Warning: technical / engineering information. Proceed only if you want to understand how this weird oil drilling stuff works. For the real basics, you can also read An Introduction to Drilling Offshore Oil Wells, a fairly simple overview of the entire drilling/exploration process.

Ok, by now you’ve probably read something about parts of oil well drilling in the media because of the BP Deepwater Horizon Oil Blowout/Spill, but you are probably having trouble visualizing how the actual drilling of complex directional wells proceeds. Directional means the well bore angles in, like the current BP Gulf spill relief wells, ie, they go down vertically then angle and are redirected towards a target.

In the video below, the target is a narrow band of oil in a geological layer. It’s an animation that shows fairly clearly the steps involved in drilling a curved well. In the relief wells, the target is a roughly 16″ steel casing, 18,000 feet below sea level, and about 13,000 feet below the ocean floor.

Cool, no? In this video you see how the basic directional drill works, how casing is inserted after the horizontal drill is removed, how cement is pumped up around the casing to create a solid bond (what the CBL, cement bond log that BP decided not to do checks, ie, that the cement is fully filling the space between the drill hole and the casing pipe.

At the end you see how a drill pipe is placed horizontally in the oil deposit, with multiple holes for accessing the reservoir’s oil along a great distance. Short in the video, but can be a mile or more.

Engineering fine points and explanations of the technology on directional drilling below the fold.

Technical Explanations of Directioinal Drilling Technologies

This is from today’s theOilDrum.com daily discussion thread, BP’s Deepwater Oil Spill – Improving the Clean-up – and Open Thread:.

Thread starter in
sub-thread only

Choice technical explanations:

ROCKMAN on June 21, 2010 – 6:39am Permalink | Subthread | Parent | Parent subthread | Comments top

ski — I think that was 200′ horizontally. Maybe another 2,500′ or so of hole to drill. And maybe one more csg set. And I think the latest plan is to drill past the blowout hole, measure exactly where it is using magnetometer, plug back and re-drill to the intersept. But plans do change.
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Petey Wheatstraw on June 21, 2010 – 6:43am Permalink | Subthread | Parent | Parent subthread | Comments top

Hey, Rockman,

How do they steer the drill bit?
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ROCKMAN on June 21, 2010 – 7:20am Permalink | Subthread | Parent | Parent subthread | Comments top

Petey — I find a good hard slap on the side of the head works well. No…wait…that’s with geologists…not drill pipe. Actually one basic technique is really simple: imagine you pushing a stick along a layer of sand the ground. The stick is straight to it goes straight. Now use a stick with a bend on the end of it. Push it and it turns in the direction of the bend. Want the stick to go in the opposite direction? Just flip it over and push.

The bottom hole drilling assembly (BHA) has such a “bent sub” on the bottom. There are electronic instruments in the BHA that indicate the orientation of the sub so it can be pointed in the direction you want it to go. The drill pipe doesn’t rotate in this situation. The bit turns via a “mud motor”: the drilling mud flowing thru the drill pipe turns only the drill bit. We call this “sliding”. But what if you want to go straight? Simple: rotate the drill pipe as you go forward. This way the bend is constantly going thru a 360 degree cycle so there is no prejudice to direction. We call this “rotating”. When the RW gets close we may see a lot of reference to “sliding” and “rotating”. There are other steering system designs but in the end they work pretty much the same: point the bit in the right direction and the rest will follow.

But remember Mother Nature is always in charge. You can angle the drill pipe to go right and it might go left. There’s is always a directional plan. And then there’s what Mother will allow.
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Frontier_Energy on June 21, 2010 – 7:59am Permalink | Subthread | Parent | Parent subthread | Comments top

Rockman:

To expand somewhat on your explanation and maybe a little clarification. The system Rockman described is a conventional downhole mud motor. The historic “bent sub” is actual an integral part of the motor.

The type of directional drilling system being used on the two relief wells, as also was used on the original wellbore is a “rotary steerable” system. No “sliding” is required as the BHA is rotated continously. There is an inclinometer behind the bit that allows real time sensing so that the well bore can be monitored real time relative to the desired well path. There is two way communication between the rotary steerable and the surface through mud pulses. “Deflection” is added on the fly, meaning that if the build is not aggressive enough you can add it as necessary and the rotary steerable will make the necessary bend angle internally needed without a requirement for stopping. This allows the directional driller to “paint the line” in regard to well path. It is the rotary steerable technology that has allowed them to make as fast a progress that they have over the past few weeks. Conventional mud motors and “sliding” directional technology would have taken much longer.
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aliilaali on June 21, 2010 – 9:10am Permalink | Subthread | Parent | Parent subthread | Comments top

put my 2 cents in the basket here as well…

there are many advantages for a rotary steerable as opposed to a mud motor….since there is no sliding there are many advantages interns of

1- better hydraulic performance interms of leading to better lift for cuttings to surface and such
2- better weight transfer leading to higer penetration rates
3- reduced well bore tortuosity — resulting in a cleaner wellbore with better LWD/MWD data
4- better downhole positioning
5- lesser BHA trips for correction runs
6- issues like ledging are minimized which are really the bane far as bent-sbus are concerned with high dogs in current applications

AutoTrack (BHI), PowerDrive (SLB) and Geo_pilot (Sperry) are three big ones ….

