The report by American, British, Indonesian and Australian scientists is published this week in the academic journal 'Earth and Planetary Science Letters.' It outlines and analyzes a detailed record of operational incidents on the drilling of a gas exploration well, Banjar-Panji-1.
The well is operated by the Indonesian oil and gas company Lapindo Brantas, which has confirmed that the published data is correct.
Lead author, Professor Richard Davies of Durham University's Centre for Research into Earth Energy Systems, published research in January 2007 which argued the drilling was most likely to blame for the eruption of the Lusi mud volcano on May 29, 2006.
This theory was challenged by the company that drilled the well and some experts who argued that the Yogyakarta earthquake two days before the eruption, which had an epicentre 250 kilometers (155 miles) from the mud volcano, was the cause.
Professor Michael Manga of University of California, Berkeley and graduate student Maria Brumm undertook a systematic study to test the claims that the eruption was caused by this earthquake. They found that none of the ways earthquakes trigger eruptions could have played a role at Lusi.
Professor Manga said, 'We have known for hundreds of years that earthquakes can trigger eruptions. In this case, the earthquake was simply too small and too far away.' The new report concludes the effect of the earthquake was minimal because the change in pressure underground due to the earthquake would have been tiny. Instead, scientists say they are '99 percent' certain drilling operations were to blame.
Professor Davies explained, 'We show that the day before the mud volcano started there was a huge ‘kick’ in the well, which is an influx of fluid and gas into the wellbore. We show that after the kick the pressure in the well went beyond a critical level.'
'This resulted in the leakage of the fluid from the well and the rock formations to the surface - a so called 'underground blowout.' This fluid picked up mud during its accent and Lusi was born.'
He said chances of controlling this pressure would have been increased if there was more protective casing in the borehole.
Professor Davies said, 'We are more certain than ever that the Lusi mud volcano is an unnatural disaster and was triggered by drilling the Banjar-Panji-1 well.'
Lusi is still flowing at 100,000 cubic meters per day, enough to fill 53 Olympic swimming pools. It appears that the flow will continue indefinitely and so far all efforts to stem the flow have failed.
Recent research in which Davies was involved showed it is collapsing by up to three meters overnight and could subside to depths of more than 140 metres, having a significant environmental impact on the surrounding area for years to come.
Indonesian President Susilo Bambang Yudhoyono has declared the 400 hectare area inundated by the mud flow as a disaster-prone area unfit for human habitation.
The president authorized a flow of mud to be pumped into the Porong River where it is washed into the Java Sea. Pumping of sludge started on October 16, 2006.
On November 23, 2006, eleven fatalities were reported from the explosion of a gas pipe caused by the mud flow. The accident occurred due to subsidence of the ground, up to two meters (6.5 feet), due to the outflow of mud causing a dike to collapse resulting in the rupture of the state-owned Pertamina gas pipeline. The gas sent flames into the sky and according to the local people, the heat could be felt one kilometer (0.6 miles) away.
A network of dams and barriers has been erected to contain the mud. On September 26, 2006 a barrier failed, resulting in the flooding of more villages. Further strengthening of the dam system appeared to contain the sludge and no further reports of breaches were received until January 4, 2008 when a dike collapsed after a dispute with a landowner prevented reinforcement before the onset of the rainy season.
Professor Manga said, 'While this is a most unfortunate disaster, it will leave us with a better understanding of the birth, life and death of a volcano.'