A retrofit wireless gauge was installed into a dry gas well and provided downhole pressure and temperature data for over a year before being retrieved. This paper discusses the installation of the system, a comparison of modelled vs. actual telemetry characteristics and a critical analysis of the gauge performance with lessons learnt.
Troll is Norway's largest gas reservoir and has been on-line since 1996. Production is expected to continue for many decades. Declining pressure means that surface compression is required to drive production and an accurate understanding of reservoir pressure and decline is critical to meeting contractual gas deliveries and achieving recovery targets. As an alternative option to retrofit permanent downhole gauges, the operator was seeking intervention based solutions to enable data collection if permanent gauges are not available.
The pressure and temperature data from the installed gauge was compressed and transmitted via a pressure pulse telegram to surface. The telemetry system used a down-hole choke to cause a small reduction in flowing wellhead pressure which was recorded using the existing wellhead sensors. Interpretation of the telegram was performed by accessing the wellhead sensor data to provide daily downhole pressure and temperature readings.
To ensure successful telegram detection through large seasonal variations in production rate a pressure regulation protocol was introduced to the wireless gauge. Differences in the predicted and actual transient flow response meant that modifications to the downhole pulse were initially required before an acceptable pulse amplitude was achieved.
As the system did not require any topside modifications it was envisaged that no interaction would be required with the asset production team. However, after an intermittent flaw was discovered in the telegram protocol a production management response was implemented to mitigate any damage potential. The application of the wireless gauge ultimately produced clearly detectable pressure pulse telegrams sent daily for 428 days in service.
Thousands of wells today use downhole pressure gauges to optimize production and provide data used to maximise recovery. As lifetime of producers may extend past the practical service life of these gauges intervention based solutions can provide a viable alternative. Proving, evaluating and improving this technology will result in more robust systems, greater service life and increased data quality enabling the benefits to be realised in a wider range of applications.