Lost Circulation is exactly what it says: You are losing your circulation of fluids, in this case in a well. Hence, you have a work string in the well and circulate fluids down this string while the returns come up the annulus (normally, you can also circulate the other way in some cases).
If you do not get returns, or you get partial returns, you have Lost Circulation. This is something that normally happens during the drilling phase of a well and most likely on exploration wells or wells drilled where one of the formations are somewhat unknown.
In principle lost circulation is possible any time you are circulating well in some way, it can be during completion or production phase or during a work-over operation also. In all cases the losses are leaking off to a low-pressure and permeable formation somewhere in the well.
Lost circulation can in some cases be very dangerous if it leads to a well-control issue. This can happen in those cases where you lose fluids to one formation, then your fluid level in the well drops and you may get below the pore pressure in another formation. If that happens, you can have a situation that can be very difficult to control. You have fluid coming out of one formation and disappearing into another one. In the worst cases this could lead to a blow-out.
During drilling, you depend on the mud coming out the annulus at a certain velocity so it can carry the cuttings with it and clean the well. If you do not have full returns, you may not be able to clean the hole properly and cuttings could accumulate in the annulus. This could lead to more losses and even stuck pipe in some cases.
Losses during drilling can also by itself lead to the drill pipe getting stuck. If a long part of it is sucked into the side of the wall, it does not take a huge differential pressure to make it very hard to get the pipe moving again. This is why any driller never stops moving the pipe for very long.
Large losses is also very expensive, some mud or drilling fluids can cost many hundred dollars per barrel. If you then lose a few hundred barrels per hour, the bill climbs rapidly. In severe cases, you may also not have enough materials onboard the rig to make up new mud to fill the hole to keep up with the losses, or you simply run out of materials.
If you anticipate losses in a a formation, you can lower the mud weight before entering this formation.
During drilling, some losses is very common and they do not really any problem other than the density and the viscosity of the mud can be adjusted to lover the pressure it exerts on the formation. You can also adjust circulation rates and speed of drilling.
Otherwise, lost circulation material can be added to the mud, these materials will normally filter off onto the leaking formation and plug or partially plug the off to reduce or eliminate your losses. There are many lost circulation materials, various types and particle or material sizes to cure various types (size) of leaks. There are also chemical solutions that are designed to cure upon contact with each other or the formation.
Cement is the most common sealing material for heavy and large losses. It is usually the first remedy to be tried after the normal lost circulation material the mud company provides can no longer stop the losses. This is because cement is well known, easily available, at least on a drilling rig, and a low cost material.
Despite that cement is a cheap material to pump, it can become expensive to use if it does not work. It is not uncommon that multiple cement plugs must be pumped before the losses are cured, this could take several days and possibly damage the well in other places. The cement has particles that bridges off in small cracks and pores, that can make it hard to squeeze off and seal small leak paths. In addition, a cement slurry is heavy, it gets diluted in water, particles settle, so that it may not work at all. Then if it is contaminated with hydrocarbons, it may not ever set up.
Download Free Ebook: Guidelines for setting Cement Plugs
In some cases, you can plug back quickly the formation and run casing and cement it in place on top of the low pressured formation, then you can lower the mud weight to what the lower pressured formation can handle before continuing. Now running casing is expensive and of course, you can only run some many strings in a hole. If the completion enables closing off a zone by closing a production sleeve, it can be very easy to seal off a zone. If it does not, zonal isolation becomes more complicated. Intervention in the well of some sort is then necessary.
Wellcem have proven multiple times that ThermaSet resin technology is more successful in sealing these types of issues than for example cement. Therefore, the Wellcem solution is gaining popularity as a better lost circulation solution in severe type of cases.
From the Well Integrity Blog: The resin curing process
As ThermaSet is not miscible with water, it will not mix with or be diluted by water or water-based fluids but reach the treatment area intact and undiluted. ThermaSet resin will be tailored to each application and the density can be adjusted from 0.7 to 2.5 SG to maximize the placement efficiency and match the drilling fluid so as not to exert additional hydrostatic pressure on the formation, nor allow gravitational forces to intermix the resin with other fluids.
We do not rely on plugging the loss zone, but rather penetrate it and cure inside the zone. This ensures a better and more robust seal and new losses when you start drilling again is much less likely than having the formation sealed with cement.
The placement methods are tailor made for each well and allows for pumping of the material through the bit. For ThermaSet, the efficiency in sealing off the formation and stop the losses in just one treatment is very common.
ThermaSet can handle large amounts of hydrocarbon contamination and still cure into a strong material. This means it will work well in formations that are both water or oil wet, it will penetrate the leak paths and formation and form a robust and long-lasting seal.
Our unique method is developed over many years with a large number of jobs done. If you want to get more insight to resin-based solutions like ThermaSet for Lost Circulation, click here:
You may also want to learn more about other areas of application like Control Lines and more, just scroll down ->
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The primary objective is to ensure that no leaks to surface exist and that no formation fluid migration occurs even many years after the well has been abandoned.