Measuring Subsea Well Annulus Pressure and Temperature

In one of last week’s Offshore Technology Conference-related posts, I mentioned the announcement of a Roxar downhole wireless pressure-temperature (PT) sensor system for the annulus B region. If you’re not in the oil and gas business, terms such as annulus, tubing, and casing sound like a lot of jargon. I thought I’d try to decode the jargon to describe what this new sensor does.

Wikipedia has a great analogy of an oil and gas well annulus:

An easy way to visualise this would be to stand a straw (purple in diagram) straight up in the center of a glass of water. All of the water in between the straw and the sides of the glass would be the annulus (yellow area in diagram), with the straw itself representing the drill string and the sides of the glass representing the formation.

This Wikipedia entry goes on to describe the differences in the A, B, and C annuli:

In a completed well, there may be many annuli. The ‘A’ annulus is the void between the production tubing and the smallest casing string. The A annulus can serve a number of crucial tasks, including gas lift and well kills. A normal well will also have a ‘B’ and frequently a ‘C’ annulus, between the different casing strings. These annuli do not normally have any connection to well bore fluids, but maintaining pressure in them is important in order to ensure integrity of the casing strings.

Oil Well Tubing, Casing, and AnnulusI reviewed a presentation given by Emerson’s Terje Baustad at OTC. Until the release of this pressure and temperature sensor, there was no direct access to the annulus B region for these measurements. The measurements were inferred through simulations or assumptions about the formation. Prior methods of pressure testing were cumbersome and came at the expense of production / injection uptime.

Having accurate pressure and temperature measurements can help avoid gas/oil migration to the seabed, injection gas escaping through failed casing, and loss of the barrier where the annulus is sealed off. Discovering these issues without these measurements is a time-consuming, fault detection process. With this downhole wireless PT sensor, the B annulus can be continuously monitored up to once per second to provide positive verification of the casing pressure barrier integrity. Changes in the sensor readings provide early warning of any abnormal conditions in the subsea well.

Terje shared how this sensor connects to Roxar Integrated Downhole Network ¼” electrical cable, which provides power and communications from the sensors to the wireless transponder. This network supports up to 32 downhole sensors measuring annulus pressure and temperature, tubing pressure and temperature, tubing density and pipe orientation, tubing permittivity including flow velocity, and choke position.

The sensor was designed to have no impact on the integrity of annulus barrier, handle pressures up to 10,000 psi and temperatures up to 302degF. These sensors are designed and tested for a minimum of 20 years of continuous operation.

The technologies that are required for the oil and gas production industry continue to amaze me. Imagine how many other applications require 20 years of service in those kinds of pressures and temperatures. The best part is that these technologies can help oil and gas producers more quickly spot and correct problems before they escalate.

Posted Tuesday, May 10th, 2011 under Downhole, Measurement, Oil & Gas, Subsea.

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