Hard vs Soft Shut In

7/27/2019 Hard vs Soft Shut In

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Sedco Forex

Research & Engineering

Hard or Soft Shut-in : Whichis the Best Approach ?

Te c h n o l o g y Tr a n s f e r

HARD- SOFT shut- in?

Sedco Forex March '93

INTRODUCTION

There is now reasonable acceptance through-out the industry for the use

of a hard shut-in procedure following the detection of a kick. Sometimes,however, there may still be disagreement (particularly at remote locations)over which method to use. For this reason Sedco Forex Research andEngineering have investigated the hard/soft shut-in issue in order toprovide clear reasons why the hard shut-in policy is recommended inthe Sedco Forex well control manual.

The investigation had three main phases :

Experimental data was obtained by shutting-in on a simulated kick using

a test well facility in Aberdeen. The well was instrumented with downholepressure measurements and the tests simulated drilling into anoverpressured formation. Theoretical analysis of the transient pressures produced when the BOPis shut was developed with Schlumberger Cambridge Research.

An engineering report was produced and, following some further analysis,this was later expanded into an SPE paper which was presented at theFebruary 1993 IADC conference in Amsterdam. Some copies of this paperare included at the end of the presentation package.

The following material describes the results of the HARD/SOFT shut-instudy and is suitable for presentation to clients and at training courses.


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HARD - SOFT shut- in ?

Hard orSoft Shut-in ? Several shut-in procedures in use :

variants of "Hard", "Soft"

Historical approaches uncertain

Varying preferences results in confused drillcrews

Require safety of personnel and well

Several procedures to shut-in the well after detection of a kick arein use today. Mostly these consist of small variations in one of two

main approaches - the "hard" or "soft" shut-in.

Historically operators have had varying preferences for whichshut-inapproach is adopted. Often this has been based of dubiousassumptions for which there has been little supporting evidence.

Even worse is the situation which often occurs in practice wherethe operator and drilling contractor have conflicting procedures forshutting in the well after detection of a kick.

This results in added confusion to an already stressful situation.

It should always be remembered that the final requirement is toprovide optimum safety of personnel while maintaining safety ofthe well.


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Hard shut-in

Choke manifold valveCLOSEDat BOP closure

Advantages

stops influx in shortest time

simplicity

Perceived Disadvantages

"water-hammer" pressure pulse

In the case of a HARD shut-in the choke valve is closed when theBOP is closured. The choke is normally in the closed position duringdrilling mode.

This has the following advantages:

(a) The influx is stopped in the shortest possible time andtherefore minimises the pressure when circulating out the kick.

(b) This procedure is simple and quick - there is normally noneed to change any valve alignment.

The perceived dis-advantage is that a pressure pulse or waterhammer effect is produced in the well-bore when the BOP is closed.This has been thought to cause possible formation damage.

"Water-hammer" is a general term describing pressure wavesproduced through liquids in pipes when the flow is rapidlystopped by closing a valve. It can sometimes be heard in domesticwater systems - if a tap is closed rapidly a bang can be heardthroughout the pipe system.


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Soft shut-in

Choke manifold valveOPENat BOP closure

Perceived Advantages

reduced pressure pulse

Disadvantages

increased influx volume

greater complexity

In the Soft shut-in method, the choke valve is open at closureof the BOP.

This has the following advantages and disadvantages :

The advantage of a reduced pressure pulse in the well-bore when theBOP is closed.

The main disadvantages are that :

(1) A larger influx is obtained due to the delay in fully shutting inthe well. Note that this can be a severe disadvantage, especiallywhen the flow from the formation is large, causing higherpressures when the influx is circulated out.

(2) The soft shut-in is more complex due to the requirement ofensuring valve alignment before the BOP is closed. Rememberthat all this takes place at a stressful time, just following thedetection of a kick.


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To investigate the effect of the additional time required to use thesoft shut-in compared to the hard shut-in, a commercial kick simulatorwas used to evaluate the consequences of the soft shut-in on thetotalkick volume.

The kick is detected at 10 bbl in both cases and the pumps areshut-down for a 30 second flow check.

The difference between the soft and hard shut-ins is a delay of 50seconds required to open the failsafes and close the choke.

This is a best case estimate and the time difference between thetwoshut-in methods will be greater in many practical field situations.

The effect of the 50 second additional time to perform the soft shut-

in results in 1.8 bbl or 12.5 % additional influx in the annulus.

The well is then shut in for a 20 minute period after which the chokepressure for the soft shut-in case exceeds that for the hard shut-incase by 60 psi.

A greater time difference in shutting-in the well will produce a largerinflux, resulting in higher maximum choke pressure.


