(476e) Thermodynamic Models for Leak Detection and Inventory Verification of Natural Gas In Salt Cavern Storage

Leak detection and inventory verification of natural gas storage is an important requirement for the safe and reliable operation of underground storage facilities.  In depleted oil field and aquifer facilities the injection and withdrawal of gas is essentially an isothermal process: the gas temperature remains constant at the geothermal temperature of the storage medium.  Natural gas exhibits more complex, transient behavior in salt caverns: the gas temperature increases during injection and decreases during withdrawal.  During idle periods, the gas in the cavern either cools or warms towards the geothermal temperature.  Depending on the gas storage operations, the cavern gas temperature can deviate considerably from the geothermal temperature.  Furthermore, the gas under certain circumstances can temporarily cool or heat the cavern walls.

Sophisticated salt cavern storage models are available for simulating the thermodynamic and heat transfer processes that determine the gas pressure and temperature histories.  These models, however, require a great deal of input information, can be cumbersome to operate, and may be prohibitively expensive for certain storage managers. They are also inadequate for leak detection and diagnosis.

This paper presents a simple thermodynamic analysis that permits the analyst to predict the gas storage conditions immediately following injection or withdrawal.  The model uses the compressibility factor equation of state for the gas and treats the cavern as a perfectly insulated volume.  Thus, the model should be used only to determine storage parameters either during or immediately after injection or withdrawal.

The success of the thermodynamic model is demonstrated by comparing its performance with both field measurements and with the predictions generated by the Salt Cavern Thermal Simulator, a software package used by the natural gas industry to predict storage behavior.  Estimation of the gas state during injection/withdrawal or immediately thereafter is shown to be adequately predicted through thermodynamics alone; however, finite rate heat transfer effects dominate during idle periods.  The thermodynamic model is also a powerful means for detecting leaks in the salt cavern facility. Criteria for leak detection are derived from the thermodynamic model and demonstrated with field data obtained from a salt cavern failure.