Abstract
The storage of natural gas in underground caverns is a common storage method. However, subsurface performance during injection and production rates differ from what is observed on surface. This paper reviews a unique well intervention solution conducted in The Netherlands that enabled the placement of downhole pressure and temperature gauges, with the successful simultaneous acquisition of accurate data from multiple depths during a 40-day period in a salt dome gas cavern.
This intervention was performed to obtain pressure and temperature data during injection and production of gas in and out of the gas cavern. The data was acquired by installing five memory gauges spaced out with 0.160-in. slickline at various intervals over 275 m. This arrangement enabled the customer to gather data from multiple depths while continuing to inject and produce gas at very high rates through the 9⅝-in. production tubing above the gauges. The unique assembly needed to perform such a job while under pressure required a specific rig-up process to install and retrieve long assemblies.
The purpose of the test was to verify the accuracy of the model with subsurface data for an extended period to predict gas behavior during injection and production. Although real-time data transmission through a cable to a surface-readout system was the preferred method of data acquisition, this was not possible because the cable would cross the tubing retrievable safety valve, rendering it inoperable during the test period. After 40 days, the memory gauges were successfully retrieved from the cavern. The acquired data was then entered into the production software to validate the model and, ultimately, to optimize gas injection and production rates. The suspension of the memory gauges in this manner provided an accurate pressure profile across the gas cavern that was not affected by the injection and production flow.
Equipment was specifically designed and developed to enable memory gauges to be hung below the end of the tubing. This assembly provided minimal restriction to gas flow and a platform to enable the installation of memory gauges suspended more than 275 m below the end of the tubing. Because the rates of injection and production change as the level of gas varies within the facility, collecting relative data at multiple points simultaneously is very desirable.
