Lost circulation whole mud (whether to depleted reservoirs or to natural/induced fractures)is one of biggest causes of well kicks. In the wellbore, fluid levels can decrease and this lowers the hydrostatic pressure. Once hydrotatic pressure is less than formation pressure, it will cause a flow from the formation in permeable zones. Figure 1 illustrates loss of fluid level into a weak zone which will lead a well control incident if a wellbore is not filled up on time with correct mud weight.
Lost circulation can happen for a number of reasons and we’ve detailed four possibilities below;
Annular Circulating Friction
When drilling near the fracture gradient of the formation with a heavy mud, the pressure added by circulating friction should always be considered. Especially in small holes with a large drill pipe or with stabilizers inside the protective casing, this added pressure can be significant. In many cases, the pumping rate will need to be reduced to then decrease the circulating pressure. Often, where high gel fluids are used, the problem quickly becomes acute when attempting to break circulation in this way.
Entering the Hole Too Quickly
When the drill pipe and bottom assembly are lowered too quickly, this can also cause a loss of circulation (this includes the reamers, drill collars, and bit). Although somewhat similar to swabbing, it’s effectively in reverse as the weakest formation is targeted by the piston action as it forces the drilling fluid into new positions. If the pipe is much larger than the hole and the string has a float in it, the problem is made even worse. Whenever weaker formations have been exposed, or even if heavy mud has been employed to counter high formation pressure, particular care will need to be taken when running pipe into the hole. Many commercially available programs now exist for surging calculations which makes the whole process easier than ever before.
Balled-Up or Sloughing Tools
To restrict the flow of fluids in the annulus, it’s possible to partially plug the annulus by sloughing shale. By imposing a backpressure on formations, a breakdown can quickly occur as long as the pumping continues. Stabilizers and other large drillstring components are common locations for annular plugging; the chances of encountering this type of lost circulation can be reduced with efforts to reduce balling. Read more details about balled up bit below.
Excessive Mud Weight
Finally, fluid levels in the hole can decrease and circulation can be lost whenever the fracture gradient (of the weakest exposed formation) is outweighed by bottomhole pressure. Why? Because the effectiveness of the hydrostatic head will decrease when acting against those formations that haven’t broken down. Assuming the mud levels drop severely, the BHP can fall below formation pressure and the well will start flowing.
Lost circulation should be avoided at all costs; if returns cease, measured volumes of water should be pumped into the hole and this should minimize the loss of hydrostatic pressure. For the Drilling Site Supervisors, they need all volume measures so they can calculate the weight of mud required to support the formation before fracturing occurs. After gaining returns, the well will need to be checked to ensure it isn’t flowing alone.
We hope this article help you prevent lost circulation in the future. Please feel free to share your thought in the comment box below.
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Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.