When the standard well control method (drillers' or weight and wait) are not suitable, alternative methods can be considered: Stripping, bullheading, volumetric, off-botton, no circulation, etc
1. Stripping
This procedure is recommended when the drill string is partially or completely out of the hole. The principle of this technique is to maintain the BHP constant with the well closed and while running drill pipe. This will be achieved by using combined stripping and volumetric method (control of the gas migration while pipe is stripped in the hole).
This technique should be used first as the well can be most effectively killed with the bit on bottom. It is to be noted that the option to run in the hole with the well open must never be attempted even if the bit is closed to bottom.
1.1 Requirements
The following conditions must exist for successfully accomplishing a kill operation by using a stripping Technique:
Rig designed for Stripping
- Stripping tank available and calibrated
- Trip tank available and calibrated
- Surge bottle available (If not can the annular opening chamber be vented)
- Bop designed for stripping Ram to Ram or Annular to Ram
- BOP space out compatible with stripping operations
- Facilities to fill up the drill string
- BOP spare parts available
Knowledge of procedure and equipment.
1.2 General procedure
Main symbols and units used
- SICP (bar) Initial Shut-In Casing Pressure
- Vinf (litre) Initial Volume of Influx (pit gain)
- P-step (bar) Working pressure increment
- V-step (litre) Volume of mud in open hole/DC annulus Corresponding to the hydrostatic of P step
- P-saf (bar) Allowance for loss of hydrostatic pressure in Influx rises from below the bit to around the DC
- Cap OH/DC (litre/m) Open hole / Drill collar annular Capacity
- Cap OH (litre/m) Open hole capacity
- MW1 (SG) Initial mud weight
- Influx density (SG) Estimated Influx densit
Stripping through the Annular preventer
1. After shutting in the well, install an inside BOP (Gray valve), record SICP and determine the influx volume Vinf
2. Determine the working pressure increment Pstep. Convenient values for Pstep are between 5 and 10 bars (50 and 150 psi) in accordance with scale division available on the pressure gauge (i.e. 100 psi, 10 bars…etc).
3. Calculate Vstep volume of mud in the open hole/DC annulus corresponding to the hydrostatic of Pstep
Vstep= (Cap OH/DC x 10.2 / MW1) x Pstep
4. Calculate Psaf, allowance for loss of hydrostatic as the influx rises from below the bit to around the drill collars calculate as below:
Psaf = (Vinf / Cap OH/DC – Vinf / Cap OH) x (MW1-Influx density) / 10.2
5. During the stripping preparation allow SICP to build up to Pchoke:
Pchoke = SICP + Psaf + Pstep
6. Apply the lowest practical closing pressure to the annular preventer while avoiding leakage.
7. Commence stripping at 2 min/stand. Allow SICP to build up to Pchoke (if not already achieved during stripping preparation) without bleeding off any mud
8. Continue stripping, once Pressure choke is reached bleed into the Trip tank (via choke and MGS) to keep Pressure choke constant.
Mean while bleed off from the Trip tank to the Stripping tank the volume of mud corresponding to the closed end displacement of each stand stripped in the hole.
Continue stripping maintaining Pressure choke constant and until the total volume drained in the Trip tank = Volume step
9. Once the measure volume on the trip tank equals Volume step, close the choke and allow Pressure choke to rise by Pressure step by means of stripping drill pipe in the hole (Pressure choke2= Pressure choke + Pressure step).
The closed end pipe displacement volume should not be bled off during this operation.
10. Continue stripping, once Pressure choke2 is reached bleed into the Trip tank (via choke and MGS) to keep Pressure choke2 constant.
Mean while bleed off from the Trip tank to the Stripping tank the volume of mud corresponding to the closed end displacement of the stripped in drill pipe.
