Sawolo et al. (2009) the Lusi mud volcano controversy: Was it caused by drilling?

Abstract

1. IntroductionThe Lusi mud volcano in Sidoarjo, East Java, was first noticed bylocal villagers at 5 am on the 29th May 2006. It started to erupt150 mfromtheBanjarPanji-1gasexplorationwell(Fig.1)twodaysafter the Yogyakarta Earthquake (5:54 am 27th May 2006), hasdisplaced 13,000 families and led to13 fatalities. The trigger for themud volcano has been the subject of significant debate (Davieset al., 2007, 2008; Manga, 2007; Mazzini et al., 2007; Tingayet al., 2008).The Sawolo et al. (2009) paper assesses and then dismisses thepossibility that there was a subsurface blowout (breakdown of thestructural integrity of the well) caused by a kick in the well (aninflux of water or gas from surrounding formations) whichoccurred on the 27th and 28th May 2006. For the subsurfaceblowout to have occurred, the pressure of the fluid (drilling mud,water, gas) in the unprotected section of the well has to exceed themaximum pressure the well can tolerate, which is estimated bya pressure test known as a leak-off test (LOT). To reach thisconclusion Sawolo et al. (2009) estimate what we deem to be anunrealistically high leak-off pressure (LOP) and unrealistically lowpressure within the borehole during the kick.Here we counter the main arguments made by Sawolo et al.(2009), pointing out inaccuracies, incorrect interpretations anddeviations from the daily drilling report (the factual account ofdaily operations). We also take this opportunity to describe for thefirst time direct evidence that the well was the cause of the mudvolcano. Lastly we show that their claim of an earthquake trigger isnot supported by the mud log data they present.2. What pressure could the well tolerate?The estimated LOP proposed by Sawolo et al. (2009) is 16.4 ppg(19.27 MPa/km) measured at 1091 m (1 ppg¼1.175 MPa/km). Indetermining the leak-off pressure (LOP), industryaccepted practiceis to take the inflexion point on a pressure build-up curve (Bell,1996; Enever et al., 1996; Addis et al., 1998; Jorgensen andFejerskov, 1998; Okland et al., 2002; Raaen et al., 2006; van Oortand Vargo, 2008). Based upon the pressure versus time plot(their figure 11), using this method the leak off was 15.8 ppg(18.57 MPa/km). The rationale stated by Sawolo et al. (2009) fornot interpreting the LOP by the conventional method is thatinterpreting leak-off pressure is less reliable when using oil-basedmuds and they suggest that the ‘fracture closure pressure’ shouldbe used instead.The fracture closure pressure (FCP) is generallyconsidered to beequaltotheminimum principalstressmagnitude andthus equaltothepressurerequiredtoopenanypre-existingfractures.Hence,theFCPcanbeanaccuratevaluetouseasformationstrength.However,the 16.4 ppg (19.27 MPa/km) value suggested by Sawolo et al.(2009) as the ‘fracture closure pressure’ is in contravention of alltechniques for estimating FCP. FCP is determined by carefullymonitoring the pressure decay in the well after the pumps areturned off (Enever et al., 1996; Jorgensen and Fejerskov, 1998;Raaen et al., 2006). The FCP can then be estimated from the pres-sure decay curve by a variety of methods, with the double tangentor root time methods most commonly used (Enever, 1993; Raaenet al., 2006). These techniques all require the pressure decay tobe monitored for a long duration after the pumps are shut-in(generally >10 min; Enever et al., 1996; Jorgensen and Fejerskov,