Guidelines for Volumetric Stripping Rev.1 Feb.2004

Volumetric Stripping Guidelines

Low Volume Swabbing while POOH.

Form. Pressure Mud gradient

Trip Margin

Rev: 1 Prepared by TWH/5, date: 14.02.2004

Volumetric Stripping Guidelines Introduction: A study of blowout statistics (1200 case histories), revealed that even though the drilling industry continually learns more about how to handle an unstable well, it seems that problems of influx detection, kick handling and well pressure control do not change much with time. Data indicates that most frequently blowouts occurred during activities of actual drilling (pressure prediction, drilling in a field we think we know), tripping (swabbing, losses due to surge pressures) and circulating (ECD ignored, losses). Secondly it was noticed that the blowout frequency rose when activity levels increased due to shortage of competent drilling personnel. Thus in most cases a blowout could have been prevented. Rarely has a blowout been caused by equipment failure. It is therefore just as vital as it ever has been to continue training & assessing drillingcrews in the principles of well control. Between 20-40% of all recorded blowouts are the results of kicks taken during roundtripping. Human errors are the main causes of these kicks and the contributing factors leading to this can be summarised as: ƒ Misinterpretation of hole condition. ƒ Misinterpretation of mud condition. ƒ Pipe pulling speed. ƒ Incorrect interpretation of Triptank data (tapered string). ƒ Incorrect procedure applied. The accepted policy in most areas was, to run the bit back as close and fast as possible to bottom as long as the flow is not excessive. Uncertainty arises over the decision as to what is and what isn’t an excessive flow. Secondly the most important, but often forgotten, factor is the position of the influx in relation to the drillstring. If the drillstring is run unexpectedly into the influx, a severe loss of hydrostatic head will occur. The result of this may be a further loss of (over)balance with the producing formation and a rapid increase in influx (second) volume and ultimately higher closed-in surface and casing shoe pressures. In view of the above it becomes obvious that the correct procedure should be to close in the well at the first indication of flow. If strictly adhered to, closed-in pressures will in all cases be at a minimum and will leave more options open for a decision on how to kill the well, particular with a “weak” casing shoe. Additionally, it cannot be stated often enough that preventing is always better than curing an event. Everything possible should be done to enable early detections of kicks. Crews should be continuously reminded and trained of the correct procedure when swabbing is induced and on the importance of the correct use of the Triptank whilst pulling-and running pipe out/in the hole. Throughout this document the “worst-case’ scenario is described, a Low Volume Swabbed Gas influx, since it will migrate. Nevertheless a fluid kick, Oil or Brine, in an over-pressured reservoir can also give a tremendous loss of Hydrostatic-Head when the swabbed influx is displaced behind the Bottom Hole Assembly (BHA) while running into it. It only simplifies the operational since it will not migrate as fast an Gas influx. This document is a summary of the PD-03 ‘Stripping Course’-hand-out and other published documents on this subject and should serve as a tool for supervisors at all levels, how to focus on procedural and equipment requirements. It also captures all shortfalls as observed during assessments recently held. Note: •

•

There are excellent Well Engineering software packages, such as WfW & IDM to calculate and simulate Swap & Surge pressures with respect to pipe pulling speed and minimum Trip Margins. This document only describes the effect of Low Volume Swabbing and how to proceed.

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Volumetric Stripping Guidelines Table of Content: Subject: Introduction Table of content 1. Conditions which can lead to a Well Control event while tripping pipe 2. Example of the results when attempting to outrun a gas swab-kick 3. Preparing for a Volumetric Stripping operation. 4. Volumetric Stripping procedure 5. Attachments: 1. What to Check & Plan before POOH. 2. Guideline on how to POOH safely. 3. Volumetric control for gas migration with Bit off bottom. 4. Equipment requirements to perform a stripping operation. 5. Checklist for the combined Stripping & Volumetric operations 6. Volumetric Stripping sheet & Recording sheet. 7. Guidelines for conducting a Strip-drill

Pages: 2 3 4 4-6 7 7-8 9 10 11 - 14 15 - 20 21 - 23 24 - 25 26 - 27

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Volumetric Stripping Guidelines 1.

Conditions which can lead to a Well Control Event while tripping pipe.