11. Continue stripping in maintaining Pchoke2 constant and until the total volume drained in the Trip tank = Volume step
12. Repeat as often as necessary as shown in steps 9, 10 and 11. Fill up the drill string every 10 stands with mud from the active (Not trip tank). Adjust stripping speed to avoid surging. Stop stripping when one of the following situations arises:
- Bit is on bottom
- Gas has reached surface
- Stripping is no longer possible (BOP problem, OH resistance)
13. When the bit is on bottom, the well can be killed conventionally using the driller’s method
Stripping from the Annular preventer to rams
For this procedure the location of the tool joint in the BOP stack must be known at all times.
This procedure will be based on the actual BOP configuration on the rig, for example:
- 18-3/4” annular
- Upper 5” pipe rams
- Shear rams
- Middle variable rams
- Lower 5” pipe rams
1 Follow the above Stripping through annular preventer procedure to step 6.
2 Strip the DP until the tool joint #1 reaches the top of Annular preventer.
3 Close the Middle pipe rams, keep 1500 psi of closing operating pressure.
4 Bleed off through the choke line the trapped pressure between Annular preventer and middle pipe rams.
5 Decrease pipe rams closing operating pressure from 1500 to 500psi.
6 Open the Annular Preventer
7 Strip the DP until the tool joint #1 reaches the middle pipe rams.
8 Close the Annular Preventer with 1500 psi of closing operating pressure.
9 Bring the pressure between Annular and rams to the current well pressure.
10 Decrease Annular Preventer operating pressure from 1500 to 500 psi.
11 Open the middle pipe rams.
12 This process will be repeated alternatively, then the general procedure in term of well control (Pressure choke) will be followed as shown in the above Stripping through annular preventer procedure
Check list before stripping
A Gray Valve has been installed above the full opening safety valve (TIW Valve)
The TIW valve is opened
Have spare Gray valve and Kelly valve available on rig floor?
SICP is recorded
The volume of influx is known
MAASP and Maximum casing pressure are known
Drill string weight in mud is known
The piston force against the drill string is calculated
The stripping sheet is filled up
The stripping tank is calibrated
The trip tank is calibrated
Will all the mud be bled off to the trip tank
It is possible to bleed from trip tank to stripping tank?
Some defoamer has been added to the trip tank?
Degasser is running
Drill string can be filled up independently from trip tank (i.e. from active pit)
Tools are available for DP/casing protector removal
Grease/oil available for DP lubrication
The Annular preventer operating pressure can be adjusted.
The Pipe Rams operating pressure can be adjusted.
Have personnel on location been assigned specific jobs and instructed on operations?
Check surge bottle pre-charge pressure & open if closed.
Have preparations been made to circulate out when the pipe is on bottom?
What was the original circulating pressure before tripping out of hole?
2. Bullheading
This procedure is recommended in very special cases and should only be used when normal circulation method for kick control is thought to be too dangerous (large influx, excessive surface pressures/gas volumes if influx circulated out, unable to strip to bottom It is to be noted that in certain cases bull heading could possibly create worst problems (i.e. severe lost circulation) than what it might solve.
One of the following hole conditions must exist for successfully accomplishing a kill operation by using a Bull heading Technique:
- Very small amount of open hole exists or the second weakest point in the open hole has a fracture/injection pressure higher than the reservoir where the kick originated.
- Influx is above the weakest point in the open hole and hardly any open hole exists.
- All of the other following conditions must exist:
- Operations have been comprehensively planned and no problems are foreseen.
- All equipment (surface and down hole) must have a higher pressure rating than the maximum anticipated Bullhead pressure (allowing for relevant fluid gradients).
- Casing Burst must exceed surface bull heading pressure plus any hydrostatic difference between Mud Hydrostatic Inside casing and Mud Hydrostatic outside casing
- Surface Mud Volumes must be at least equal to double the amount required to bullhead influx back into zone. (Generally bull heading will not be started until surface mud volume is equal to twice the entire hole volume. If this is not possible due to Rig Mud System, additional volume will be built and placed on a liquid mud supply vessel).
- In extremely critical cases, seawater may be used. Seawater will not kill well but will probably reduce surface pressures.
- Bull heading will not create severe lost circulation (i.e. Total loss of fluid column in the annulus).
- Usually it will be necessary to pump more than the calculated volume due to poor displacement of influx.
