During a lost circulation event, mud leaks into the formation rocks during drilling. Lost circulation is usually identified when the rate that mud is pumped into the well is greater than the rate at which it returns to the surface through the annulus up to and include full and complete no returns to the surface.
Lost circulation is a common problem when drilling through rocks that are naturally fractured, highly permeable, or cavernous. However, circulation loss can also be caused by induced fractures. These fractures are the result of excessive mud weight or tripping into the borehole to fast (called “surging”).
If lost circulation isn’t corrected, the level of the drilling mud in the annulus will drop as it escapes into the formation where the loss is occurring. As the mud level falls, the pressure exerted by the mud decreases because the height of the column of mud along with the mud density is what makes hydrostatic pressure within the borehole. Once the borehole pressure is less than the formation pore pressure, hydrocarbon fluids may be able to enter the well. This is a dangerous situation that could result in a blowout.
In addition, loss of circulation means that drilling equipment may not be properly lubricated by the drilling mud. This can damage drilling equipment.
There are a couple of ways to deal with lost circulation:
Blind Drilling
If the loss of mud is small enough, drillers might decide that it’s worth it to pump extra mud down the well in order to allow drilling to move forward until casing can be set. The well is kept under control by counteracting the leak with additional mud, some of which is lost to the formation. This is called “blind drilling”, and is typically frowned upon because of the increased risk of a blowout.
Plus, drillers do not know where the mud is going nor do they know where the cuttings are going potentially leading to stuck pipe.
Change Consistency of Drilling Mud
For moderate mud losses, drillers might attempt to plug up leaks in the wellbore by changing the consistency of the drilling mud using additives to make it thicker. Sometimes a fibrous and/or laminated and/or granular material is added to the mud in the hopes of plugging whatever is causing the lost circulation.
Lost circulation isn’t just dangerous – it’s expensive too. The loss of drilling fluids can add up to millions of dollars, the material used to correct the condition while relatively inexpensive can rapidly add up, and the lost drilling time will cost the operator (called “non-productive time” or NPT). If severe lost circulation can’t be corrected, the well may need to be abandoned.
Preventing lost circulation from occurring in the first place is much cheaper than trying to fix it once it occurs.
Although little can be done about natural fractures, taking care to ensure that the mud weight is kept within the mud window will prevent induced fractures from becoming a problem. The possibility of lost circulation is something that should be taken into account when developing the drilling and mud weight plan.
Images: “Lost Circulation” by Michael Black; “Mud Funnel” by Michael Black