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From Fossil To Fuel™
From Fossil To Fuel™

Episode 22 · 2 years ago

Episode 21 - Workovers, Snubbing Units and Service Rigs

ABOUT THIS EPISODE

A high level overview of the types of workover operations and why we do them.

My name is Brendan McDougall and I'm a professional engineer. Or the last decade I've worked in many different facets of the oil and gas industry. While I have a pretty solid technical background in oil to gas, I don't really know a whole lot about the other non technical departments that help run an oiling gas company. Recently I took a course to help develop my business acumen and better understand how the financial side of the business works. What a novel concept to educate the technical people on the business and financial side. I thought it would be a really cool idea to return the favor and educate the non technical people on the technical side. This is how the concept from fossil to fuel was born. Through these twenty four episodes, we will take a journey from how oil and gas was formed millions of years ago how it is refined into the fuel that runs our cars and heats our homes. Come join me on this adventure as we learn how the oiling gas industry operates from fossil to feel. The term workover refers to the process of pulling or replacing equipment down the whole or the well and making changes to a well workovers tend to be expensive relatively speaking, and so are usually only done if absolutely necessary. Perhaps some of the artificial lift equipment that we talked about previously down whole is malfunctioning and needs to be replaced, or maybe the completion technique that we used isn't working as well as it should have and needs to be redone. It could be something as simple as changing out the tubing because it's become corroded or because it's now...

...sized incorrectly. There are many different reasons for workovers, many and there are many different types of equipment used for workovers, but the two war courses of the industry, in the two will talk about today, are snubbing rigs and service rigs. Snubbing rigs and service rigs are similar in that they use joints of metal pipe that screw together to enter the well and perform whatever it is you need to do for the work over. The snubbing unit operates on wells that are under pressure, meaning that there's pressure at surface. So at the actual well headed surface, if you were to open it up, the well would flow because there's pressure pushing fluid up. The snubbing unit physically has to push the pipe for that reason, into the well through one, or usually multiple, closed blowout preventers or BOP's, to overcome the force created on the tubing by the wellboard pressure. So, if you think about it, you've got wellboard pressure pushing up. So in order to get your tubing in the well going down, you have to actually push it in and overcome that force that's pushing up because of the reservoir pressure. This act of pushing the pipe into the well against the wells pressure is called snubbing, and maybe a better visual for you to think about this is for anybody who's gone to a pool and you're playing with some sort of toy that's that floats. So whether it's a ball or one of those noodles that you can float on in the pool, anytime you take something that floats on water and you physically push it underneath the surface of the water, you can feel it pushing up against your hands. It wants to come up because it floats. So it's the same kind...

...of concept to help picture in your head what snubbing looks like. So you've now you're trying to just like you're trying to push the ball or the floating ball under the surface of the water and you have to physically push against it to get it underneath there. It's the same kind of thing with snubbing. We've got a force that's pushing against us and you have to overcome that force to push the tubing or whatever type of pipe that you're using into the well. But the service rig, on the other hand, is unable to push pipe into the well under pressure, and so therefore it can only perform workovers where the wells are not under pressure and there is zero pressure at surface, meaning that if you were to open the well, nothing would happen because there's no pressure pushing anything up. So when a well has zero pressure at surface, the well's is said to have been killed, and so it's a bit of a harsh term, but it refers to the fact that the well has lost its ability to push fluid out and is effectively dead from a production point of view. You may even hear engineers talk about killing a well. They're referring to the process of taking a well that is under pressure and bringing it to a safe state where there is no longer any pressure at surface, and this is accomplished by using something called a kill weight fluid. Kill weight fluid relies on the principle of hydrostatic pressure and to probably think of it a little bit like like a Tuger war. So if you have two people pulling on opposite sides of a rope and the two people have exactly the same strength, the rope shouldn't move, but as soon as one becomes stronger, the rope will start to move in that direction. Similarly, we have two forces acting...