During any roundtrip in a well, by definition Swabbing (dragging mud and causing a negative pressure), Low Volume Swab kick, will occur when POOH and a pressure Surge will develop when RIH. Swabbing can lead to a Well Control Event while a pressure Surge can lead to losses which could also turn into a Well Control Event. The main elements which affect this phenomenon can be summarised as followed: ƒ Poor mud conditions: - high viscosity (PV/YP) - High Gel strengths - High Fluid Loss ⇒ thick mudcake. - Use of LCM ƒ Balled BHA, stabilisers, often in soft and reactive shale/clayish formations. ƒ Hole size – BHA size & geometry. ƒ Insufficient Trip-margin, i.e. overbalance, to compensate for any natural swab pressure. ƒ Pulling speed, gel effect. And remember that swabbing can still take place when the Bit/BHA is inside the casing, particular at high angle wells, casing shoe set at inclinations above 350, where a lot of oriented drilling, stationary drillstring, had to take place ⇒ poor hole cleaning. Other causes which can lead to a Well Control event while tripping can be categorised as followed: ƒ Improper hole fill, actual less than calculated ƒ Misinterpretation of slug behaviour in a tapered drillstring ƒ Weighting material sag (on surface as well as subsurface) ƒ Filling the hole with a light mud weight (heavy rain experienced, etc)

2.

Example of the results when attempting to outrun a gas swab-kick.

The figures below, including those on the next 2 pages, are self explanatory and the conclusion can be drawn from here, that the attempt to run into a rising and expanding (gas) influx emphasises the risk attached to a kick with higher pressures. It distinct the advantages of an early close-in procedure.

Development of a Swab kick: Stage 1 0

Situation: Depth, m

Penetrated the reservoir (gas) and the 8 1/2” bit at 2800 m (TVD) had to be changed • Mud gradient = 17.5 kPa/m • Form. gradient = 16.3 kPa/m • Gas gradient +/- 3 kPa/m ⇒ Overbalance on reservoir at bottom: = 3360 kPa

500

•

1000

1500

2000

2500

2800 m 3000 Overbalance, Trip margin

3500

0

10.k

20.k

30.k

40.k

50.k

Pressure, kPa

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Volumetric Stripping Guidelines

Development of a Swab kick: Stage 2 0

Situation: •

500

Depth, m

While POOH swabbing occurred. • With the bit at 1700 m, a slight flow was observed. • A gain of +/- 1.5 m3 was measured from improper hole fill and flow (expansion). • OH filled with gas-cut mud. ⇒ Overbalance on reservoir at bottom: = 2000 kPa

1000

1500 POOH to 1700 m Accumulated gas, Influx

2000

2500

2800 m 3000 Loss of Overbalance, Trip margin

3500

0

10.k

20.k

30.k

40.k

50.k

Pressure, kPa

Development of a Swab kick: Stage 3 0

Situation: Depth, m

BHA run in hole while the well was not flowing excessively (?) • Influx displaced behind BHA, sharp increase in influx height and consequently a further reduction of overbalance. • Gain at surface +/- 4.0 m3. • OH filled with gas-cut mud. ⇒ Overbalance on reservoir at bottom: = +/- 0 kPa

500

•

1000

1500

Increase in height of Influx

2000 RIH to 2200 m

2500

2800 m 3000

Zero Overbalance, Trip margin

3500

0

10.k

20.k

30.k

40.k

50.k

Pressure, kPa

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Volumetric Stripping Guidelines

Development of a Swab kick: Stage 4 0

Situation: •

500

Depth, m

Continue RIH while expansion and accumulation of gas continued. • Well flowing excessively (?) • Overbalanced lost and consequently the well started to produce (2nd influx) • Gain at surface +/- 10 m3. • OH filled with gas-cut mud. ⇒ Overbalance on reservoir at bottom: = - (?) kPa

1000

1500

Increase in Influx Volume ⇒ height

2000 RIH to 2400 m

2500

2800 m 3000 Underbalanced !! Producing (2nd Influx)

3500

0

10.k

20.k

30.k

40.k

50.k

Pressure, kPa

Development of a Swab kick: Stage 5 Results: • Annulus pressure>5000 kPa 0

Situation: 500

•

Depth, m

Well closed due to excessively (?) flow. • Total influx vol.=15 m3. • Closed in annulus pressure 5000 kPa • Bit still 400 (TVD) m of bottom. • OH heavily loaded with gas-cut mud, 2nd Influx. ⇒ Control lost ⇒ To recover, difficult and risky

• High Stripping forces, damage to equipment.

1000

1500

2000 2400 m

2500

2800 m 3000 3500

0

10.k

20.k

30.k

40.k

50.k

Pressure, kPa

From the foregoing it can be seen that an alternative approach is desirable to return the Bit/BHA back to bottom whilst maintaining control of the well. Such an approach is described in the remaining of this document, the combined stripping and volumetric method.

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Volumetric Stripping Guidelines 3.

Preparing for a Volumetric Stripping operation.

