TIME PRESSURE
06.11 Low Choke Method - Dynamic Kill
This method of well control is occasionally proposed for handling shallow gas kicks. If it is anticipated that shutting-in a kick will result in surface pressure above the maximum allowable, the well is allowed to flow through the choke (and kill) line and surface pressure is maintained slightly below the maximum allowable value. In this way the rate of influx may be sufficiently slowed to allow well control to be regained by circulating kill fluid down the drill string.
There may be circumstances under which this technique can be implemented successfully, however there are inherent dangers. Initially bottom hole pressure is maintained at a value below the kicking formation pressure and inflow will therefore continue. The continued influx will reduce bottom hole pressure further as the annulus is unloaded. Only if kill fluid can be circulated into the annulus at a sufficient rate to overcome this unloading effect and increase the bottom hole pressure will well control be regained.
The low choke method is an attempt to out run a kicking well, and should not be attempted except for handling shallow gas kicks.
07.11 Bullheading
Bullheading is generally recommended in the following circumstances:
1. If a kick is taken with the drill string far off bottom, or with no pipe in the hole. With the pipe close to bottom, stripping in should be considered. The decision to strip, as well as the stripping procedure, must allow for the effects displacing the influx up-hole and for the effect of gas migration. If the upward force (closed in pressure multiplied by the cross-sectional area of the closed-end drill pipe) exceeds the string weight, it will not be possible to strip in.
2. If the influx has the potential for containing H2S.
3. If circulating the kick out could result in excessive gas rates through the well control system.
4. If the influx is very large, resulting in excessive surface pressures.
After shutting in the well on a potential kick, the decision of whether to bullhead or circulate out the kick must be made very quickly after considering the following subjects.
See Fig 80
1. Stabilised SIDPP and SICP – Do the pressures stabilise very quickly, indicating a kick from a high permeability formation? Is gas migration evident?
2. Influx volume and fluid type.
3. What are the estimated fracture pressure gradients for the shales and any exposed sand(s) in the open hole? How do they relate to the shoe strength (LOT)? How are shales and sands distributed in the open hole? Is fracturing the hole (with potentially ‘charged’ formations) an acceptable consequence?
4. Pressure limitations of pumping equipment, wellhead equipment, and casing shoe tests.
Fig 80
5. If a gas influx is suspected (shut in pressure continues to rise indicating migrating gas in water base system), pumping rate for bullheading must be fast enough to exceed the rate of gas migration. If pump pressures increase instead of decreasing, this is an indication that the pumping (injection) rate is too slow to be successful. This can be a problem in a large diameter hole.
6. The possibility of breaking down the formation of long open hole sections beyond the last casing shoe rather than the producing formation. This could provoke the development of an underground blowout.
Bullheading Procedure
1. Ensure that sufficient fluid of the current weight is available for the operation and that the line to the kill pump suction is clear.
2. Line up BOP and choke manifold to pump down lower kill line. Pressure test the surface equipment to above the maximum injection pressure.
3. Start the bullheading operation at a sufficiently slow rate such that the volume versus rate relationship can be monitored. Attempt to keep the rate constant during the operation and plot up volume versus rate as per leak off graph. Allow for the compressibility of the drilling fluid as the pressure is brought up to the injection pressure.
4. As bullheading continues, the surface pressures should theoretically decrease as lower density influx is displaced by higher density fluid. Surface pressures should
HALLIBURTON BOP SICP KILL LINE SIDPP SICP INFLUX HALLIBURTON BOP SICP KILL LINE SIDPP SICP GAS INFLUX BULL HEADING
be monitored and plotted at regular intervals to check that the influx is being bullheaded away. If the injection pressure does not fall it may be as a result of fluid being injected into a formation above the influx. See Fig 81
The injection pressure may increase during the operation as the permeability of the reservoir is damaged. If the injection pressure approaches the maximum allowable surface pressure, stop the pumps and allow the pressure to stabilise. Recommence at a slower rate keeping within the maximum pressure limitations.
If it becomes impossible to bullhead without exceeding maximum pressure limitations i.e. fracture pressure, the decision to continue bullheading operations in excess of this pressure will depend upon the volume of the remaining influx and the position of the bit in the hole.
5. Once the calculated volume of influx has been bullheaded back to the formation, bleed off trapped pressure and shut in the well to monitor drill pipe and casing pressures.
If the shut-in pressures have fallen, then it is a fair assumption that the operation has been partially successful. It should be remembered that if the kick was taken whilst drilling. It is unlikely that the drill pipe and casing pressures will read the same due to the dissemination of the influx in the fluid.
6. If bullheading was seen to be successful, then it should be continued until the drill pipe and casing pressure are similar. The subsequent well kill operation to secure the well will depend on how the kick was taken.
- If the influx was taken whilst drilling, then the well can be killed using the wait and weight method utilising the original shut in pressure information.
- If the pipe is off-bottom, then it will be necessary to strip back to bottom using standard stripping procedures. A circulation should then be performed, maintaining constant bottom hole pressure, to clear the hole of disseminated gas.
7. If the procedure is not seen to be successful, then consideration will have to be given to:
- Stripping back to bottom if necessary and circulating out the influx at a rate dependent on its size and the limitations of the surface equipment.
Fig 81
1: Bullheading taking place over the influx with plugging of the formation taking place. 2: Bullheading taking place over the influx.
3: Bullheading influx into the formation as pump pressure reduces.
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