These 1960s-era frac packs share many of the essential features of frac-pack treatments today: proppant packing into the reservoir formation sand at a rate and pressure greater than fracture-creation conditions, high proppant loading in the final fluid stage, a fluid and proppant pumping schedule designed for high near-wellbore conductivity, and a design that includes sand-control measures. However, frac packs then and today are separated by some key differences in the design processes, as wells as in the fluids and materials, associated well-operation methods, completion equipment, and treatment execution. Evolution of these techniques and products, especially in the last decade, has led to far more reliable sand-control results and high completion flow efficiencies. Pretreatment diagnostic pumping,7,8 computer-aided 3D fracture simulation treatment design,4,9,10 and the TSO method for increasing near-wellbore conductivity1-4,11 are critical evolutionary factors contributing to the success of today's frac packs. Advances in frac-pack practices contributing to improved results include fracpack proppant sizing to optimize conductivity and sand control, improved packers and associated service tools,12 and fracture fluid improvements. The frac-pack completions at West Clear Lake are an example of the obtainable results for gas-storage wells in high permeability, unconsolidated, waterdrive reservoirs with these advances, which have evolved into current industry-accepted fracpack practices.

The operator in this case study has increased usable reservoir storage capacity with frac packs to make the reservoir perform more like a depletion drive than a strong waterdrive. Waterdrive reservoirs typically provide a 65% recovery of gas in place (GIP), whereas depletion reservoirs typically recover 85%.13 By increasing the withdrawal rate capability of West Clear Lake from 220 to 600 MMcf/D, the rate of gas withdrawal can outrun the water contact advance in the reservoir. This allows more gas volume to be withdrawn to a lower reservoir pressure before water breakthrough requires shutting the well in. Effectively, the frac packs have enlarged the usable storage volume and provided predictable deliverability at the West Clear Lake facility.

Benefits of Frac Packing

The benefits of frac packing are well documented.1–5,9–24

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