although certain hybrid systems are available too ….something like BHI VertiTrack is a true vertical drilling system that runs a mud motor but uses three expendable pads (aligned at 120 degrees all around) in the BHA just behind the stator to realign the wellbore to true vertical by using hydraulics to push out one or a combination of pads to realign slowly….but then this is strictly a vertical bore application…

there are also closed loop steerable systems available where top drive torque is not sufficient …..
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ROCKMAN on June 21, 2010 – 9:21am Permalink | Subthread | Parent | Parent subthread | Comments top

Yeah, yeah, yeah…rotary steerables are great but where’s the excitement in letting the computer do all the work. Sure the computer usaually does a better job. I’m just old school and like the extar pressure of making the steering calls myself. Just one more justification for drowning my anxieties with a big bowl of Blue Bell. [ed: Blue Bell is ROCKMAN’s favorite ice cream]
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Frontier_Energy on June 21, 2010 – 7:12pm Permalink | Subthread | Parent | Parent subthread | Comments top

For most application on land the conventional motor does just fine. A lot of offshore applications as well. But to get the fast P-Rate needed to get to the target on this bitch -we need rotary steerables. They may be using a conventional motor once they get near the target, maybe someday we will hear the “rest of the story”.

Calling a set is not that big a deal- standing on the rig floor making the set is the challenge with a conventional motor. Using weight and differential – rocking the table to get the string moving, etc. Nothing like having a bit take a bite and make about 3 wraps downhole. You sit there and wait until everything unwinds. Then after waiting – it doesn’t unwind – so you have to pick up- work the pipe and try to get a toolface.

In the present situation we need rotary steerables at high rate of penetration zeroing in on the target. The computer does not steer the motor- the deflection is made on the fly using the inclination at the bit for reference. This is compared to a conventional motor where you get your deflection based on what you anticipate the motor yield in deg/100′. You get your survey 50 feet behind what you have already drilled and have to calculate what you think you will get on the next set. Thats all fine and good until you get a change in formation as you mention “Mother Nature”

One issue with rotary steerable is cost – not relevent in the present situation.

There’s also a very well done technical discussion of directional drilling, with images and graphics at Pirate4x4.Com forums.

Pros/Cons of Directional, High Technology Drilling Methods

The basic thing this very complex drilling technology lets us do is drain existing and new reservoirs more quickly, leaving less for the future, and bringing the future oil depletion scenario just that much closer. They can, and do, for example, target a narrow band of oil, maybe only 10 feet tall, and snake the drill along it for a mile. Obviously, the well will supply a lot because it’s a straw with a lot of holes, as you can see clearly in the last part of the above video, but if you suck at very high pressure from a long tube you simply drain the target pool that much more quickly.

An obvious pro is that such methods make intercepting a 15″ or so pipe 18k feet below sea level possible, and within reason, practical, in order to kill an out of control well spewing oil into the Gulf of Mexico, or anywhere else.

Saudi Arabia, for example, national oil company Aramco is considered a global leader in such methods, despite persistent statements that if we could just get western oil companies into the 2nd and 3rd world NOCs our oil supply issues would simply vanish. This is a false though intellectually attractive belief that seems to appeal most to those least willing to conserve or change.

Oil Drilling Factoids I’ve Picked Up

1. Vertical wells are not actually vertical, but form a very gentle corkscrew shape.

2. The straight steel casing has no trouble bending around the curves of a directional well, or the corkscrew of a standard well, because it’s steel and flexible, and the curves are very gentle.

3. Failure to center the casing in the hole, using centering tools, can cause the cement to form a channel on the side with less pressure, related to fluid dynamics. This is why the recent BP discussion re Halliburton/Congressional investigation focused on BP’s decision to not use enough centering devices on their casing.

4. Pressure in rock is simply the weight of the rock column above the point you need to know the pressure of. Water is a bit less dense than rock, which is why the ocean bottom pressure at 5000 feet is about 2250 psi. At 18,000 feet it’s about 12k PSI in the reservoir, so that’s about 0.8 pounds per foot of rock vs about 0.4 pounds per foot of water, per square inch.

5. The column of drilling mud/fluid, which can have greater or lesser densities (mud weight, more from ROCKMAN) depending on the pressure it needs to hold back, has to total more than the pressure of the oil/gas in the reservoir, or you get a blowout. Weights vary between about 14 and 20 pounds per gallon, ppg. Anything more risks physically fracturing the rock at the end of the well bore, which is a very bad thing since then the drilling mud flows into the fractures and of course, the rock isn’t forming a solid seal any more.

Pressure (psi) = 0.052 X mud column height (feet) X mud weight (#/gallon) (src).

So to kill the BP blowout, for example, you’d take:
11,900 = 0.052 x 13,000 x mud weight, which gives: 17.6 lb mud weight to kill the well, give or take.

6. BP removed the drilling mud without checking adequately for the cement job’s integrity, and also failed to properly track the rate of mud returns, which is the main warning sign of an approaching well kick, or natural gas shooting up the well bore, and which is supposed to be dealt with by pushing more mud back down to kill the kick, and sometimes even the well itself. This is exactly what BP failed to do, and it’s why most other drillers are furious at BP. Rather than spend one more day, and about 1 million dollars drill platform rent, BP decided to take a short cut and replace the mud column with sea water, which doesn’t have the proper weight to hold back the oil/gas pressure.

And that’s it for today’s tech talk. All of the above points may be slightly wrong due to my misunderstanding various subtle points, but the overall idea should be more or less right.

Tags: video

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