What Caused the ‘Lusi’ Mudflow Disaster in East Java, Indonesia?: Using Geomechanical Models to Test Earthquake and Drilling-Trigger Theories

Abstract

The ‘Lusi’ mudflow on Java is a unique geological disaster in which a new mud volcano suddenly erupted in an urban area, burying over 11000 buildings. The mudflow, which has been erupting continuously for 9 years, has displaced 40000 people and caused over US$2.7 billion in damage. Intense debate has focused on whether the disaster was triggered by a drilling kick in the adjacent Banjar Panji-1 (BJP-1) well (1 day earlier, 150m away), or whether the eruption was a natural event induced by the 2006 Mw6.3 Yogyakarta earthquake (2 days earlier, 250km away). Both theories argue that an event changed the effective stress under Lusi, with some studies proposing that high pressures during the drilling kick initiated hydraulic tensile fracturing, while the ‘earthquake-trigger’ hypothesis argues that shear stress increases caused strike-slip reactivation of the nearby Watukosek fault. Yet, neither theory has been fully quantitatively tested, as data has not previously been available on the initial state of stress and rock mechanical properties under Lusi. In this study, the pre-eruption stress and pore pressure state under Lusi is determined, and a new petrophysical log suite used to estimate rock mechanical properties. The initial state of stress is then used to test all known triggering theories, by examining the stress changes induced by the earthquake and drilling kick and determining whether fracturing or fault reactivation was likely to have occurred. The results demonstrate that the earthquake was too small, on its own, to trigger the Lusi eruption. Furthermore, this study results in a new triggering model, in which the drilling kick, and not the earthquake, caused catastrophic shear failure of the borehole wall, and subsequent reactivation of the Watukosek fault. These results indicate that the Lusi disaster is one of the most destructive examples of human-induced faulting ever witnessed.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015