BP EXPLORATION (XEU) WELL TESTING PROCEDURES MANUAL
SECTION 4100
SUBJECT WELL KILL
REVISION 0
DATE 01/10/92
WELL KILL
1. SAFETY
1.1 Standard drilling and well control practice covers all safety aspects of this operation.
1.2 Individual responsibilities during the well kill are as the BP Well Control Manual Section 1, page 10. These are summarised as:
a The BP Rep
- To organise a pre-kill meeting for all those involved in the supervision of the well kill operation.
- To provide specific well kill procedures.
- To monitor and supervise the implementation of these procedures.
- To be present on the rig floor at the start of the kill operation. Either the Toolpusher or the BP Rep should be present at all times on the rig floor during the operation.
- The BP Rep has the right to assume complete control of the work required to control the well
- To assign the responsibility of keeping a diary of events.
b The Petroleum Engineer
- Will provide technical back-up to the BP Rep.
- To keep a diary of events.
c The Senior Drilling Contractor Representative (OIM)
- Has the overall responsibility for all actions taken on the rig.
BP EXPLORATION (XEU) WELL TESTING PROCEDURES MANUAL
SECTION 4100
SUBJECT WELL KILL
REVISION 0
DATE 01/10/92
- Has responsibility for ensuring that the driller and the drill crew are correctly deployed during the well kill operation.
- Must be present at the rig floor during the start of the kill operation. Either the Toolpusher or the BP Rep should be present at all times on the rig floor during the operation.
- Has the responsibility for briefing the off duty drill crew prior to starting a new shift.
e Driller
- Has the responsibility for supervising the drill crew during the well kill operation.
f The Mud Engineer
- Has continuous responsibility for monitoring the mud system and the condition of the mud.
g The Cementing Engineer
- Will ensure that the cement unit is ready for operation at any time.
- Will operate the cement unit at the discretion of the BP Rep.
h The Subsea Engineer (where appropriate)
- Should be available for consultation at all times during the well kill operation.
- Has responsibility for checking all the BOP equipment during the operation.
j The Mud Logging Engineers
- Have responsibility for continuously monitoring the circulating system during the well kill operation.
BP EXPLORATION (XEU) WELL TESTING PROCEDURES MANUAL
SECTION 4100
SUBJECT WELL KILL
REVISION 0
DATE 01/10/92
2. PREPARATION
2.1 Prepare a Well Kill Decision tree and discuss the planned well kill with the BP Rep/Toolpusher/Driller/Downhole tools Engineer/Gauge hand.
2.2 Hook up the surface pipe work as required for the well kill.
BP EXPLORATION (XEU) WELL TESTING PROCEDURES MANUAL
SECTION 4100
SUBJECT WELL KILL
REVISION 0
DATE 01/10/92
3. OPERATING PROCEDURE
The sequence of operations in a Well Kill will vary according to :
- The Down hole test tools in the well.
- Which tools are still functioning.
- The reservoir properties.
- If the well is being abandoned or suspended.
The two example Well Kill decision trees give a broad outline of the preferred sequence of operations and likely contingencies.
Each well must be treated individually and a decision tree and programme prepared for that well.
If a well has good permeability, the simplest method of well kill is to bullhead from surface.
As a very rough way of estimating if bullheading will frac the formation is as follows:
- Estimate the productivity index (PI) of the well from surface pressure and flowrate data.
- Use the estimate of PI to calculate the injection pressure at a rate of 1 bbl/min (1440 bbl/d)
- Compare the estimated injection pressure with the prognosed formation fracture pressure.
When mud first reaches the formation the pump pressure will rise as the mud plugs the upper perforations and less of the formation is open to the hydrocarbons. Once mud is present over the whole perforated interval it will be more difficult to squeeze away fluids and the pressure will increase towards frac pressure.
At this point the fluid in the tubing will be mud, but the fluid in the sump will be a mixture of mud and hydrocarbons (because of mixing when pumping the mud down the tubing and the low velocity of the mud in the sump allowing migration of the
hydrocarbons in the sump).
To remove the hydrocarbons from the sump, the pressure should be increased to above fracture pressure and the sump volume should be bullheaded away at a high pump rate.
procedure should ensure that fluids in the fracture leak off to the formation not back into the well bore.
A plot of 'the square root of time since pumping stopped' versus 'pressure fall off' will yield a straight line as the fracture closes. Once the fracture has closed the line will curve over to the right.
The next step would be to establish the circulation path, then unseat the packer, (when a lock open tester valve is run, unseating the packer will establish the circulation path).
Flowcheck the well for losses/gains, and circulate in an LCM pill or circulate out any trapped hydrocarbons as necessary. Circulate bottoms up. If the well is taking losses, the LCM pill should be circulated in with the rams closed because the losses can hide the migration of any hydrocarbons sitting just under the packer.
