Production Casing & Tubing
So what purpose does production casing serve, when does it get installed, and what does it have to do with production tubing? Stay tuned because we’re about to give you the quick rundown on the basics of casing and tubing in oil and gas wells. Thanks for joining us for another Oilfield Basics video blog. My name is Derek Krieg and we here at Oilfield Basics are trying to build the go-to educational platform for the oil and gas industry. There’s so many awesome topics to cover and if you like what we’re doing or find this video helpful, please be sure to like, subscribe, comment, and share this with someone who you think could benefit from our materials. I’d mean the world to us. Anyways, let’s dive in.
So let’s first get the basics down pat. When the drilling rig arrives on location, it has one goal- to create a wellbore to (and through) the target formation as designed by the drilling engineer. Many times the end of the well, or TD, will be thousands and thousands of feet away from the surface. As you can imagine, one continuous wellbore cannot be drilled that entire distance. Less consolidated or loose rock at the surface could easily cave in, meanwhile high pressure formations deeper in the well could begin trying to flow. Also, we want our drinking water protected, right? This is why multiple “strings” of steel pipe called casing are inserted into the well every so often. The rig will drill down to a predetermined depth, bring the bit back to surface and then lower casing that’s just a little smaller than the open hole diameter into the well. Cement is then pumped down the inside of the casing and then is displaced to the outside of the casing with a “wiper plug” and fluid. Now that section of the wellbore is isolated and protected from any outside forces, fluids, etc.
Once casing is installed and cemented, the rig will then continue drilling with a smaller sized bit until they reach TD or need to install the next casing string. The last string of casing that is installed is called the production casing. This casing string is installed through the production or pay zone of that well. It typically is cemented into place as well, so that the completions or hydraulic fracturing treatment will be effective. During fracing operations on the shale wells, the production casing is perforated which allows the producing formation to be able to flow its hydrocarbons into the wellbore, and now the well is ready to begin producing.
Now please understand that hydrocarbons flow from the reservoir into the wellbore due to pressure difference. Since the wellbore is at a lower pressure than the formation, the hydrocarbons will flow into the well through the perforations in the production casing. The hydrocarbons will then flow up to the surface, because again, the surface pressure is much lower than the bottom hole pressure.
However, once the reservoir pressure decreases, it will not be able to successfully lift the liquids to the surface as easily because the pressure difference will be less. This is because the production casing is 4.5 or 5.5 inches wide, which is too large of a diameter to produce through because you won’t be able to hit the critical velocity (or flow rate) required to successfully transport the liquids to the surface. Thus, one well known method of solving this problem is to install production tubing. Production tubing is typically anywhere from 1 7/8″ to 2 7/8″ wide. This is literally a string of tubing that is lowered into the well by a workover rig later in the well’s life. Some companies even just install production tubing before they begin producing the well so that it won’t lose its ability to lift fluid naturally as soon.
If that seems too complex, just picture blowing through a straw. You’ll remove liquid in the straw more efficiently if the straw is a smaller diameter compared to a straw that is a large diameter. It’ll be much harder for us to blow the liquid out of the bigger straw. This is the same thing with the reservoir.
Now let’s quickly analyze this wellbore schematic. This is a well with production casing and tubing. When production tubing is installed into the well, it is typically just lowered to the curve of the lateral. Also, once tubing is installed, operators will not typically produce from the production casing unless the well is operating on rod lift. At surface, the casing wing values will be shut, and the well will be producing from the tubing through the production tree portion of the wellhead.
Note that the space between the outside of the tubing and the inside of the casing is called the “annulus.” If someone says the well is producing from the annulus, they mean it’s being produced through the production casing. This infers that the annulus is open. Some companies might utilize packers to seal off the backside, or annulus for a few different reasons. That’s a whole other video for another time, but just realize that this could be another scenario that you might run into. This kind of setup is common on injection wells, but again, is another video of its own.
Hopefully this video helped cleared up the distinction between casing and tubing in a typical producing horizontal well. If you want to learn more about well design, drilling ops, production equipment and more, check out our courses at www.OilfieldBasics.com/Learn. Also, be sure to share this video, give it a thumbs-up on youtube, subscribe to our channel, and follow us on our other social media platforms. We’ll see you in the next one.