...upon each other in the well. The reservoir pressure is pushing fluid out and up the well, but the weight of the fluid itself, or the weight of the column of fluid in the well due to gravity, is pushing back down. If the force of the reservoir pressure pushing up is greater than the weight of the fluid pushing down, then the reservoir pressure will push fluid out of the hole and the well will flow. However, if the weight of the fluid pushing down exceeds the force of the reservoir then the well will not be able to flow and will be considered dead. Now I need to make a distinction here, that if the weight of the fluid exceeds the force of the reservoir pushing up by a significant margin. What's going to happen is the column of Fluid is actually going to push fluid back into the reservoir. So we have to be careful hither when we're talking about kill weight fluid that you in a perfect world you want the force of your kill weight fluid pushing down to be equal to the force of the reservoir pushing up. Sometimes it's difficult to get that exact, so we may air on the side of being a little bit what we would call overbalanced, meaning that you want to air on the side of caution or air on the side that your kill weight fluid is a little bit higher than it needs to be, just so that when you open the well head up, you're not going to have boil and gas come flowing out at you. So once we have the kill weight fluid high enough to overcome the reservoir pressure, meaning the well is dead, we can then well, assuming that we did the math right and the well is dead,...

...now we can use a service rig to do whatever work over needs to be done. If, for whatever reason, we can't kill the well or it'll be too expensive to kill the well well, then we can rig up a snubbing rig or snubbing unit instead. And when I say too expensive, what I mean there is that the kill weight fluid, or the weight of the fluid to kill the well needs to be high enough that it overcomes the russure. Well, the higher the weight of your kill weight fluid, and by weight really it's the density. So the higher the density of your killweight fluid, the more expensive that fluid becomes, and so there's a bit of a cost benefit of analysis to figure out on whether it's cheaper to kill the well with a killweight fluid and use a service rig or just use a snubbing rig and do it while the well still under pressure. So the service rigs are kind of like mini drilling rigs, but cheaper and a little bit more mobile. The main part of the service rig, like a drilling rig, is a derek but it's on a big truck and the rest of the service rig components travel along in a vial com vehicle convoy with it. So these parts all get hooked up together on location, kind of like giant lego walks. There's a pump truck which is used to pump and store fluid. There's a dog house which serves as both the command center in the meeting room. There's a tool truck with all your tools and typically some crew vehicles to transport the rest of the crew. Like drilling rigs, the services rig crews have a hierarchy, with the driller at the top and the roughneck at the bottom, with most service rigs having a rig manager who oversees the crew. Once the service RIG is set up, it can pick up individual pieces, or joints as we call them, of pipe in the Derek and screw them together using...

...what are called power tongs. Power tongs are kind of like giant wrenches that clap onto the metal pipe and screw them together using hydraulics to provide the torque. A snubbing unit works in a similar way, but the setup is much different because once the pipe connections have been torqued up, now you need an apparatus to actually push them into the wealth to overcome that force. So, in respect to controlling the flow of the well boarcas remember, it's under pressure, snubbing units will need multiple pressure control mechanisms to eliminate any flow or spill while pushing or pulling the tubing from the well board. A snubbing unit can be quite tall for this reason, because if you've got all of these multiple pressure control mechanisms that are going to help contain the pressure and contain the fluid, these are going to be stacked on top of one another. So snubbing units, for that reason, are usually quite tall. In all snubbing applications a plug has to be installed at the bottom of the tubing string and this is just to prevent and flow from the well board coming up through the tubing. And so again I'll see if I can give you a bit of a visual. Hopefully you've got a pen with you. But if you've got a pen with you and follow along with me on this one, and you hold the pen in your mouth and you're holding one end in your mouth and you're holding the other end with your fingers and you're trying to blow the pen out of your mouth. So the pain is the tubing. The pressure you create with your mouth trying to blow it out is the same as the well bore pressure trying to blow the tubing out of the hole. The seal of your lips on the pen would be kind of like the pressure control mechanisms or bopeas...

