Lost circulation can be one of the more serious problems that can arise during the drilling of an oil well or gas well. Circulation is said to be lost when the drilling fluid, known commonly as "mud", flows into one or more geological formations instead of returning up the annulus.
The consequences of lost circulation can be as little as the loss of a few dollars of drilling fluid, or as disastrous as a blowout and loss of life, so close monitoring of tanks, pits, and flow from the well, to quickly assess and control lost circulation, is taught and practiced. If the fluid in the wellbore drops due to lost circulation (or any other reason), hydrostatic pressure is reduced, thus allowing a gas or fluid, which is under a higher pressure than the reduced hydrostatic pressure, to influx into the wellbore.
Another consequence of lost circulation is called "dry drilling". Dry drilling occurs when fluid is completely lost from the well bore without actual drilling coming to a stop. The effects of dry drilling can be as minor as destroying a bit to as serious as major damage to the wellbore requiring a new well to be drilled. Dry drilling can also cause severed damage to the drill string, including snapping the pipe, and the drilling rig itself.
Lost circulation falls into two main categories:
- "Minor losses" - This is where losses are between 6 and 470 barrels (1 to 75 m3), and remains within those amounts, or is ceased, within 48 hours.
- "Severe losses" - This is where losses are greater than 470 barrels (75 m3), or it takes greater than 48 hours to control or cease the lost circulation.
Total losses may also occur, where the return of fluids is completely lost to the surface. This may fall into either the minor or severe losses categories, depending on the amount of losses and the time involved in regaining circulation.
Although preferred, ceasing lost circulation completely is not always possible or required. Controlled losses allows drilling to continue while keeping the wellbore full, preventing an influx of gas or fluid into the wellbore, known as a" kick", which can lead to a blowout.
A number of options are available when lost circulation occurs, depending on the severity.
- Minor losses may be controlled by increasing the viscosity of the fluid with bentonite and/ or polymers, or with the addition of other additives which typically includes sawdust.
- Severe losses will require increasing the viscosity of the fluid with bentonite and/ or polymers and the addition of other additives which typically includes sawdust.
Total losses can be regained through conventional use of increased viscosity and additives, or through use of unconventional methods such as pumping of golf balls, tree branches, rags, additive sacks, and other items in conjunction with, or followed by, a high viscosity fluid. If total losses occur and circulation cannot be regained, several options are available, depending on the operational requirements and depth being drilled in relation to desired production geological zones. Continuing drilling while pumping drilling fluid is one option, though continued drilling while pumping water is less costly and more often used. Sometimes the cuttings from continued drilling will aid in reducing or stop losses altogether. A third option is to cement the zone where the losses occurred, and to drill through the cement and continue drilling the well. This third option is very often the most cost effective if severe losses occur, as lost circulation can sometimes not be controlled with conventional or unconventional methods.
- The most common additive used to control or cease lost circulation is bentonite, in that it inherently will seal small holes or fractures. Bentonite, in higher concentrations, increases viscosity, and therefore slows the fluid flow into the surrounding rock. Although bentonite is the most common additive used, it sometimes is not used at all, depending on the current fluid being used and the current depth of drilling in relation to desired production zones.
- Polymers are also sometimes used to increase the viscosity. Though these are more costly, they are more compatible with several types of fluid systems.
- When it comes to additives which physically plug or seal the losses, there are a great deal of them. Sawdust, flaked cellophane, crushed or ground gypsum are all used.
- Other common, and cheaper additives, that are used are shredded newspaper and cotton seed hulls. Cotton seed hulls are less preferred as they may cause wear to pump swabs and springs. Both of these are generally only used when either fresh or brine water is being used for the drilling fluid.
- When drilling in salt formations, brine water is typically used as it is more difficult to dissolve more of the salt preventing the formation of "washouts". Washouts not only contribute to loss of circulation, but can jeopardize the integrity of the wellbore itself.
Several factors are considered in what additives are used:
- Hole size currently being drilled.
- Drilling fluid in use. The additives must be compatible.
- Depth of the well in regards to geological stability.
- Depth of well in relation to the desired production zones. Plugging a production zone is not a desired outcome.
- Drill bit nozzles sizes. If the additive(s) will not go through the drill bit, they cannot be used.
- Other drill string mechanical equipment such as a mud motor or MWD tools. If the additive(s) will not go through the drill string, they cannot be used.