When planning a Trip, for what ever reason, options should be considered on how to prevent an unexpected Well Control event. Attachment #1 & 2 could be used as a guide to prevent such an event from happening or how to proceed if swabbing did occur. If a Well Control event develops and gas migration takes place, it’s important to prevent any further escalation. Attachment #3 could be used as guides how to Volumetric control excess pressures due to gas migration while preparing for the Stripping operations. When planning for a possible Stripping operation, Attachment #4 could be used as a guide on what should be the considered requirements for equipment when conducting such operations. When preparing for a Stripping operation, Attachment #5 could be used on how to perform an overall check prior to commence the operation. Attachment #6 could be used as a guide how to collect and record all required data to ease operations. Attachment #7 could be used as a guide how to perform and train crews for an unplanned Stripping operation.

4.

Volumetric Stripping procedure.

Should a well kick whilst tripping, it is important that the decision taken is made with great care to minimise the exposure to a Well Control event. To this end, it is essential that rigs are suitable rigged-up to implement the Volumetric Control method and combine this method with Stripping operations. If a swab kick is detected at an early stage, wellhead pressures will remain manageable and as such stripping should be possible trough any Bag-type preventer. The use of a Bag-type preventer allows Tooljoints to pass through the packing element due to its design. To minimise wear, the pipe should be lubricated and closing pressure applied to the preventer should be kept to a minimum whilst avoiding leakage. Additionally, Drillpipe/casing protectors should be removed and the Drillpipe (and Tooljoints) surface to be smoothened from tong & slip marks. Regardless of the method used to strip pipe into the hole, to enable effective pressure control, it is important to measure all the fluid movements on surface and from the wellbore accurately. Influxes that have entered the well could be gas and migration may take place. If there is no migration it could mean that a fluid, oil or formation water, is swabbed but it also could mean that conditions are as such that gas is still in liquid phase and can brake-out higher up the hole. As long as we face a liquid phase, as pipe is stripped in, the volume of mud released from the wellbore should equal the closed-end displacement and the surface pressure should remain the same. Note: With an influx at liquid phase, gas, oil or formation water, we still apply the same principles, i.e. Pchoke=Pann+Ps+Pw, to maintain a controlled bottom hole pressure. If a gas influx migrates, the surface pressure will increase further after the initial build-up is completed. In this case it is necessary to bleed off additional fluid to let the gas expand and thus reduce the chance of formation break-down. This is achieved by maintaining a constant choke pressure whilst the pipe is stripped in and thus combining the Stripping and static Volumetric Control method. When the Bottom Hole Assembly (BHA) enters the influx, a dramatic loss of Hydrostatic Head (HH) will take place, difference in gradient of mud with respect to the gradient of the influx. This loss of HH should be anticipated and additional back pressure should be added. To simplify and allow easy operations the following assumptions have been made, all of which gives “worst case” scenario: ƒ An influx will be treated as a gas influx, i.e. adding extra back-pressure (Pw) allowing migration and expansion. Page 7 of 27 Rev: C:\Documents and Settings\1\Мои документы\My Work Data\Drilling Electronic Books\Well Control Reference Documents\Guidelines for Prepared by TWH/5, date: 14.02.2004 Volumetric Stripping Rev.1 Feb.2004.doc

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Volumetric Stripping Guidelines ƒ ƒ

The influx remains as a continuous slug, bubble, occupying the entire annular cross section. Loss of HH (Ps) will be added at the very start of the stripping operations and it’s assumed that the influx will be opposite the smallest annular section, i.e. opposite the Dc’s in the BHA.

