During well kill operation, crews should always be vigilant since complications can actually occur at any stage. If there’s a discrepancy in the kill plan, it needs to be noted immediately. For example, pressure gauges may stop working; they therefore should be monitored carefully. If there’s a failure, back-up gauges need to be made available during a well control operation.
In this article, it will describe common problems and complications during well kill operation which are plugged bit nozzle, plugged choke, choke washout, pump failure and string washout.
The alertness in determining early possible kick indicators in well control is of the utmost importance to prevent a well control incident. Careful observance and positive reaction to these signs will keep the well under control and prevent the occurrence of a well control situation.
The various signs that have been recorded as early warning indicators are not consistent in all situations. The signs however may have to be used collectively as one indicator may not accurately provide the warning of getting into an underbalanced situation. Even though the series of signs may change between wells, early warning indications can be found from the following list.
- Increase in drilling rate of penetration.
- Increase torque and drag.
- Decrease in shale density.
- Mud property changes.
- Increase in cutting size and shape.
- Increase in trip, connection and/or background gas.
- Increase in the temperature of the return drilling mud.
- Decrease in D-exponent.
Known as an underbalanced condition, this occurs when, in the wellbore, formation pressure is higher than the hydrostatic pressure and this lead to a well control situation. To overbalance formation pressure, the required hydrostatic pressure is normally provided through an adjustment in drilling fluid density. Hydrostatic pressure loss can occur for a number of reasons;
- ECD loss
- Surface drilling fluid dilution
- Cement density reduction
- Drilling process releasing formation fluids
- Weighting material movement from mud cleaning equipment
- Drilled cuttings or mud weighting materials settling
Since a reduction in the density of mud returns is sometimes happened, most wells are designed to have sufficient overbalance to encounter the small reduction of mud density and this should prevent a kick. However, if there is significant mud weight reduction, an investigation shall be performed to find any root cause and provide any preventive actions.
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.
Figure 1 – Lost Circulation in Drilling Leading to a Well Control Situation
Primary well control is the most important barrier while drilling and completing any wells so it is imperative to ensure that the primary well control is effectively maintained.
When various precautions and procedures have been followed, effective primary well control can be achieved. These procedures can be seen below;
Using a trip sheet (an accurate log), it is possible to maintain tripping both in and out of the well. A trip sheet can help to record the volume of mud that not only enters the well but also that is displaced when tripping. During tripping, the changes in mud volume can be measured using a calibrated trip tank.
For any steel removed, a specific amount of mud is entered into the well when the tripping pipe or drill collars from the hole. To ensure proper well monitoring, tripping may need to be stopped whenever the volume of removed steel significantly outweighs the volume of mud required. After stopping, consideration should also be made towards returning back to bottom in order to condition the mud (and find the cause of the issue). At all times, the drill floor should have the required crossover subs and a full opening safety valve readily available. Continue reading