Underground Gas Storage Glossary - Innovative Energy Consulting ...

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Innovative Energy Consulting Pty Ltdnecessary to operate and protect the storage project, including producing wells ina buffer zone around the project.Cushion gas: The quantity of natual gas held within the confines of theunderground gas storage container intended as permanent inventory required forreservoir management purposes and to maintain an adequate Minimum StoragePressure. Cushion Gas is rarely, if ever, produced and the amount of Cushiongas required is dependent upon the reservoir quality, number ofinjection/withdrawal wells, gas withdrawn schedule, amount of compression, andfield operating parameters. The Cushion Gas volume may consist of recoverableand non-recoverable in-situ gas volumes and injected gas volumes. One of thelargest cost components for an UGS project is the purchase of Cushion Gas priorto placing the facility into service. UGS facilities tend to have a long operating lifethereby destroying the present value of the ultimate recovery and sale of CushionGas. One of the ongoing optimizations of any UGS project is to balance theCushion Gas requirements and costs related thereto with the costs of additionalinstalled compression that would be required to lower the Cushion Gasrequirement. The ratio of Cushion Gas to Working Gas Capacity or to Total GasCapacity has a significant impact on the economics of storage operations due tothe upfront capital and carrying cost of this gas inventory. Typically Aquifer gasstorage has a very high Cushion Gas requirement (80% of the Total GasCapacity), Depleted reservoir gas storage has the next highest requirement (30%to 70% of the Total Gas Capacity) and salt cavern gas storage has the lowestCushion Gas requirement (typically 20% to 30%).Cycle: One storage cycle is the theoretical time required to completely inject andwithdraw the working gas quantity for any given underground gas storage facilityor the turnover time for the working gas capacity rating of the facility. The cyclerate of any storage facility is usually expressed in cycles per year and is thenumber of times the working gas volumes can theoretically be turned over eachstorage year. The cycle rating for Porous Storage varies from 1 to 6 per yearwhile that for Salt Cavern Storage are as high as 12 per year.Days Service: The Days Service is another manner in which to describe thecapability of an UGS facility or the rights of a storage customer pursuant to aFirm Gas Storage Service contract to cycle working gas capacity. The DaysService refers to the number of days required to completely withdraw theMaximum Working Gas Inventory associated with a facility or a storage contract.The following table matches Days Service with Annual Cycling Capability:# Days Service Annual Cycling Capability10 1830 960 490 2150 1Underground Gas Storage Terminology 4
Innovative Energy Consulting Pty LtdDeliverability: see Withdrawal RateDelta-Pressuring: The practice of operating an underground gas storagereservoir at a maximum pressure greater than the discovery pressure associatewith that reservoir, for the purposes of increasing both the Working Gas Capacityand Withdrawal Rate. While not applicable to every gas storage reservoir,generally accepted Delta-Pressuring in the gas storage industry is up to 160% ofthe hydrostatic pressure gradient. The fracture pressure for the region, theintegrity of the sealing formations, spill points, containment properties, etc haveto be considered and quantified in order to obtain regulatory approval to Delta-Pressure a storage reservoir. As discussed under Hydrocarbon Pool, the bestcandidates for Delta-Pressuring are Hydrocarbon Pools formed by stratigraphictraps, particularly those with a Discovery Pressure Gradient that is much lowerthan the Hydrostatic Pressure GradientDepleted Reservoir Storage Field: A sub-surface natural geological reservoir,usually a depleted gas or oil field, used for the primarily for the storing andcycling of natural gas. The economically recoverable reserves have usually beennearly or completely produced prior to the conversion of the reservoir to gasstorage operations. The integrity of the reservoir has been demonstratedthroughout the primary production stage of operations and then verified byextensive gas injection/withdrawal testing.Depth Top of Structure/Cavern Roof Depth: The true vertical depth from thesurface down to the top of the storage formation or salt cavern roof.Discovery Pressure Gradient: The pressure gradient of the original pressure ina given reservoir at the time of its discovery. The Discovery Pressure Gradient ofa reservoir is commonly equal to the Hydrostatic Pressure Gradient; a reservoir isclassified as over-pressured if it was discovered at a Discovery PressureGradient that exceeds the Hydrostatic Pressure Gradient and a reservoir isclassified as under-pressured if it was discovered at a Discovery PressureGradient that is less than the Hydrostatic Pressure Gradient. For the purposes ofUGS operations, it is important to note that an under-pressured reservoir is bydefinition well sealed, depletion drive, and the Delta-Pressuring of the reservoirhas a more pronounced effect on the amount of Gas In Place that the reservoircan hold relative to the original gas in place.Displacement or Backhaul: Describes the notional transaction of gas flows intoor out of a gas storage facility, as the case may be, due to offsetting coincidentalinjection and withdrawal requests or nominations by two storage customers atany time.Underground Gas Storage Terminology 5
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