Underbalanced Drilling

How to torpedo an underbalanced drilling project

This tongue-in-cheek article looks at all the bad logic used in carrying out a UBD operation. It is based on conversations with operators and drillers, derived from real-world experience

Les Skinner, Cudd Pressure Control, Inc., Houston

There has been a long list of papers written over the past several years on the correct way to set up underbalanced drilling, completion and workover projects. However, nobody, to the author's knowledge at least, has written an article on how to keep a successful one from occurring. Here are some guidelines to guarantee that a successful underbalanced operation will never occur on your watch.

THE SCENARIO

You have just completed your 30th year as a drilling engineer for an operator with a variety of fields. Some of them are partially depleted. There is a need for a number of replacement wells to maintain production and meet corporate goals. All of your experience has been with overbalanced drilling, but your boss tells you it's time to get into underbalanced drilling to avoid near-wellbore reservoir damage.

You have been asked to run the project although you know little, if anything, about underbalanced drilling. You admit you know even less about underbalanced completions and live-well workovers. You also know that if this UBD stuff catches on, you'll be saddled with it for the remaining 5 – 10 years of your career. As an old dog, you really don't give a rip about learning new tricks. So, how does one go about killing a UBD project?

PRESCRIPTION FOR FAILURE

Here are 11 sure-fire steps to prevent future UBD work. This may not kill the first project, but the second one doesn't stand a chance:

1. Do not go through a rigorous candidate selection process. All the literature talks in glowing terms about the success of UBD all over the world. To make a good candidate selection, time and money to analyze prospects must be expended. That requires getting someone experienced with candidate screening procedures. Instead of investing the money necessary to make a good candidate selection, just tell management that UBD works everywhere on every type of well. Then, just pick one – preferably one that's already on the budget and scheduled to spud soon. That way, they won't have a chance to change their minds.
2. Don't plan the UBD operation. Now, after at least 10 good years of UBD work, there is enough industry experience somewhere to drill underbalanced without all that modeling. Nobody believes models anyway, so just go for it and don't let the reservoir or UBD performance modeling interfere with the job. Also, without a model, there's no way to tell whether the well conforms to the model, so there's no accountability for a bad result. It just didn't work out. As long as it's underbalanced, there really isn't any difference between drilling 100 psi underbalanced and 1,000 psi, is there?
3. Don't worry about underbalanced well control issues. You know there's an IADC-certified course on UBD, but you haven't taken the course and you don't really see a need to do that for one well. If the well starts kicking too hard, just bullhead heavy mud down the annulus. After all, that's all that was needed the last time there was a kick in the annulus. Worked fine then; don't see why it won't work again. After the well is under control, you can always go back to underbalanced drilling. A little mud never hurt the formation. Besides that, you've been doing real well with overbalanced drilling for years (except for that pesky lost circulation problem and stuck pipe on the last 10 wells).
4. Don't train the rig crews on underbalanced operations. Rig crews will do what they're asked to do. Just because there's a kick in the hole, it doesn't mean they need to do anything different than they always do on a rig. Get the cheapest rig and let the drilling contractor deal with educating his crewmembers. You can always let old Earl take care of the rig. He's been supervising rig operations for over 40 years. He'll mill on the crews a little, and they'll do whatever he says. If they get scared, they can just quit. The drilling contractor can always get more hands.
5. Don't spend any time or money on drilling fluid design. Most underbalanced operations use gasified fluids according to the articles in the literature. So, just pick a vendor and pump nitrogen into the conventional mud system. There's probably some old surplus mud in storage that can be purchased inexpensively. The nitrogen can provide all the underbalance necessary. If the well won't flow while drilling, one can always slow the mud pump down a little or turn up the nitrogen a notch or two. Then, just let 'er rip. Maybe foam is a good option since it has been used some in the past for UBD. Design? Just throw some soap in the mud and pump it downhole. It will suds up as it goes down the drill pipe. No problem.
6. Don't bother with MWD, LWD or PWD information. If gasified fluid is used for drilling, conventional pulsed data will not work. So just don't run the downhole tools – that saves money. The pulsed information is never used anyway. Besides, this is supposed to be a straight vertical hole. You can just run a packed hole assembly and drill like crazy. That increased penetration rate with UBD is really nice. Just pour the weight to it and get deeper cheaper. That should really impress management! There is something now called electromagnetic data transmission from downhole measuring tools, but there's no need to spend money on data we don't really need in the first place.
7. Limit surface separation equipment to only what can be rented down the road. The industry has used gas busters to handle kicks for 60 years. Now the people that do this stuff are using four-phase separators and other high-tech stuff. Just put all the fluid in a big pit and let it gravity segregate after the gas is out of the system. Pressured separators? Not necessary. It will settle out eventually and, if we get a little oil or condensate on the ground, the spill control guys can always clean it up.
8. Don't worry about HAZID and HAZOP plans. This isn't going to take long with the high penetration rates, and there's not really going to be much exposure to hazards while underbalanced drilling. So there's really no reason to go through an exhaustive process of identifying hazards and preparing steps to deal with them. Just ignore them, they might just go away – then all the time spent on these silly plans won't be wasted.
9. Don't worry about safety systems on the rig. The nitrogen generation equipment and separation equipment is going to be so simple on this first project that we don't really need any indicators of remote equipment problems on the rig floor. Just keep drilling and let the vendor that rents us the equipment worry about keeping it running. Heck, you don't even need anyone there to operate it. If it goes down, just bypass it and go straight to the pit. Then, just call them out in the morning.
10. Once the well is drilled, kill it for logs and completion work. A little heavy mud in the hole should be OK to permit rigging up logging tools and running the liner. You can cement it just like in the past. You can even hold a little back pressure on it so there won't be any gas coming up the backside. Formation damage due to mud resting on reservoir rock, unprotected by filter cake, is really not a problem. The filter cake will eventually build up, so don't worry about it. You can always get rid of the skin with some acid. You can perforate it underbalanced, too. Should take care of any skin since it will all be near the wellbore face.
11. Don't test the well to find out if UBD was successful. Drill it and get it online. If it makes more than other nearby wells, just choke it back. If it produces too much, management will want to drill more of these, and the company really doesn't want to gut the reservoir. So just hold 'er back a little and they'll never suspect a thing. Write a memo to the production guys and place a maximum flowrate on the well. After all, flowing a UBD well will be just like an overbalanced well, and sand production cannot be tolerated. Oh, yes – and no bottomhole pressure buildup tests either. They're really not that important, and they might indicate some remaining skin.

CONCLUSION

So there you have it: a perfect recipe for failure. Follow these steps, and you will not have to drill a second UBD well. Management will hate UBD so badly after all this that they're bound to let you go back to overbalanced drilling, which everyone knows is better and safer. No need to change the corporate culture to take advantage of a new technology, you know. We've been successfully drilling wells for a long time. UBD? Well, we've never done it that way before.  

THE AUTHOR
	Les Skinner, is a well control engineering manager for Cudd Well Control, a division of Cudd Pressure Control. He earned a BS degree in chemical engineering from Texas Tech University in 1972. He has 20 years' oilfield experience with major/ independent operators as a drilling/ production engineer, plus 11 years with well-control companies, covering 14 US states and 13 international countries. He has designed/ supervised drilling/ completion programs for deep and horizontal wells. He has worked on several major blowouts, including those in Kuwait during the Gulf War. Mr. Skinner is a licensed professional engineer in Texas. He has published several technical papers and holds three patents. He is a member of SPE, TSPE, NSPE and AIChE.