Procedure: 1. If Swabbing takes place, perform a flow check and use the flowchart in attachment #2 as a guide on how to proceed. 2. If a swab kick is detected, close-in the well by installing a RHKC on the string, close same and use the hard shut-in method to close-in the annulus. Record the influx volume and monitor, record, pressures at 2 minutes intervals to determine the stabilised pressures, Pann and Pdp. 3. Continue recording pressures at 5 minute intervals to see if migration takes place. 4. Complete the Volumetric Stripping sheet as per attachment #6. While preparing for a Stripping operation, use the Volumetric Control method (see Attachment #3) to control the well if gas migration takes place. 5. If a Stripping operation has to be conducted, reduce Bag-type preventer closing pressure, use attachment #4 as a guide, and open Surge-bottle. 6. Have a safety meeting, brief crews and complete all checks, use attachment #5 as a guide, to start Stripping operations. 7. Install Gray-valve on top RHKC. Open RHKC slowly to check that Gray-valve is sealing. If a nonported float valve is installed in the BHA, most likely no pressures will be seen below the RHKC. Consider to install a circulating device to confirm the string is not plugged and record pressures before commencing to strip down. From this pressure reading it’s even possible to optain an indication if the Influx is below or above the bit. 8. Install a stand of Drillpipe, smoothen Drillpipe tong and slip-marks and remove protectors when installed. Lubricate Tooljoints. 9. Ensure Choke-line & Manifold are lined up to discharge bled-off mud into the Trip-tank. 10. Ensure Trip-tank is +/- 30% filled and starting volume recorded. 11. Allow Pchoke to rise till Pchoke1=Pann + Ps + Pw, whilst stripping the first stand. Pann = Initial closed-in annulus pressure before second build-up. Ps = Allowance for the loss of Hydrostatic Head (HH) when BHA is run into influx. Pw = Working pressure to allow expansion. See attachment #3 for calculations on Ps and Pw (∆V) 12. Fill each stand stripped with mud from the active mud-system. 13. Avoid excessive surge pressures on the well as well on the Bag-type closing system when passing Tooljoints through the packing element. 14. Whilst stripping pipe, the volume increase due to Closed-End (CE) displacement of Drillpipe and expansion volume of influx are purged into the Trip-tank. After Stripping the entire stand the CE volume will be bled into the Strip-tank. Thus ensuring that any increase in the Trip-tank is due to expansion of the gas which reflects the loss of HH in the well. 15. Maintain Pchoke1 constant whist stripping pipe until the actual volume increase (⇒ excluding CE volume) in the Trip-tank equals the equivalent working volume, ∆V, converted from Pw. 16. While continuing stripping the next stand, allow Pchoke to rise to Pchoke2 = Pchoke1+Pw . 17. Continue stripping pipes and repeat steps 12-16 as often as necessary to allow migration and Volumetric control until the Bit is back on bottom. 18. Back on bottom, prior to conducting the “Drillers Method” killing operations, close Ram-type preventer to have full integrity on BOP equipment when the influx reaches surface. Line up mud discharge from Choke-manifold through Separator. 19. After the well is back on primary control and before opening the Bag-type preventer, be aware that trapped pressures could still be in the BOP-stack and below the Gray-valve. 20. Ensure that equipment is reset to normal operations since you still have to attempt to POOH again!

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Volumetric Stripping Guidelines Attachment #1:

What to Check & Plan when planning to POOH Drilling Stopped Why?

A Bit change

Case of Open Hole

E-line Logging

(min. exposure time)

(ext ended exposure time)

(extended exposure time & interference of equipment.)

No

No

Continue with Attachment #2

Reservoir exposed?

Reservoir exposed?

Continue with Attachment #2

Yes

Reservoir exposed?

Yes

Check & Plan ‰ ‰

‰

‰

‰

‰ ‰

‰ ‰ ‰

Top Hole Drilling without BOP protection? ⇒ Shallow Gas, pull wet and consider to pump out of hole? Soft shale’s/clays drilled? ⇒ Increases chance of a balled-up/packed assembly! ⇒ Increased chance for High Volume Swabbing! Mud conditions as per program? ⇒ Gels to a minimum to minimise Low Volume Swabbing, also during E/line logging (Check tool sizes and logging speeds wrt hole size)? ⇒ Gels to a minimum to avoid losses when RIH, especially when POOH for Casing? ⇒ Drilled with a low Fluid Loss (FL) minimised Mudcake ⇒ To prevent weighting material settling (Barite-sag)? ⇒ Volume, height of planned viscous pill not excessive? Hole conditions and clean? ⇒ Correct mudweight not masked by cuttings? ⇒ Cutting-bed on high deviated wells? ⇒ Planned to pump out of hole when O/P’s are experienced? Hole size in relation BHA size, number of stabilisers in BHA, minimum annular clearance? ⇒ Consider to pump out of hole to +/- 100 m above exposed reservoir to minimise Low Volume Swabbing trough reservoir? Tapered drillstring used? ⇒ Slug size, displacement & position pre-calculated? Gas shows on connections, drilling and on previous trip? ⇒ Trip Margin adequate? ⇒ ‘Swab’-trip considered? ⇒ Spotting a viscous pill above reservoir considered? Reservoir High permeable? Monitoring equipment in good working conditions? Extra training for personnel required? ⇒ HPHT conditions! ⇒ UBD conditions!

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Volumetric Stripping Guidelines Attachment #2:

Guideline on how to POOH safely.

“Colour coding”: = safe = be Alert = unsafe, close-in

Condition mud and Circulate clean. Flow Check POOH and confirm hole conditions. Check for Swabbing; externally, annulus, and internally, drillstring!

