In March, the SPE Board of Directors approved the publication of a technical report on the calculation of worst-case discharges (WCDs). The report, “Calculation of Worst-Case Discharge,” documents the consensus from an SPE-sponsored summit held in March last year.
At the summit, 68 subject matter experts met in New Orleans with the goal of improving the methods of calculating and reporting WCD scenarios. The attendees—representing operators, regulators, academia, and service providers—developed the report, which was made available for comment for 30 days and edited to include comments before being approved by the Board.
The focus of the technical report is on the calculation of WCD rather than well design or intervention. Its primary application is in the US Gulf of Mexico, although the report may be used for wells elsewhere.
Deterministic methods are proposed because of the wording of regulations and requirements for detailed well design and response planning. Probabilistic modeling and statistical analysis are unacceptable. All reservoir properties, the report says, should be best-estimate, success-case values based on sound geology, geophysics, and engineering judgment. All calculation parameter values should be explained and justified to ensure consistency and transparency. Parametric sensitivity is recommended for identifying the major variables and their impact on the WCD calculation.
Topics covered in the technical report include reservoir properties, inflow modeling, outflow modeling, total volume, special cases, and reporting. Future improvements could include flow correlations for high rates in large-diameter pipes, sonic velocity effects, and probabilistic methods.
According to the report, the flow scenario should be modeled over the duration of the spill to determine when the highest, single-day flow rate from the well occurs, which may or may not be the first day. In multiwell situations, the report says, it is important to remember that the WCD well may or may not be the first well drilled on the block or in the field. Each potential well location must be assessed and the WCD determined by the planned well that has the highest WCD flow rate.
The US Bureau of Ocean Energy Management defines WCD as “the single highest daily flow rate of liquid hydrocarbon during an uncontrolled wellbore flow event,” that is, the average daily flow rate on the day that the highest rate occurs under worst-case conditions (e.g., a blowout). It is neither the total volume spilled over the duration of the event nor the maximum possible flow rate that would result from high-side reservoir parameters, nor a distribution of outcomes. It is a single value for the expected flow rate calculated under worst-case wellbore conditions using known (expected) reservoir properties.
The full report is available at no cost at www.spe.org/publications/techreports.