Lost Circulation

What is Lost Circulation?

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.

What are your options?

If you anticipate losses in a a formation, you can lower the mud weight before entering this formation.

Minor adjustments

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.

Adding material

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

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

Plug back

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.

Resins

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