Well Control - Combined stripping and volumetric method

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When the drillstring is partially or completely out of the hole and a kick is experienced, every effort should be made to safely return the bit to bottom whilst at the same time maintaining well control. The well can be most effectively killed with the bit on bottom.

The string needs to be "stripped" into the well preferably through the annular preventer. This should be a smooth and efficient operation. It requires knowledge of equipment and procedures used by all crews.

When applying the combined stripping and volumetric method, estimates of worst case pressures applied to the wellbore can be calculated using the well control formulae which are applicable to the volumetric method.

Important points to note when stripping are as follows:

·Install an inside BOP above the RH kelly cock. Open the kelly cock prior to stripping and check if the inside BOP is not leaking.

·Have a kelly cock and inside BOP available on the rig floor during stripping operations.

·Remove all drillpipe/casing protectors.

·Lubricate the string with grease. Ensure the tooljoints are smooth.

·Apply the lowest practical closing pressure to the annular preventer whilst avoiding leakage. Watch the flowline for any leakage. Any returns to go back into the trip tank.

·If necessary, use defoamer in the trip tank to ensure precise mud volumes are measured.

·Accurately measure and record mud volumes bled-off. A calibrated trip tank and separate stripping tank must be installed and used.

·Keep the string full. Measure and record mud volumes used to fill the string.

·Monitor the marine riser of a subsea BOP stack for gains and take the effect of heave and tidal changes into account when stripping.

·Plot the choke pressures vs. time on graph paper and check for rapid DV volume changes in order to try to identify when the string has entered the influx or when the influx has entered the choke line of a subsea BOP stack.

·The packing element of an annular preventer must be allowed to breath slightly when a tool joint passes through. The pressure regulator valve of the BOP control unit should be set to provide and maintain the proper control pressure. Recommended BOP closing pressures can be obtained from the manufacturer's BOP operating manuals. Strip drills will build confidence and confirm recommended BOP closing pressures.

·A surge bottle connected to the closing line of the annular preventer will improve effective BOP control during stripping tool joints through the annular preventer. Slow tool joint stripping speeds reduce surge pressures and prolong the packing unit life. Stripping speeds should not exceed 1 m/sec (3 ft/sec). It is also recommended to vent the opening chamber control line of the annular preventer (Cameron D-type) to improve stripping tool joints through the preventer. When a subsea BOP stack is used, a surge bottle must be installed on the bag preventer opening and closing lines.

·Depending on the well pressure, and/or if there is no surge bottle installed, it may be necessary to use the annular and ram type preventer in sequence in order to pass the tool joint through the annular preventer. It should be noted that this alternative is very time-consuming.

"Strip drills" are required to train crews and to evaluate the stripping characteristics of the BOP in use.

It is important that the correct procedures to be used are implemented as soon as possible in case of a kick whilst tripping. A "stripping checklist" should be available on each rig to assist supervisors during this well control operation.

Procedures concerning the combined stripping and volumetric method are discussed for the following conditions:

·Bit off bottom.

·String out of hole.

1 Bit off bottom

·Close in the well, record Pa and determine Vinflux.

. Whilst preparing for stripping, allow the closed-in annulus pressure to build-up to Pchoke.

·Commence stripping. Allow the choke pressure to build up to Pchoke = Pa + Ps + Pw without bleeding off any mud.

·Once the required choke pressure is reached, Pchoke is kept constant whilst drillpipe is stripped in the hole. Excess pressure is bled off via the choke manifold into the trip tank. The closed-end pipe displacement of each stripped-in stand of drillpipe is drained from the trip tank into the stripping tank. The string is stripped in the hole until a volume DV1 has accumulated in the trip tank.

Instead of bleeding off mud and gas via the mud-gas separator into the trip tank, it can also be bled off into an auxiliary tank, depending on rig design and equipment set-up. In that case, the closed-end pipe displacement volume of each stand is drained into the trip tank and DV1 is measured in the auxiliary tank.

In principle, Ps and DV1 values are calculated as per formulae above. However, more accurate calculations may be used Section 2.3.8.1 depending on the following circumstances:

·size of influx volume;

·MAASP;

·position of influx in relation to the string;

·hole configuration (e.g. liner or full string of casing).

·Once the correct mud volume (DV1) has entered the trip tank, the choke is closed and the annular pressure is allowed to rise by P w by means of stripping drillpipe in the hole. The closed-end pipe displacement volume should not be bled off from the trip tank into the stripping tank during this phase of the operation.

