Calculating the blowout rate of oil and gas wells is commonly one of the first steps in environmental impact assessment, contingency planning and emergency response. The blowout rate is a direct measure for the economic and environmental damage caused by a blowout and an indicator for the effort required to regain control over the well. Hence a simulator was developed to estimate blowout rates.
This simulator was validated for field cases by comparing calculated blowout rates with estimates based on observable phenomena such as flame length and heat release rates. This limited validation to onshore and platform well blowouts, which are usually governed by critical outflow conditions at surface, i.e. ambient pressure is considerably less than the wellbore pressure just upstream of the outflow. For subsea wells, blowing out against the substantially higher pressures at seabed, this does not apply. The blowout rate is determined by the total system performance from inflow at sand face to outflow at seabed.
To validate the blowout rate calculations under these conditions, data were collected on high rate well flow through an annulus against elevated surface pressures, resembling subsea blowout conditions. A comparison of the measured rates with the calculated rates demonstrated that the rates could be predicted with a high accuracy, provided the mechanical condition of the tubulars is properly taken into account. Default assumptions for the hydraulic roughness of the tubulars lead to over-estimates of the blowout rates and consequently worst case estimates for the environmental and economical damage caused by a blowout.
The number, which is invariably attached to potential or actual blowouts, is the blowout rate. In the first place the blowout rate is a measure for the damage caused by a blowout, since there is a direct relation with:
The loss of reserves,
The amount of hydrocarbons released to the environment, the size of the area affected and the cost of clean-up operations after the event (Oudeman 2005),
The complexity of the efforts to regain control over the well.
More difficult to quantify, in case of an actual blowout, is the impact of issues such as negative publicity, loss of reputation and credibility. A representative example is the infamous 1979 blowout of Ixtoc-1 in the Gulf of Mexico (Lugo 1981), which led to an oil spill of an estimated 3.5 million barrel (Figure 1, from the archives of Emergency Response Division, Office of Response Restoration, National Ocean Service, National Oceanic and Atmospheric Administration), fouling a considerable part of the Texas coastline. Between August 6 and September 13, 1979, approximately 24,000–32,000 bbl of oil were deposited onshore in Texas, mainly on the beaches.
To generate an estimate for the blowout rate, some form of nodal analysis is generally applied, matching the inflow performance of the well to the vertical lift performance (Oudeman 1998). For onshore and platform well surface blowouts, the blowout rate is often controlled by the sonic outflow conditions, since the pressure in the well will exceed atmospheric pressure by a factor two or more. This makes accurate modelling of the total system performance less important.
This does not apply to blowouts at seabed of subsea wells. The wells blow out against the hydrostatic pressure of the water column at the mudline. In most cases this pressure will determine the flowing wellhead pressure of the blowing well. Sonic conditions will not develop at the wellhead and the total system performance will have to be taken into account to obtain an accurate estimate for the blowout rate.