⋅ Continue POOH and consider pumping a slug only when back in the shoe. ⋅ Have regular Flow checks. ⋅ When O/P’s are experienced swabbing can still occur!

No

Yes

Swabbing? Yes

In Annulus

No

Yes

Flow?

⋅ Flow check. ⋅ Evaluate situation.

Close in and evaluate situation.

Bit close to bottom?

Swabbed close to a possible reservoir or in Top-hole? Yes

Yes

Yes

⋅ Circulate and condition mud. ⋅ Flow check. ⋅ RIH, circulate and condition mud as required. ⋅ Flow check. ⋅ Wash out of hole and clear tight spots(s). ⋅ Continue and follow plan.

⋅ Circulate and condition mud. ⋅ Flow check. ⋅ Wash out of hole and clear tight spots(s). ⋅ Continue and follow plan

No

Sufficient Trip Margin?

No

No, i.e. extended exposure time!

In drillstring; ⋅ Fill up, check fluid level and circulate drillstring volume to homogenous mud. ⋅ Flow check.

No

⋅ Prepare to Strip to bottom. ⋅ Strip to bottom. ⋅ Circulate Influx out, use Drillers method (P Standpipe = Cst).

⋅ Use Drillers method to circulate out any possible influx in annulus. ⋅ Flow check. ⋅ RIH, circulate and condition mud as required. ⋅ Flow check. ⋅ Wash out of hole and clear tight spots(s). ⋅ Continue and follow plan. Bit change?

Yes, i.e. short exposure time!

Remember: ƒ ƒ

ƒ

ƒ

Every situation stands on it own. Perform Volumetric bleed when gas percolates up. Don’t start Stripping operations when not 100% prepared. Stripping back to bottom is a non-standard operation. Evaluate carefully and Instruct crew thoroughly, repeat same on crew changes. This flowchart is a guide for standard operations, modify it when your operations are non-standard, i.e. UBD, HPHT, etc.

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Volumetric Stripping Guidelines Attachment #3 Volumetric Control for Gas migration with Bit off Bottom. Any time a well is closed-in due to signs of a possible influx observed while POOH and if this influx contains Gas, the influx will start to migrate to surface ⇒ increase of pressure in time will be observed. This increase in pressure should be monitored closely and controlled, while the Rig team prepares themselves for this Well Control event, this to prevent any possible fracturing of formation and in particular around the previous casing shoe. How to control this increasing pressure, while maintaining a bottomhole pressure preventing any further escalation (2nd influx), is described below and should be used to train staff on the Rig. Other applications where this application could be used for are: ƒ Pipe is near surface and the weight of string insufficient to perform stripping operations. ƒ Pipe is completely out of the hole pre-empting a stripping operation. ƒ Annular BOP damaged and stripping through pipe-rams is not an option (no double set of piperams or spacing for Tooljoint not sufficient) ƒ Circulation through the drillstring not possible due to plugged nozzles, forgot to open RHKC when stripping had commenced, etc.

Equipment Arrangement for Volumetric Control Returns from Flowline back into Trip-tank

Opening line

Annular Preventer

Stripping Bottle

Closing line Choke venting into Trip-tank Ram-type Preventers not included!

Gray Valve Trip Tank, calibrated!

RHCK

Choke

Stripping Tank, calibrated!

BHA

Swabbed Influx

Next pages will describe: ƒ The migration and volumetric control of a closed-in well containing a gas-influx with the Bit off bottom.

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Volumetric Stripping Guidelines Percolation or Migration speed: When no pressure is released on surface, the gas-influx volume will not change and the pressure at the top of the, assumed, bubble will remain Cst, Boyle’s law: PxV=Cst. ƒ Since the gradient of gas is far less than the gradient of any mud system, ρgas +/- 2-3 kPa/m and ρmud >10+ kPa/m, the influx will migrate, change-over of medium, to surface. ƒ Since a volume is a function of Area x Height (AxH), Boyle’s law in this case becomes also a function of height which has to be translated into Hydrostatic Head (HH). ƒ Subsequently, when an influx migrates in time, it will come around several drillstring components and consequently the height of the influx will change accordingly. In relation to this the equivalent hydrostatic head will change ⇒ rapid increase in choke pressure when migrating across your BHA. This at the same time can indicate where about the influx could be positioned. ƒ See figure below for a schematic explanation.

Pchoke

Pchoke

Pchoke

H3

H2

H1

Influx around BHA

Influx partly around BHA and in OH

Influx below Bit

•Influx volume remains constant, Pchoke raises constantly and aggressively when getting around the BHA. •Height of influx changes due to capacity changes of hole configuration. (H1