...blow up preventers that are controlling flow from the well bore so that no oil and gas escapes, the same as no air escapes from your mouth, and your fingers are the snubbing unit holding onto the pipe and preventing it from being forced out of the well. So you're trying to push the pain into your mouth and overcome the force of yourself pushing it out with pressure in your mouth. Hopefully that helped paint a little bit of a picture. It does when I do it. If not, then just ignore what I just did there and we'll walk through how it actually works. Maybe that will make it a little bit clearer. So what happens is snubbing units do this using what are called traveling slips and stationary slips. So the slips are just devices that use little metal teeth to grip and hold the pipe. So if you're snubbing pipe into the well or you're pushing it into the well, what happens is the traveling slips will grip the top of the pipe and, once they've gripped it, push it into the well that's under pressure. These traveling slips are attached to to usually ten to twelve foot long hydraulic cylinders, and it's the cylinders that give the snubbing unit the ability and the power to overcome the force created on the tubing by the pressure on the well pushing it up. So, once the traveling slips engage and push the length of pipe in, the stationary slips, which are at the bottom, will now grip the pipe to prevent the wells pressure from pushing the pipe back out. So now that you've got your stationary slips holding the pipe in place that you just pushed in, the...

...traveling slips can then let go and move back up and grip the next section of at this time, once the traveling slips have gripped on to make sure that the pipe doesn't get pushed out, the stationary slips can let go and the traveling slips will push the next section of pipe into the well and it continues on like that. If you're snubbing pipe out of the hole, then it just works in reverse. The traveling slips will grab the pipe at the bottom and pull it up. The stationary slips will then grab the pipe so the traveling slips can come back down and grab the next section of pipe, the pipe that service Riggs and snubbing rigs are pushing and pulling. is typically the production tubing string. Not Always, but most of the time. And, like I mentioned before, it may need to be replaced for many reasons, but most often due to corrosion or resizing. And you're usually just you're probably wondering why would you resize it? Without getting into too much detail, you can resize your tubing string depending on how much production decline you have, and so it really just gets back to increasing your flow rate by minimizing the area or diameter of the pipe. So again, I don't want to go too far down the road there, but we would effectively resize the tubing string depends on how much your production rate has declined. Sometimes so we have equipment down whole whether it's some sort of artificial lift that we have to get out, and if it can't be pulled by wire line, then it's going to have to be done by a snubbing rig or a service rig, and most often it's a service rig. If we need to kill the well, then we'll need water or a dense or killweight fluid. Sometimes waters enough to do...

...the job, but if it's not kill weight fluid can be brine, which is the technical term for salt water. And if you didn't know, salt can be added to water to make it heavier, which is why you can float in certain bodies of salt water but you'll sink in a pool. And it's also why I talked about earlier why kill weight fluid can be expensive if you need a higher density, and it's just because, depending on what type of salt or really what kind of ion you have making up the brine, they can be more expensive to make. So typically heavier kill weight fluids are more expensive. Sometimes the tubing is fine and all of your artificial lift equipment is in good working order, but the reservoir just isn't producing at the rates that you're expecting. It could be that something has happened to the reservoir to plug it up or that it has been interfered with its ability to produce oil and gas. Either the completion technique that you tried has failed, or perhaps it's time to have it redone or do something new. Perhaps the wall was completed many years ago and new technologies have come out that can deliver much higher production rates and because of that, now you can justify the expense ultimately, though, it really just depends on the engineer's interpretation of what's going on down there and, as usual, the cost benefit analysis of doing the workover. Hey, guys, if you like today's episode, make sure you subscribe to the podcast. Unlike most podcasts that release an episode every week or two, I did all twenty four at once, Netflix style, so you can listen...

...to them all right now if you just hit subscribe. If you like today's episode, make sure you leave me a comment or thumbs up, or you can email me at from fossil to fuel at GMAILCOM, or look me up on Linkedin. I'm Brendan McDougall.

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