The Incident

On the morning of 21st August 2009, a minor expulsion of gas (Burp) was reported to have escaped from the H1 well. The burp subsided quickly and normal operations had resumed aboard the Montara WHP. The burp was considered to be an insignificant anomaly from the well at the time.

However, two hours later, the H1 well exhibited a tremendous kick that expelled well fluid, oil and gas from the top of the well, hitting the underside of the West Atlas drilling rig before cascading into the sea.

The well continued to leak till 3rd November 2009 for a total of 74 days into the Timor Sea. The Australian Department of Resources, Energy and Tourism carried out an investigative survey and concluded that the total volume of the Montara Spill could not be exactly determined. However, they estimated the volume to range from 1.2 to 9 million US Gallons.

Patches of hydrocarbon sheen could be observed over the surface of the ocean and have been reported as large as 90,000 sq. kms.
The first four attempts by PTTEPAA to plug the oil leak failed. However, the fifth attempt on 3rd November 2009 succeeded when ~ 3400 bbls of kill mud was pumped into the H1 well through a relief well.

The first four attempts by PTTEPAA to plug the oil leak failed. However, the fifth attempt on 3rd November 2009 succeeded when ~ 3400 bbls of kill mud was pumped into the H1 well through a relief well.

During the recovery phase, on 1st November 2009, a fire broke out on the West Atlas drilling rig and engulfed the rig in flames. On a fortunate note, the fire is believed to have burnt off a portion of the hydrocarbons until the well was sealed.

Aftermath.

Spill Response.

Several attempts were made to stop the flow of oil from the H1 well into the Timor Sea. On 21st August 2009, the Australian Marine Oil Spill Center mobilized aircraft and equipment to disperse chemical dispersants on the surface of the water.

Starting on 23rd August 2009, a Hercules aircraft sprayed 10,000 liters of chemical dispersant onto portions of the oil sheen. Several simultaneous spraying operations were underway with the dispersants being used as the primary response to the oil spill.

Dispersants from Vessels commenced on 30th August 2009 and continued till 1st November 2009. A total of ~ 184,100 liters of chemical dispersants had been sprayed from aircrafts and vessels between 23rd August to 1st November 2009.

Killing the H1 Well.

On 11th September the West Triton Jackup Rig arrived at the Montara field to drill a relief well. By 1st November 2009, the West Triton had successfully drilled a relief wellbore to intercept the H1 well (After 4 unsuccessful attempts to intercept).

On 1st November 2009, while pumping heavy kill mud through the relief well into the H1 wellbore, a fire broke out on the West Atlas drilling rig and delayed operations. However, by 3rd November, a total of ~ 3400 bbls of heavy kill mud had been pumped into the H1 well to stop the oil leak.

Upon sealing away the source, the fire engulfing the Montara WHP and the West Atlas drilling rig eventually died down.

PTTEPAA continued to hold active pressure on the H1 well using the heavy mud and brine until the well was cemented to form an effective barrier against future flow.

Root Causes

Investigations carried out following the Montara incident have revealed that the primary cause for the blowout was a failed 9-5/8” casing shoe. It is believed that hydrocarbons entered the failed well barrier (A cemented 9-5/8” casing shoe) and flowed to surface through the 9-5/8” casing string.

In addition, the inquiry has also revealed other discrepancies and errors that had led to the incident.

The following factors have been identified to have sequentially contributed to the Montara blowout and oil spill:

H1 Well Suspended in April 2009.

• When the H1 Well was suspended in March 2009, none of the well’s integrity barriers complied with PTTEPAA’s Well Construction Standards.
• The Cemented 9-5/8” Casing Shoe had not been pressure tested as per the company’s Well Construction Standards.
• Records indicate a critical issue during the 9-5/8” casing cement job when the cement was extensively over-displaced resulting in a ‘Wet Shoe’. The observation had been recorded in the Daily Drilling Reports (DDR) but was not considered a critical observation at the time.
• No practical confirmation had been observed or recorded that the top and bottom plugs had bumped while cementing.
• As per the H1 well’s original drilling program, TWO Pressure Containing Anti-Corrosion Caps (PCCCs) were supposed to be installed on the well – For the 13-3/8” and 9-5/8” Casing Strings.
  • However, the PCCC for the 13-3/8” Casing was never installed.
  • The PCCC installed for the 9-5/8” Casing was not pressure tested and verified as per the company’s Well Construction Standards.
• Key personnel were under the impression that the fluid left inside the 9-5/8” casing created an overbalance over the pore pressure at the time. However, the properties of the fluid inside the casing had not been monitored to ensure a significant overbalance as per the Construction Standards.

H1 Well Re-Opened in August 2009.

• When the West Atlas returned to the WHP, it came to light that the 13-3/8” PCCC had never been installed. Due to the absence of the PCCC, the threads of the 13-3/8” casing had corroded.
• In order for the 13-3/8” casing threads to be cleaned, the 9-5/8” casing PCCC had to be temporarily removed from the well.
• The primary implication here is that following the removal of the 9-5/8” casing PCCC, the H1 well was left exposed with only ONE Untested Well Barrier to arrest uncontrolled formation fluid flow – The Cemented 9-5/8” Casing Shoe.
• The H1 well was left in an unprotected state while the crew proceeded to carry out other activities as part of the planned batch drilling process on the Montara WHP. The blowout from the H1 well occurred 15 hours later.
• The initial ‘Burp’ of hydrocarbons that had occurred 2 hours prior to the actual blowout had been ignored.
• A critical evaluation of the anomaly was not carried out.

Therefore, the investigation concluded that the absence of tested and verified well barriers in the H1 well was the primary cause of the blowout and oil spill.

Conclusions.

The Montara incident was the direct result of failure to ensure well integrity in the H1 well. Human error, specifically, lack of concern with respect to testing and verification of well barriers, plus Failure to consider & evaluate an observed anomaly form the well were the primary causes of the incident.

The Montara blowout and oil spill was closely followed by The Macondo blowout and oil spill on 20th April 2010 that had a much larger magnitude, severity & after effects.