As is typical in the oil and gas industry, the first step to decommissioning a well is to obtain the required permits from the governing authority. Typically, the permit application process requires that the operator has a detailed plan about how the well will be plugged. This plan is usually required when the initial permit to drill is issued, but it may need to be revised due to changes in technology since production began or unexpected geologic conditions.
Once the required permits have been obtained, operators will begin the process of removing casing from the well. The value of the casing is often enough in itself to recoup the cost of plugging the well, which typically ranges between $50,000 and upwards of $100,000 for onshore wells.
Removing Casing
Because casing integrity is key to effective plugging of a well, only casing located above the location of the shallowest cement can be recovered. Usually, a tool is first lowered to determine the top of the cement. Next, explosives or cutting tools are used to sever the pipe above the cement plug. Finally, powerful hydraulic jacks are used to break the pipe free and lift it from the well, one segment at a time.
Once removed, the used casing might sell for 50-70% of the price of new casing.
Most jurisdictions have strict guidelines that require that certain zones of any abandoned well be plugged. Typically, these zones include parts of the well within a certain distance of, or within, any aquifers, the former producing zones of the well, and the part of the well closest to the surface.
After plugging, the well should consist of cement plugs alternating with drilling mud. The plugs prevent the migration of fluids, and the drilling mud provides stability and ensures that pressures within the well are properly contained. The uppermost part of the well is often cut off and plugged at a level below “plow depth” to ensure that it doesn’t pose a hazard to future land users. Uses of the land beyond farming were not always foreseen at the time of plugging and have sometimes posed problems for real estate developers many years later.
During the plugging process, a regulator is often required to be on site to witness the plugging, make sure it is done correctly, and record the details of the process in case of future problems.
In the early days of the industry, and prior to most regulation, operators plugged wells with scrap material, tree trunks, cloth, and other junk, which were topped with a bag or two of cement. Modern well plugging operations depend on sophisticated techniques that ensure that the well will remain sealed for hundreds of years.
The most common technique in use today is referred to as the balanced plug method. The video below will show you how this method works and why it is effective.
Transcript
Balanced Plug Jobs – Alfred William Eustes III – Colorado School of Mines
When the producing life of a well is finally over, operators are required to plug the well.
This protects groundwater supplies from contamination by fluids rising from deeper layers, protects the atmosphere from escaping methane, prevents accidental explosions, children falling in the hole and who knows what else? It should always be done and regulations demand it.
The ultimate goal of a plug job is to fill the various sections of the well completely with cement, but it’s a bit more complicated than just pumping lots of slurry into the well.
First off, they usually don’t cement the entire hole. The hole probably cannot hold an entire column of cement weight.
They are required to cement specific sections of it, and these specifics vary from state to state.
In addition to that, if the cement is contaminated by things in the well, it won’t set and seal properly and an improperly sealed plug isn’t a whole lot better than no plug. Preventing contamination of cement by drilling fluid or other contaminants or debris is one of the goals of a balanced plug job.
By the time we’ve reached the cementing stage of the plug and abandonment phase, all casing that can be removed from the well will have been salvaged. It’s still valuable, even as scrap.
Now, something to remember is that we never want to have an open hole with nothing in it if we don’t have controls on the surface to shut off flow. So when we engage in a plugging operation, we’re going to put drilling fluid in the hole to offset pore pressure, just like we do when drilling a well.
Once the hole is deemed ready, it’s time to plug it.
If the plug isn’t going to start at the bottom of the hole, a metal or plastic bridge plug can serve as the foundation for the cement plug. We may just leave drilling mud in the hole beneath that.
A bridge plug is run into the hole, which is still filled with drilling mud, mind you, and at the predetermined depth, it is set. When that happens, it clamps onto the walls of the well, sealing off the hole from movement of anything in either direction. Bridge plugs are secure, but to really secure the well, cement is added to reinforce the plug for the long haul.
To do the cementing, a length of drill pipe or drill string is lowered to the intended depth of the bottom of the cement plug. This may be directly above the bridge plug.
Often, the drill pipe will be tipped with a several hundred foot long, small diameter pipe called a “stinger”. The reason for using a stinger is that it can be pulled out of the cement slurry without disturbing it very much, as compared to a bigger pipe. It adds a layer of complication ‘cause sometimes cement and other fluids have difficulty flowing through the small diameter stinger.
Here’s how the plug actually happens.
Several different fluids are pumped through a drill string and stinger until they exit the bottom of the string and rise up the annulus to the desired height. A spacer fluid, with a density between the density of the cement and that of the drilling mud, is usually pumped in first.
This pushes the drilling mud in the annulus up out of the well before the cement arrives so as to minimize the contamination of the cement.
The cement is then pushed through the pipe to fill the annulus and the inside of the stinger. Before the cement hydrates and sets up, the stinger is slowly lifted from the hole and the cement inside and outside the stinger mixes.
Because the materials on the inside and outside of the stinger are mixed, the boundaries between the cement, and the spacer and the mud inside the stinger need to be the same as the boundaries in the annulus.
Since we typically pump in one direction, down the inside of the pipe, the fluids are put in the well in sequence – first drilling mud, then spacer fluid, then a measured amount of cement, then more spacer and finally more mud.
The challenge lies in pumping the exact amount of each fluid so that the levels inside and outside of the stinger are the same; that’s why it’s called a balanced plug job.
An additional complication occurs when the stinger is removed from the cement – the level of fluids in the well falls to compensate for the volume of the metal making up the pipe itself.
Fortunately, since the size of the borehole and the stinger are known, and the metal volume of the stinger pipe is known, cementers can calculate exactly how much spacer, mud and cement to pump in sequence to balance the plug.
Typically, operators underbalance the plug – they pump slightly less mud into the stinger at the end of the plugging operation then their calculations require.
This makes up for the tendencies of the fluid to move up with the drill pipe as it’s removed because they can’t escape through the stinger fast enough. This process increases hydraulic head and can result in the offset of fluid boundaries outside of the stinger. By under-displacing, operators can cancel out this effect.
If all goes well, a plug like this should last hundreds of years, maybe thousands. We really don’t know. We haven’t been around long enough to find out, but we think that if they’re done properly, they’ll provide security equivalent to the level found in the original rock.
Removing Equipment
Once the well has been plugged, operators typically have a certain amount of time to remove any remaining surface equipment. This equipment could include tanks, separators, broken machinery, and scrap. Once the surface equipment is removed, the surface reclamation process can begin.
Images: “Plugging Application” by Top Energy Training