Drilling in the Chukchi Sea: Is Shallow Safer?

Shell's Fennica icebreaker on it's way to Burger J. Capping stack in front

Since 2010 Royal Dutch Shell has consistently argued that drilling in the Arctic is no big deal; that challenges in the Arctic are far less intense than those encountered by BP's Deepwater Horizon in the Gulf of Mexico that resulted in the most catastrophic and damaging oil spill in our nations history.

Shell's cavalier attitude towards Arctic drilling is reflected in the following remarks made by Shell's top Executive for the Arctic, Ann Pickard, in a recent interview with Bloomberg Business News:

"the Burger J prospect lies beneath only 140 feet of water, and its crude oil reservoir is under relatively low pressure as these things go.

In contrast, BP’s ill-fated Macondo well lay beneath a mile of ocean and was under extremely high pressure.

“The blowout scenario is quite different than the case of BP’s Macondo,” Pickard says. Burger J “is the kind of thing we’ve done all over the world for decades.”

Of course the Gulf of Mexico is not prone to massive ice flows, fierce out of the blue storms or raging winds. Nor is it summer foraging ground and nursery to a large proportion of the planet's walrus, beluga and bowhead whale populations that have sustained Native Alaskans for millennia.  Neither is it warming, according to various estimates, 2-4 times faster than the rest of the planet as a direct consequence of our overindulgence of fossil fuel.

Setting aside these apparently trival differences, we asked a very narrow question in terms of expected well pressures and Worse Case Discharge scenarios: is off shore drilling in the shallow Chukchi Sea really safer than drilling in the deep waters of the Gulf?

This is what we found.

Chukchi Sea Not a Low Pressure Field

To begin with, using depth of water as an indicator of wellhead pressure, as Shell's top Arctic Executive does, is backwards.  Five thousand feet of water weighs much less than five thousand feet of compressed subsurface strata (rock and mud). For two equally deep wells, the one that has significantly deeper water but correspondingly less subsurface strata will have lower bottom-of-hole formation pressure, not higher, as Pickard implies.

It's not clear that the Burger J well will be shallower than the Macondo well since Shell declined to make their targeted well depth public, claiming it's Shell's "proprietary" information.

Shell's most recent Worst Case Discharge (WCD) estimate (see Table below) predicts an initial flow rate of 23,100 bbls/day; barely 37% of the initial Macondo flow rate of 62,000 bbls/day (1).

Table 2.g-1 from From Shells March 2015 Exploration Plan

Different geology between the Chukchi and the Gulf of Mexico can account for some difference in pressures, but no conceivable geological differences could account for Shell's calculation that pressures in the Chukchi would be less than half of the pressures in the Gulf of Mexico. Especially when the very abundant natural gas deposits that Shell is telling it's investors will fill their pockets are taken into account.

Assuming the WCD flow rate would be roughly proportional to the pressure difference between Macondo and Burger J, a more realistic WCD flow rate from Burger J would be between 50,200 and 92,400 bbls per day -- as compared to Shell's estimate of 23,100 bbls/day and of the Macondo's 62,000 bbls/day.

It should be noted that Shell's capping stack is only rated for 25,000 bbls/day maximum.

The Chukchi Sea is also thought to have a much greater proportion of gas to oil than the Gulf of Mexico where the gas pocket sank the Deepwater Horizon. It would follow that the Chukchi also has a greater risk of disaster than the Macondo prospect resulting in a greater magnitude WCD.

The Deepwater Horizon in flames before it sank.

Math Geek Alert: Our Calculations

The formation pressure is roughly dependent on depth and how heavy everything above it is (as well as a number of geological factors which can affect the pressure, but generally...).

We can calculate and compare the minimum expected formation pressures of the Deepwater Macondo and Burger J by adding the pressure of the water plus the pressure of the subsurface strata for each (sea water weighs .44 pounds per square inch (psi) per foot of depth, and subsurface strata (layered zones of sandstone/slate/shale, etc) weighs around 1.15 psi per foot of depth).

The Deepwater/Macondo Well

The Deepwater Horizon was in 5,067 feet of water, and the well was bored through an additional 13,293 feet into the subsurface strata below the bottom of the ocean.  Therefore, the well went to a total depth of 18,360 ft.

Therefore, the formation pressure at the bottom of the Macondo well was ~17,516 psi

The math: (5,067 ft x .44 psi) + (13,293 ft x 1.15 psi) = 17,516 psi

The rate of discharge at the wellhead is roughly proportional to net formation pressure at the wellhead (all else being equal).  To get that number, we subtract the ocean bottom water pressure and the weight of the column of oil (let's assume .40 psi per foot of depth) in the well from the formation pressure at the well bottom.

Therefore, net formation pressure at the Macondo wellhead was ~9,969 psi

The math: 17,516 psi - [(5,067 ft x .44 psi) + (13,293 ft x .40 psi)] = 9,969 psi

The Chukchi Sea/Burger J Well

Shell is drilling the Burger J well as we write.  It can't drill into the zone where oil and gas is expected to be encountered until the famed Fennica arrives with the capping stack (a second-tier emergency device intended to stop an oil spill in case the blowout preventor fails).

The ocean is 140 feet deep at the Burger J site.  We can estimate that Shell will drill to at least 11,000 ft and not more than 20,000 ft -- the rating limit of both of Shell's drill rigs (although Shell hasn't publicly revealed the maximum depth they intend to drill -- this information is tagged "proprietary").

Therefore, the minimum net formation pressure at the Burger J wellhead is ~8,083 psi

The math:

(140 ft x .44psi) + (10,860 x 1.15 psi) = 12,489 psi
12,489 psi - [(140 ft x .44 psi) + (10,860 ft x .40 psi)] = 8,083 psi

This turns out to be around 81% of the wellhead pressure at Maconda.

The maximum net formation pressure at the Burger J wellhead is ~14,895 psi

The math:

(140 ft x .44 psi) + (19,860 ft x 1.15 psi) = 22,900 psi
22,900 psi - [(140 ft x .44 psi) + (19,860 psi x .40)] = 14,895 psi

This turns out to be about 149% of the wellhead pressure at Maconda.

Taking the relative formation pressures as a rough measure of relative initial flow rates, we get a potential Chukchi WCD flow rate of between 50,220 and 92,380 bbls/day.  Compare this to Macondo's initial flow rate of 62,000 bbls/day.

The math:

62,000 bbls/day  x 81% = 50,220 bbls/day
62,000 bbls/day  x 149% = 92,380 bbls/day

The math on how many days it would take Shell to shut off a blown out well-head from a large, high pressure reservoir is anybodies guess.

If, like with the Deep Water Horizon, a random spark ignites the gas causing the entire rig to explode and sink, the shallow ocean turns out to have a vastly different consequence.

Shell's Noble Discoverer Drilling Rig (drilling Burger J now)

From top to bottom, both of Shell's drill rigs are taller than the Chukchi Sea is deep at the well site.  In case of a fire and explosion, the rig would have nowhere to go but on top of the spewing, gushing well.  In which case, that capping stack Shell's been waiting for will be useless. 

The math:

Add the regulators (BOEMs) own calculation of a 75% probability of a catastrophic oil spill (should the Alaskan Arctic lease fields be fully developed) with the hard scientific fact that Arctic hydrocarbons must stay in the ground if we are to avoid 2 degrees Fahrenheit of planetary warming, and you get 100% chance of undoing Alaska's subsistence-based coastal village cultures and ensuing global chaos.

References

(1) Assessment of Flow Rate Estimates for the Deep Water Horizon/Macondo Well Oil Spill, National Incident Command, Interagency Solutions Group, Flow Rate Technical Group.