Drilling made volcano spew mud
Drilling for gas most probably caused the eruption of an Indonesian mud volcano, forcing the evacuation of thousands of people, scientists report.
"[The eruption] appears to have been triggered by drilling of over-pressured porous and permeable limestones at depth of around 2830 metres below the surface," says the study, the first published on what caused the eruption.
The study, which appears in the February issue of the journal GSA Today, adds that the volcano has been disgorging 7000-150,000 cubic metres of mud every day since it erupted in May last year.
Such pressures, coupled to the local geology, suggest the flow "will continue for many months and possibly years to come", warn the UK researchers, led by Professor Richard Davies from the University of Durham.
In the coming months, sag-like subsidence several kilometres wide will occur, and around the main vent there is likely to be "more dramatic collapse", forming a crater, the study adds.
An area of at least 10 square kilometres around the volcano will be uninhabitable for years, and over 11,000 people will be permanently displaced, it says.
The volcano, known locally as Lusi, has been spewing steaming mud since May last year, submerging four villages, fields and factories.
It erupted from a gas well near Surabaya, East Java. As many as 13,000 people have fled their homes.
Satellite images tell the story
The experts analysed satellite images of the area for their study.
They say that seepage of mud and water are usually a preventable hazard when exploring for oil and gas.
"It is standard industry procedure that this kind of drilling requires the use of steel casing to support the borehole, to protect against the pressure of fluids such as water, oil or gas," Davies says.
"In the case of Lusi, a pressured limestone rock containing water, a water aquifer, was drilled while the lower part of the borehole was exposed and not protected by casing.
"As a result, rocks fractured and a mix of mud and water worked its way to the surface. Our research brings us to the conclusion that the incident was most probably the result of drilling."
History repeats itself
Davies says the case in Indonesia is similar to one that happened offshore of Brunei in 1979.
"Just as is most probably the case with Lusi, the Brunei event was caused by drilling and it took an international oil company almost 30 years and 20 relief wells and monitoring before the eruption stopped," he adds.
Last week, Indonesia's coordinating minister for social welfare, Aburizal Bakrie, whose family firm controls Lapindo Brantas, the firm that operated the gas well, said the volcano was a "natural disaster" unrelated to the drilling activities.
"It is not because of the Lapindo drill case but it is because of the quake," he says, referring to a tremor near the ancient city of Yogyakarta that killed around 6000 people two days before the volcano erupted.
But this latest study rules out this scenario.
Quake ruled out
It concludes that the quake was not to blame, mainly because two days lapsed before the mud volcano erupted, and no other mud volcanoes occurred in the region after the tremor.
According to the Geological Society of America, mud volcanoes are extrusions of water and mud that form a cone shape. They can be metres to kilometres wide and metres to hundreds of metres thick.
They most commonly occur when tectonic plates collide (such as Azerbaijan), in front of deltas (the Mississippi) and due to the collapse of sediments (Niger Delta).