Triggering of the Lusi mud eruption: Earthquake versus drilling initiation

The Lusi mud volcano in East Java has erupted unabated for almost 2 yr, flooding an area of 7 km² and displacing more than 25,000 people. Despite its disastrous impact, the mechanism for triggering the Lusi eruption remains highly controversial; two distinct mechanisms have been proposed. One hypothesis suggests that the eruption was triggered by the M_w 6.3 earthquake that struck Yogyakarta (250 km from Lusi) two days before the eruption. However, an examination of static and dynamic stress changes and stress transfer mechanisms indicates that the Yogyakarta earthquake was at least an order of magnitude too small to reactivate faults and open fluid flow pathways under Lusi. An alternate theory suggests that Lusi was triggered by a blowout following drilling problems in the nearby Banjar Panji-1 well. Blowouts result from an inability to control pore fluid intakes into the borehole and typically occur when the drilling window (fracture pressure minus pore pressure) is approximately zero and when there is insuffi cient protective casing of the well bore. Pore and fracture pressure data from Banjar Panji-1 indicate that the well had a narrow drilling window of only 0-2.3 MPa. Furthermore, two planned casing points were skipped during drilling, resulting in 1742 m of unprotected borehole. The combination of hazardously narrow drilling window and long uncased borehole would have made drilling problems in Banjar Panji-1 difficult to control, placing the well at high risk of blowing out. Furthermore, well-bore pressures following drilling problems in Banjar Panji-1 reached magnitudes in excess of the fracture pressure and thus were sufficient to create fluid flow pathways in the subsurface. Therefore, we suggest that no viable method is known by which the Yogyakarta earthquake could have triggered the mudflow and that a blowout in the Banjar Panji-1 well was the most likely mechanism for triggering the Lusi eruption.