The Underground Blowout

Grace, Robert D.

doi:10.1016/b978-0-12-812674-5.00008-0

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“…Abnormally high pore-fluid pressure, commonly referred to as overpressure, is a common occurrence within sedimentary basins, occurring when the pore-fluid pressure is greater than the hydrostatic pressure expected at a given depth (Neglia, 1979;Mann and Mackenzie, 1990;Osborne and Swarbrick, 1997;Tingay et al, 2007). Encountering unexpected overpressure zones during drilling operations can pose a significant risk to both human life, the environment, and a well achieving its technical objective; such zones can result in an influx of high-pressure gas or fluid into and up the wellbore (known as a "kick") and, in a worst-case scenario, a "blowout" (Grace, 2017). Accurate prediction of pore pressures when drilling petroleum wells fundamentally underlies safe drilling operations; the lack of adequate understanding and subsequent response to higher-than-expected pore pressures during drilling of the Banjar Panji-1 well in Java, Indonesia, was a contributing factor to the blowout and the flow of the Lusi mudflow that suddenly erupted in an urban area, burying over 11,000 buildings (Tingay, 2015).…”

Section: Introductionmentioning

confidence: 99%

Overpressure transmission through interconnected igneous intrusions

Schofield¹,

Holford²,

Edwards³

et al. 2020

Bulletin

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In situ overpressures in sedimentary basins are commonly attributed to disequilibrium compaction or fluid expansion mechanisms, although overpressures in shallow sedimentary sequences may also develop by vertical transfer of pressure from deeper basin levels, for example, via faults. Mafic sill complexes are common features of sedimentary basins at rifted continental margins, often comprising networks of interconnected sills and dikes that facilitate the transfer of magma over considerable vertical distances to shallow basinal depths. Here, we document evidence for deep sills (depths >5 km [>16,000 ft]) hosting permeable, open fracture systems that may have allowed transmission of overpressure from ultradeep basinal (>7 km [>23,000 ft]) levels in the Faroe-Shetland Basin, northeast Atlantic margin. Most notably, well 214/28-1 encountered overpressured, thin (<8 m [<26 ft]), and fractured gas-charged intrusions, which resulted in temporary loss of well control. Although the overpressure could reflect local gas generation related to thermal maturation of Cretaceous shales into which the sills were emplaced, this would require the overpressures to have been sustained for unfeasibly long timescales (>58 m.y.). We instead suggest that transgressive, interconnected sill complexes, such as those penetrated by well 214/28-1, may represent a previously unrecognized mechanism of transferring overpressures (and indeed hydrocarbons) laterally and vertically from deep to shallow levels in sedimentary basins and that they represent a potentially underrecognized hazard to both scientific and petroleum drilling in the vicinity of subsurface igneous complexes.

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