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To investigate the water hammer pressure pulse experimentally,

Sedco Forex in collaboration with other Schlumberger companiesconducted a series of full scale experiments using a 1430m test well.

The well was instrumented with pressure transducers at surface, at708 m and at the bit at 1430m. This enabled accurate measurementof the water-hammer pressure pulse at surface andin the wellbore.

Gas was introduced at the bit through coiled tubing. Care was takento ensure that conditions were realistic. For example, a pressureinjection profile was used to simulate drilling into a formation of 200

psi (1.38 MPa) overpressure and permeability of 100 mD(9.87e-14m2).

For both the hard and soft shut-in cases, 10 bbls were injected intothe annulus at which stage the annular preventer was closed.

P T

P T

BOP 10,000 psi

9 5/8 Hanging CasingNitrogen Injection in

1" Coiled Tubing String

5" Drill Pipe

6 1/2" Collars (270')

Packer

8 1/4" Tri-Cone Bit

2322' (MD) - 2304' (TVD)

4700' (MD) - 4640 (TVD)

(Measured from

Rig Floor)

Pressure and

temperature

measurement

Pressure and

temperature

measurement


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The surface pressure measurements obtained during the hard and softshut-in procedures are shown in this slide. The main points are :

Closure of the BOP produces a "water hammer" pulse after whichpressures continue to rise until bottom hole pressure (BHP) stabilises.

In the case of the soft shut-in the pressure is lower until the chokeis closed, after which the pressures build in a similar way to that forthe hard shut-in except that the influx volume is now greater.

To examine the pressure pulse in more detail, the bottom part of thediagram shows the time range between 110 and 150 seconds expanded.

The pulse amplitudes are 57 psi for the hard shut-in case and 20 psi inthe soft shut-in case.

Note that the water-hammer pressure pulse occurs BEFORE there isany significant increase in annular pressure.

In both cases (soft AND hard shutin) the pressure pulse amplitudes aresmall compared to the net pressure rise after bottom hole pressure hasstabilised.

[ Note however that you cannot see the final shut-in pressure for thesoft shut-in case as it does not start to build towards the final value untilafterthe choke is closed at 230 seconds]


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The downholepressure measurements corresponding to thesurface measurements shown in the previous slide are shown hereat the casing "shoe" (708m) and at the bit (1430m).

The water hammer pulse amplitude is reduced to a slight shoulderon the pressure ramp at 130 seconds. This shoulder is about 36 psiin amplitude and is less than the surface pulse amplitude due toloss of energy of the wave as it travels down the annulus.

The effect of the water hammer pulse is even less significantcompared to the normal annular pressure build than at surface.

It could also be noted that there is a reduction in pressure at the

bit BEFORE the well is shut-in due to reduction in hydrostatic headas gas enters the annulus.


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Why is the amplitude of the pressure pulse so small ?

The flow of gas/fluid from the formation causes a mud flow at velocityU1 from the annulus (see left-hand wellbore figure).

The BOP acts as a valve. If the valve were to close instantly (middlediagram) then a pressure pulse is produced which propagates downthe wellbore. The pulse amplitude is given approximately by the firstequation (this is described in detail in the enclosed SPE paper). "c" is thevelocity of sound in the mud, "p" is the mud density and "u1" is the mudvelocity at surface due to the influx from the formation (The mud pumpsare not active at this stage).

In practice the BOP does not close instantly and there is an effectiveclosure time, "Tc", during which there is an increasing constriction of theannulus.

For most practical situations, during the BOP close time "Tc", some ofthe surface pressure wave will have travelled down the well, reflectedfrom the bottom and returned to surface. tr is the round trip travel time.

The effect is to reduce the pressure wave amplitude by the ratio "tr/Tc"according to the bottom right equation.


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Results Summary

Surface measured water hammer :

P(hard) = 57 psi,P(soft) = 20 psi.

Surface theory :

P(hard) = 50 psi,P(soft) = 16 psi.

Difference at casing shoe < 36 psi

Simulated effect of additional influx :

60 psi higher surface pressure

12.5 % higher pit gain

This is a summary of experimental and theoretical pressure pulseamplitudes for the experimental tests and corresponding theoreticalpredictions.

Measured results for hard and soft cases are as shown.

There is good agreement with the theoretical predictions which arederived in detail in the paper.

At the casing shoe depth of 708m, the amplitude of the water hammerpulse is even lower than at surface. It is less than 36 psi, whichrepresents a small shoulder on the normal pressure build-up.

The simulated effect of the additional influx was shown in slide 5. This

resulted in 60 psi higher annular pressure which is GREATER than theamplitude of the water hammer pulse.

