jordan_Outsourcing_book_temp.qxd 13/11/2009 11:12 Page 63
Well Blowout Rates in California Oil and Gas District 4 – Update and Trends
Table 2: Comparison of Selected Well Blowout Rates for 1991–1998 versus 2001–2008
Well Construction Steam-injection Wells Inactive Wells
1991–1998 2001–2008 1991–1998 2001–2008 1991–1998 2001–2008
Coded blowouts 24 8 15 1 4 1
Normalised no. of blowouts 28.1 8.9 18.3 1.0 4.7 1.0
Basis 40,411 45,900* 108,190 100,000* 388,393 516,000*
Rate (%) 0.070 0.019 0.017 0.0010 0.0012 0.00019
Rate 1 per 1 per 1 per 1 per 1 per 1 per
1,400 wells 5,200 wells 5,900 well-years 100,000 well-years 83,000 well-years 520,000 well-years
p-value 0.00059 0.000087 0.086
*2008 data were not available, so the 2001–2007 average was substituted.
well servicing after 2005. The one inactive well blowout was similar to established criterion by the number of comparisons. This is called
inactive well blowouts in the 1991–2005 study.
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It was a steam-driven Bonferroni’s correction. Eight rate comparisons are tested in this
blowout from an abandoned well that created a depression by paper (included those presented below), so the p-value criterion of
displacing a large quantity of earth onto the surrounding land surface. significance with Bonferroni’s correction is 0.625%. The p-value was
Site restoration required backfilling the depression with this soil and calculated using the same approach as in the 1991–2005 study.
6
re-compacting it.
As shown in Table 2, the blowout rates for well construction and steam
The rates of different types of blowouts from 1991 to 1998 and from injection wells in operation decreased significantly. The p-value for
2001 to 2008 are listed in Table 2. Each rate is compared across the these is less than 0.625%. Consequently, the 2001–2008 blowout
two time periods through the p-value. This is a measure of the rates of one per 5,000 well construction events and one per 100,000
likelihood of two quantities, in this case blowout rates, coming from steam-injection well-years is the most current for the District, rather
the same normally distributed population. A p-value less than 5% is than the values in Table 1. The blowout rate from inactive wells also
typically considered to indicate that two quantities are likely derived apparently declined. Inactive wells include shut-in wells and
from two different populations. This is termed a statistically plugged and abandoned wells. The decline in the blowout rate from
significant difference. When making multiple comparisons, however, these wells is not statistically significant, though; the p-value is greater
there is an increased likelihood that any one comparison will meet the than 5%. The inactive well blowout rate of one per 150,000 well-years,
criterion of significance. One approach to resolve this is to divide the as given in Table 1,
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should be considered current for the District.
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