Only when the BP Rep is satisfied that losses are under control and that gas levels are acceptable may the string be pulled.
4.2 Bleed off and Bullhead
This is a variation on straight bullheading that may be appropriate in some circumstances, eg gas or gas condensate wells, high GOR oil wells, oil wells with large ID tubing (4-1/2") or well where it is desired to reduce the volume bullheaded to the formation.
- The tester valve is designed to allow it to open when the pressure above the valve is greater than the pressure below the valve.
Experience suggests that in oil wells, bleeding off surface pressure will remove up to 75% of the hydrocarbon volume from the tubing. In gas wells bleed off will remove over 95% of the hydrocarbons.
This top up fluid will be reversed out, followed by oil and then mud, it is simplest to use water as the top up fluid as this can be discharged into the sea.
After the reversing valve has opened the top up fluid and oil should be reversed out over the burners. When mud is seen at the bubble hose the flow should then be diverted down the kill line to the degasser.
Note that with a Halliburton DST string the use of the Omni valve for circulating at this point would result in pressure being trapped between the Omni valve and the tester valve.
To avoid this, cycle the Omni valve into the well test position and operate the APR-M (single shot reversing valve).
Where the tester valve is not fitted with a lock open facility it is not possible to circulate out any hydrocarbons trapped between the packer and the tail pipe. To remove the
- After unseating the packer, wait 1 - 2 hours to allow the hydrocarbons to migrate up past the packer and above the open reversing valve. Circulate the annulus via the reversing valve. Repeat the sequence until gas levels are acceptable.
Bull heading twice the sump volume down the annulus and allowing the pressure to fall off until fracture closure has been observed should be the quickest method of well kill. If after bull heading there are still hydrocarbons in the annulus, the fallback position is to allow hydrocarbons to migrate, then circulate, repeating as necessary.
If the migrate circulate method is used, running in hole several metres can help the trapped hydrocarbons past the packer and speed up the migrate/circulate process.
4.5 Leaking Tester valve
If the tester valve is leaking the 'Bullhead from Surface' well kill is the simplest approach.
This relies on either the leak being large enough to pump through the tester valve, or the tester valve lock, or open/close facility still working. If the tester valve leak is small and the valve can not be cycled open, then the 'Reverse circulate and Bullhead Down the Annulus' well kill should used.
Note that a leaking tester valve may prevent a tubing pressure operated multi reversing valve from being cycled. In this event operate the single shot reversing valve.
4.6 Some models of the Schlumberger PCT tester valve can not be opened if the pressure above the valve is significantly greater than the pressure below the valve. In such cases careful consideration must be given regarding when to lock open the tester valve.
4.7 If the well is being suspended instead of abandoned it may not be acceptable to fracture the formation with drilling mud.
BP EXPLORATION (XEU) WELL TESTING PROCEDURES MANUAL
SECTION 4100
SUBJECT WELL KILL
REVISION 0
DATE 01/10/92
4.8 It can often be seen that after bullheading more than the required tubing volume that an amount of gas is still present at the top of the tubing, even though oil has been displaced. This is because to displace gas downwards, back into the formation, the upward migration speed of the gas has to be exceeded. Even if this is done, the resultant speed of the gas downwards is roughly the mud velocity less the migration speed. Considerably more than the tubing contents must be pumped at sufficient a rate to ensure that 'surface' gas is directed back into the formation.
Once this is understood, more efficient ways of removing gas can be undertaken and the element of surprise at seeing gas in the tubing after what seemed a good kill is removed. It is also important to remember that the tubing could have gas distributed along its length, because its downward velocity was less than the mud velocity. The position of a gas bubble in the tubing could therefore be above the mud/oil interface. It is usually far more efficient to bleed off then top up the tubing to remove surface gas, within the constraints of tubing collapse. The possibility of gas within the tubing following a well kill reinforces the reasons for circulating at least one hole volume before pulling the test string.
Action on Driller : on hearing the General Fire Alarm, shut the ESD valve from the nearest station after phoning the control room and confirming the alarm is genuine.
B. Action on Testing Crew : shut in at choke manifold; if safe to do so, bleed off all process equipment to flare.
C. Action on Wireline Crew : if wireline is in hole get toolstring above the downhole tester valve as soon as possible; pull out of hole if the situation permits.
D. Further action on Testing Crew : be prepared to shut in at the downhole tester valve and subsea test tree at any time and open the ESD valve to vent off gas to flare if the situation warrants it.
A. Action on Driller : Close the divertor element to divert gas via overboard lines; inform BP Drilling Representative and Toolpusher of action.
B. Action on Testing Crew : if no wireline in the hole or leak is large, shut in at subsea test tree.
C. Action on Wireline Crew/Driller : if wireline in the hole and leak is small, get toolstring above