Monthly Safety Awareness - March 2021

11-03-2021

During our client’s recent experience in abandonment operations on a geothermal well, the first cement plug was placed inside the casing at the bottom of the production casing shoe. The well was considered a gassy well and gas tracking through the cement was considered a risk.

Extreme Wellhead Pressure from Heating of Trapped Water

After the cement plug had set, the well was fully shut in to prevent any potential for gas flow to the wellsite at surface. The wellhead pressure rose to 51 bg, and this pressure was bled off at surface. The site was then vacated for the night. Overnight the wellhead pressure rose and when the service crew returned the next morning, a wellhead pressure of 183 bg (2650 psi) was discovered. This exceeded the rated working pressure -155 bg (2250 psi) - of the wellhead and also placed significant stress on the anchor casing of the well.

This pressure was bled off safely and no identifiable damage was found on the wellhead. Abandonment operations resumed.

What was the cause?

When the well was shut in, water was trapped inside the well between the wellhead, casing and cement plug. Heating of this water would normally have led to thermal expansion of the water, but as it was inside a closed container with no place to bleed off, this instead raised the pressure inside the well significantly.

The increase in water temperature required to produce this pressure was calculated as being only 15 degC.

Lessons learned

Heating trapped water inside a vessel with no means of bleed off pressure can lead to significant pressure inside the vessel as the water is unable to thermally expand.

Always leave a path for any vessel or casing to bleed off volume as it heats, or put in an air-cap to absorb the increase in volume.

ModuSpec wishes to thank our client for sharing this safety awareness alert.