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Well Control - Stripping Method

This article describes the keys elements for the stripping method for well control

This procedure is recommended when the drill string is partially or completely out of the hole. The basic 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.

 

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
  • Knowledge of equipment

General procedure

Main symbols and units used

SICP                     Initial Shut-In Casing Pressure (bar)

Vinf                       Initial Volume of Influx (pit gain)  (litre)

P-step                  Working pressure increment  (bar)

V-step                  Volume of mud in open hole/DC annulus  (litre)

Corresponding to the hydrostatic of P step        

P-saf                    Allowance for loss of hydrostatic pressure in (bar)

Influx rises from below the bit to around the DC

Cap OH/DC               Open hole / Drill collar annular Capacity   (litre/m)

Cap OH                      Open hole capacity    (litre/m)

MW1                     Initial mud weight     (SG)

Influx density     Estimated Influx density    (SG)

 

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
  1. 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).
  1. 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

  1. 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

  1. During the stripping preparation allow SICP to build up to Pchoke:

Pchoke = SICP + Psaf + Pstep

  1. Apply the lowest practical closing pressure to the annular preventer while avoiding leakage.
  1. 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
  1. 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

  1. 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.

  1. 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. 

  1. Continue stripping in maintaining Pchoke2 constant and until the total volume drained in the Trip tank = Volume step
  1. 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)
  1. 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

 

(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?