It is recommended to strip the complete stand in the hole for each phase of the operation (e.g. whilst maintaining Pchoke constant, or when increasing Pchoke by Pw) to simplify the bleeding off process from trip tank to stripping tank and to improve the accuracy of DV1 measurements which directly results in improved bottom hole pressures.

As a result of stripping the complete stand, higher than required choke pressures will occasionally be obtained which should be taken into account when the next Pw increment needs to be added.

·Steps 4 and 5 are repeated as often as necessary, until one of the following situations arises:

-the bit is back on bottom;

-gas has reached surface;

-stripping is no longer possible (excessive pressures, BOP stack problems, open hole resistance, etc.).

Stripping is then stopped and the well killed conventionally, if the influx is above the bit.

The chance of having to kill the well with the bit off bottom is small, since the migration rate of gas in mud is such that the bit can be stripped back to bottom before gas has reached surface. Migration rates of gas in workover fluids are much higher and should be taken into consideration before deciding to start stripping pipe in the hole.

When the bit is back on bottom, the well can be killed conventionally ("Driller's Method"), using the following procedure:

·Fill up the string and circulate the string contents. Referring to the most recent pump test, start pumping at a constant reduced rate whilst maintaining Pchoke constant until Pst has stabilised. Maintain Pst constant thereafter. Circulation pressures should be slightly higher (± 350 kPa or 50 psi) as compared to circulation pressures experienced before the kick occurred due to the pressure drop across the inside BOP.

If Pst is much higher than expected, maintain that higher pressure. If Pst is lower than expected (gas may have entered the string), maintain Pchoke constant until Pst has reached its expected value. Maintain Pst constant thereafter.

·After the string contents has been pumped, close in the well and check for the trapped pressures. However, do not bleed off all the standpipe pressure via the choke manifold, but leave ca. 350 kPa (50 psi).

·Continue killing the well using the "driller's method".

Safety factors to obtain sufficient overbalance are incorporated in the killing method as described above.

An estimate of the overbalance can be made by calculating more accurate Ps and Pw values (i.e. influx opposite drillpipe instead of opposite DCs). The following safety factors provide the overbalance:

·The difference between Ps actual and Ps assumed.

·The difference between Pw actual and Pw assumed (applies for all Pw increments encountered).

·The increased hydrostatic head of the mud due to stripping drillpipe without bleeding off the closed-end pipe displacement volume (gas influx compressed; assume influx around drillpipe).

·BHP = D x r1, is being re-established when the bit has passed through the influx and the string is full of mud.

2 String out of hole

If the string is out of the hole when an influx is detected and the closed-in surface pressure allows lowering the first stands of DCs or drillpipe into the well, stripping should be initiated since it will improve the well control situation. The kelly or top drive may have to be used in conjunction with singles for extra weight.

The maximum surface pressure that can be overcome by the weight of the first stand, ignoring the friction between the annular preventer and the string, is calculated as follows:

maximum surface pressure =weight of first stand in mud / cross-sectional area of the stand

The force acting downwards will rapidly increase when more pipe is stripped in the hole, hence higher surface pressures can be allowed for.

The procedure to enter the string back into the well is as follows:

1.Install an inside BOP (Gray valve or float valve) on the bottom of the first slick stand of DCs or drillpipe. Use a bit without nozzles to reduce the chance of plugged nozzles.

2.Lower the stand to just above the blind/shear rams and close the annular preventer.

3.Open the blind/shear rams and strip through the annular preventer. Allow the choke pressure to increase by Pw and maintain constant thereafter.

4.Fill the string with mud.

5.If DCs are used instead of drillpipe, continue stripping the slick BHA and maintain a constant choke pressure. Do not use more than three stands of DCs.

6.Allow the choke pressure to increase to (Pa+Ps+Pw) without bleeding off any mud when stripping the first stands of drillpipe.

7.Continue the combined stripping and volumetric method as described in the previous Section "String off bottom".

If it is not possible to strip the string into the well and gas migration is indicated, the volumetric method or bullheading may have to be employed.

Re-entering a closed-in subsea well with a drillstring on a floater is complicated and not always possible, because of the heave and the distance to the subsea BOP stack. The heave should allow the string to move freely with the bit between blind/shear rams and annular preventer. In order to avoid buckling of drillpipe in the marine riser, DC weight should be used to get the string back into the hole.

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