October 25, 2005

DOWNHOLE EXPLOSION/ SOUR GAS BLOWOUT

During a recent workover of a well, a downhole explosion occurred that caused a catastrophic failure of the well's production and surface casings at a depth of about 18 metres (m) from surface. The result was an uncontrolled release of produced water, sour natural gas, and a small amount of oil at surface. The well blew out of control for 30 days, from December 12, 2004, to January 10, 2005.

The Alberta Energy and Utilities Board (EUB) has determined the following from an investigation into the incident:

• A mixture of air and hydrocarbons and an ignition source were required for this explosion to occur. It was determined that air was introduced into the wellbore during the previous day's well servicing operations (acidizing). When the acid job was completed, the crew pulled the tubing and acid packer cup tool out of the well. The well and equipment were secured for the night.

• On the morning of December 12, 2004, a rig crew member was opening a valve on the wellhead to take a routine wellhead pressure reading when the downhole explosion occurred. The recorded wellhead pressure was 7000 kilopascals (kPa). The ground around the wellhead lifted, creating a huge crater adjacent to the well and the service rig. Over 100 joints of tubing, which were racked in the service rig derrick, fell into the crater. Sour gas and wellbore fluids flowed to surface. The crew members escaped without injury.

• During the workover of this well, the daily well servicing reports indicated that water was pumped down the production casing at a rate of 0.2 cubic metres (m³) per minute to maintain a fluid balance in the well. Pumping at this rate was required to prevent formation fluids from flowing to surface, but was not sufficient to prevent air from being drawn into the wellbore. Air and hydrocarbon mixtures are created when a well goes on vacuum and air is drawn into the wellbore.

• The licensee surmised that a possible source of ignition was unstable oxidized hydrocarbon products. The decomposition of these products, which can take place during sudden changes in pressure (e.g., opening a valve), can yield sufficient energy to create a source of ignition. Although the actual source of ignition may never be known definitively, it is worth noting that the explosion occurred in conjunction with service rig personnel opening a valve on the wellhead to take a pressure reading. Similar situations have occurred during other incidents of this nature.

In order to prevent the recurrence of a similar incident, the EUB recommends that

For more information about this matter, please contact one of the following:*

• Paul Saulnier, EUB St. Albert Field Centre, (780) 460-3809

• Paul Bothwell, EUB Operations Group,
  Calgary, (403) 297-8995

• An EUB Field Centre listed below:

Bonnyville (780) 826-5352
Medicine Hat (403) 527-3385
Drayton Valley (780) 542-5182
Midnapore (403) 297-8303
Grande Prairie (780) 538-5138
Red Deer (403) 340-5454
High Level (780) 926-5399
St. Albert (780) 460-3800
Wainwright (780) 842-7570

*To call toll free, first dial 310-0000