3. Off-Bottom
An off-bottom well kill (off-bottom well kill means circulating mud of the appropriate weight with the bit at the present depth) will ONLY be attempted if bull heading and stripping to bottom cannot be carried out in a safe or practicable manner.
Due to the fact that this procedure is abnormal in the extreme, every care must be considered independently and a rigid procedure should not be used. However, written guidance is needed and this follows in the form of an outline procedure plus key points which must be considered prior to attempting an off-bottom kill.
3.1 Outline Procedure
1. Record all relevant well control data.
2. Calculate kill mud weight required to kill well with bit at current depth.
3. Is formation strength (at the open hole weak point) capable of withstanding the equivalent mud weight (EMW) resulting from the circulation of the kill mud? If not, then reduce kill mud weight to an acceptable level to prevent open hole fracture whilst at the same time reducing surface pressures enough to permit a stripping operation.
4. Circulate the kill mud (or partial kill mud) maintaining constant pressure (+ safety margin) at the bit using a drill pipe pressure schedule.
5. Once the well is killed (or partially killed), strip into the hole (possibly in stages conditioning the mud) to bottom.
6. Circulate the well to a stable condition with revised drilling mud weight.
3.2. Key Points to be Considered
The following points should be considered and clearly covered in the detail programme put together prior to any off-bottom well killing operation:
- Where is the influx? (If influx is below the bit then SIDPP=SICP providing there is no slug in the pipe and no trapped pressure in the mud column).
- When calculating kill mud densities use sufficient safety margins (regarding P-pore and P-frac) to allow for surging, ECD and temperature effects on mud density.
- Ensure equipment being used is adequate for pressure and temperature ranges anticipated (this includes low pressure gauges when low pressures are expected).
- Allow pressures to stabilise
- Calculate expected gas volumes at surface after expansion and ensure surface equipment adequate.
- Make allowances for gas migration. Migration rate (m/hr) = pressure increase (bar/hr) / Mud gradient (bar/m)
- Ensure a clear record is kept of events (particular volumes of different density mud in the well and bleed off volumes.
- Drill pipe pressure is not a reliable guide to bottom hole pressure during killing operations if the bit is not on bottom.
- Crews cannot be certain how much mud and how much influx is below the bit.
- If sufficient drill pipe pressure is not applied inflow can occur under the bit with a reduction of hydrostatic gradient and an increase in drill pipe pressure. The normal control action by the choke operator, to open the choke to restore the "pre-planned" drill pipe pressure, leads to further inflow.
- The actual response of the well to the kill should be monitored to allow corrective action to be taken. An example is shutting down kill pumps at predetermined intervals during circulation to record static drill pipe and choke pressures.
4. No-circulation possible
This procedure is recommended when circulation cannot be established (i.e. string, choke lines plugged) and when the string is completely out of the hole and when stripping operation cannot be followed. The basic principle of this technique is to maintain the BHP constant with the well closed, this will be achieved by bleeding off a certain volume of mud in order to allow and control the gas expansion/migration.
4.1 Outline Procedure
1. After shutting the well in allow the shut in SICP to build to SICP2 where:
SICP2= SICP + S + Pressure step
Where:
- . SICP= Initial Shut-in Casing Pressure
- . S = Safety margin pressure increment
- . Pressure step= Working pressure Increment
Convenient values of S and Pressure step are between 50 and 150psi (5 and 10 bars)
2. Calculate the Volume of mud V1 where:
V1= allowance for loss of hydrostatic corresponding to Pressure step
Example: For 6.75” hole (capacity: 23.09 litre/m) Mud weight= 2.32 SG
and Pressure step= 5 bars, Volume V1 will be:
V1= (5 x 10.2 / 2.32)23.09 = 507 litres
3. Bleed off Volume V1 to the trip tank keeping SICP2 constant
4. Allow the new shut in annulus pressure SICP2 to build to SICP3 where:
SICP3= SICP2 + Pressure step
5.Bleed off Volume V1 to the trip tank keeping SICP3 constant
6.Follow the same procedure for SICP4, SICP5….etc