Note that the 60 psi is potentially more serious as it ADDS TO the finalshut-in pressure, whereas the water-hammer pulse occurs BEFORE anysignificant annular pressure increase.

Remember that there was also a predicted 12.5% increase in pit gaindue to the difference in shut-in method (assuming a difference in timingof only 50 seconds, which is a best case example).


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When is a Hard Shut-in Hard ? No reduction inP for tr > Tc :

BOP closure is very rapid (fast ram operation).

Hole is very deep. Depth limit for pressure reduction :

hole depth < 6750 m (for Tc = 10s).

For the experiment, if there was NO reflected wave :

P 120 psi

P is stillless than the final shut-in pressure.

When does a hard shut-in produce the maximum pressure amplitude ?

A previous slide showed that the pressure pulse amplitude is reducedby the ratio tr/Tc, so there is significant reduction in the pulse amplitude

if the pressure wave round-trip time "tr" is less than the effective BOPclosure time "Tc". What does this mean in terms of hole depth ?

The pressure wave velocity was derived as 1350 m/s (see paper).

For the experiment described in this presentation, an annular preventerwas used. Taking a conservative estimate for effective BOP closure timeas 10 seconds implies that there will be significant pressure pulsereduction for hole depths up to over 6700 m. This covers most operationalsituations and in practice it will often be more than this producing an even

lower pressure pulse amplitude.

However, even if there is no significant pressure pulse reduction dueto the upward travelling wave (eg extremely rapid ram operation), thenthe pressure pulse amplitude is still less than the typical shut-in pressure.

For example, there was 120 psi predicted "water-hammer" in the casestudied in the experiment compared to an annular shut-in pressure of over200 psi.


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Conclusions

Theory and experiment show small "waterhammer" pulse in practical situations

SOFT shut-in

little improvement to pressure pulse

significant effect from additional influx

HARD shut-in

"water-hammer" smaller than shut-inpressure rise

formation exposed tolowernet pressure

Main conclusions are as outlined on the slide. The issues relatingto SOFT and HARD are again separated following the schemeused in slides 3 and 4.

The HARD/SOFT shut-in issue has been investigated theoretically,through the use of a kick simulator and experimentally using down-hole pressure measurements. All the results are consistent.

The effect of the pressure pulse produced during the hard shut-inwas shown to be less than the higher pressures produced by theadditional influx in the soft shut-in case.


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Field Implications

Results favour HARD shut-in

HARD shut-in results in :

- minimum confusion

- less influx volume

- lower annular pressures

Safety of personnel and equipmentwithout

risk to the well.

Final conclusions emphasising the field implications.

The aim is to maintain safety of personnel and equipmentwithout risk to the well.


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It could be useful to finish with the default choke valvearrangement for drilling mode as shown in the Sedco Forexwell control manual.

After all, this is what it all boils down to in practice....... !


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Possible Questions

What if contractor and operator disagree

on shut-in procedure ? decide at pre-spud meeting.

Higher mud velocity than during experiment ?

more important to shut-in rapidly

pulse is larger but is still likely to be smallcompared to shut-in pressure rise.

Effect of closing choke in soft shut-in ?

pressure pulse produced

effect is a delayed water-hammer.

Finally, some possible questions..............

Q - What happens if the operator and contractor disagree on shut-inprocedure ?A -The shut-in procedure should always be agreed at the pre-spudmeeting as it is very important that there are no conflicting requirements

during the kick detection/shut-in operations. Sedco Forex will recommendthe use of the hard shut-in procedure for the reasons described in thepresentation.

Q -What happens if the influx is larger and gas expansion produces ahigher mud velocity than during the experiment ?A -Firstly, the large influx is most likely to be produced by a high flow ratefrom the producing formation so it is even more important that the well isshut-in as rapidly as possible without any further influx. We have shownthat the pulse amplitude is small in comparison to the final shut-in

pressures and this is even more likely to be the case for a large influx.

The pressure pulse amplitude is governed by the equations shown in slide9 and a higher mud velocity will produce a larger pressure pulse. However,since the amplitude also depends on the ratio tr/Tc and the gas/mudinterface will reflect some of the pressure pulse causing a reduction in tr,the pulse amplitude is likely to be reduced from the maximum value.

Q -Does closing the choke in the soft shut-in produce a pressure pulse ?A -Yes, using the soft shut-in only delays when the pulse occurs. The

pressure pulse when closing the choke can be large as the choke canclose more rapidly than the BOP. Note : It may be worth making this pointanyway.

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Hard vs Soft Shut In