Review of flow rate estimates of the Deepwater Horizon oil spill

Abstract

The unprecedented nature of the Deepwater Horizon oil spill required the application of research methods to estimate the rate at which oil was escaping from the well in the deep sea, its disposition after it entered the ocean, and total reservoir depletion. Here, we review what advances were made in scientific understanding of quantification of flow rates during deep sea oil well blowouts. We assess the degree to which a consensus was reached on the flow rate of the well by comparing in situ observations of the leaking well with a time-dependent flow rate model derived from pressure readings taken after the Macondo well was shut in for the well integrity test. Model simulations also proved valuable for predicting the effect of partial deployment of the blowout preventer rams on flow rate. Taken together, the scientific analyses support flow rates in the range of ∼50,000-70,000 barrels/d, perhaps modestly decreasing over the duration of the oil spill, for a total release of ∼5.0 million barrels of oil, not accounting for BP's collection effort. By quantifying the amount of oil at different locations (wellhead, ocean surface, and atmosphere), we conclude that just over 2 million barrels of oil (after accounting for containment) and all of the released methane remained in the deep sea. By better understanding the fate of the hydrocarbons, the total discharge can be partitioned into separate components that pose threats to deep sea vs. coastal ecosystems, allowing responders in future events to scale their actions accordingly.

Fig. 1

Location of the Macondo well/Deepwater Horizon spill in the Gulf of Mexico ∼50 miles (80 km) southeast of the Mississippi Delta. (Modified from the US Geological Survey)

Fig. 2

Schematic diagram of damaged riser at the Macondo well spill site. Most hydrocarbon release occurred in the areas highlighted by black rectangles, emanating from the kink in the riser immediately above the blowout preventer (BOP) stack and the open end of the riser/drill pipe before June 3 and through the lower marine riser package (LMRP) after the damaged riser was cut away.

Fig. 3

Daily averages of GOR as a function of oil produced. The general trend indicates that the GOR drops as a greater percentage of the total flow is produced to the surface but with considerable scatter. If the entire flow was captured, the GOR would match the true GOR of the well. The horizontal line at a GOR of 1,600 is equivalent to the surface GOR of the IGT-8 sample taken by WHOI on June 21, which was obtained at the point of exit at the wellhead and is taken to represent the true GOR of the Macondo reservoir fluids escaping from the well. Assuming that GOR samples acquired at the surface would trend linearly to the actual GOR (IGT-8 sample collected by WHOI at well head), then the intercept should indicate the total oil flow rate on June 21. The best-fitting linear trend to the GOR data as a function of surface oil yield indicates that, if BP had been able to capture the total flow at a GOR of 1,600, then the oil captured would have been 57,000 BPD on June 21. The 1 SD uncertainty on the best-fitting line to the GOR data allows the flow rate at the GOR of 1,600 to be between 48,000 and 66,000 BPD. (Modified from ref. .)

Fig. 4

Summary of flow rate estimates from Table 1. The continuous curve represents the August model for the evolution in flow rate throughout the oil spill incident obtained by extrapolating the 53,000 BPD estimate from DOE at the time that the capping stack was closed (12) back to the beginning of the incident using the reservoir depletion model of Hsieh (13). In this extrapolation, a flow rate increase of 4% was estimated to have occurred when the riser was severed, and a decrease of 4% was estimated when the capping stack was installed. The stippled band represents a ±10% uncertainty in the August flow rate model. Compared with this August model are flow rate estimates from in situ ocean data plotted as a function of the day that the data for that flow rate were collected. Flow rates were typically reported at later dates. The postriser cut estimates all used data obtained on event day 45, but they are slightly offset from each other in time for ease of viewing. The upper bounds of the postrise cut UCSB estimate is shown as an arrow where it goes off the chart. The PIV estimates from the various sources are pooled together, with the thick part of the bar showing the range of the means and the thin part showing the range of the SD.

Fig. 5

Hydrocarbons (oil and natural gas) escaping from the end of the riser tube after it was severed on June 3 immediately above the Macondo well BOP stack. (Modified from BP video from ROVs.)

Fig. 6

Schematic diagram of possible well flows modeled by the well modeling teams from the DOE National Laboratories. (A) Scenario 1: flow initiates in the annular space between liner and casing, flowing through a breach at the top (in the seal assembly) into BOP and then riser; depending on flow restrictions in BOP, some flow may reenter the 9 7/8-in casing to flow down to enter the drill pipe. (B) Scenario 2: flow initiates in a breach of the 7-in casing, flowing up the casing. Some flow enters the drill pipe, and some continues up the casing to BOP. (C) Scenario 3: flow initiates in the annular space between liner and casing, entering a breach in 9 7/8-in casing and continuing to flow up inside the casing. Some flow enters the drill pipe, and some continues up the casing to BOP. [Modified from Guthrie et al. (14).]