Hard Versus Soft Shut-In

Technology Transfer HARD - SOFT shut-in?

Hard or Soft Shut-in : Which is the Best Approach ?

Sedco Forex Research & Engineering © 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 and Engineering have investigated the hard/soft shut-in issue in order to provide clear reasons why the hard shut-in policy is recommended in the 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 downhole pressure measurements and the tests simulated drilling into an overpressured formation. • Theoretical analysis of the transient pressures produced when the BOP is 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 the February 1993 IADC conference in Amsterdam. Some copies of this paper are included at the end of the presentation package. The following material describes the results of the HARD/SOFT shut-in study and is suitable for presentation to clients and at training courses. 1

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Technology Transfer HARD - SOFT shut-in ?

Hard or Soft Shut-in ? ● Several shut-in procedures in use : • variants of "Hard", "Soft" ● Historical approaches uncertain ● Varying preferences results in confused drill crews ● Require safety of personnel and well Sedco Forex Research & Engineering © Sedco Forex March '93

• Several procedures to shut-in the well after detection of a kick are in 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 which shut-inapproach is adopted. Often this has been based of dubious assumptions for which there has been little supporting evidence. • Even worse is the situation which often occurs in practice where the operator and drilling contractor have conflicting procedures for shutting 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 to provide optimum safety of personnel while maintaining safety of the well.

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Technology Transfer HARD - SOFT shut-in ?

Hard shut-in ● Choke manifold valveCLOSEDat BOP closure ● Advantages • stops influx in shortest time • simplicity ● Perceived Disadvantages • "water-hammer" pressure pulse

Sedco Forex Research & Engineering © Sedco Forex March '93

• In the case of a HARD shut-in the choke valve is closed when the BOP is closured. The choke is normally in the closed position during drilling mode. • This has the following advantages: (a) The influx is stopped in the shortest possible time and therefore minimises the pressure when circulating out the kick. (b) This procedure is simple and quick - there is normally no need to change any valve alignment. • The perceived dis-advantage is that a pressure pulse or “water hammer” 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 waves produced through liquids in pipes when the flow is rapidly stopped by closing a valve. It can sometimes be heard in domestic water systems - if a tap is closed rapidly a bang can be heard throughout the pipe system.

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Technology Transfer HARD - SOFT shut-in ?

Soft shut-in ● Choke manifold valveOPEN at BOP closure ● Perceived Advantages • reduced pressure pulse ● Disadvantages • increased influx volume • greater complexity

Sedco Forex Research & Engineering © Sedco Forex March '93

• In the “Soft shut-in” method, the choke valve is open at closure of the BOP. This has the following advantages and disadvantages : • The advantage of a reduced pressure pulse in the well-bore when the BOP is closed. • The main disadvantages are that : (1) A larger influx is obtained due to the delay in fully shutting in the well. Note that this can be a severe disadvantage, especially when the flow from the formation is large, causing higher pressures when the influx is circulated out. (2) The soft shut-in is more complex due to the requirement of ensuring valve alignment before the BOP is closed. Remember that all this takes place at a stressful time, just following the detection of a kick.

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• To investigate the effect of the additional time required to use the soft shut-in compared to the hard shut-in, a commercial kick simulator was used to evaluate the consequences of the soft shut-in on the totalkick volume. • The kick is detected at 10 bbl in both cases and the pumps are shut-down for a 30 second flow check. • The difference between the soft and hard shut-ins is a delay of 50 seconds required to open the failsafes and close the choke. • This is a “best case” estimate and the time difference between the twoshut-in methods will be greater in many practical field situations. • The effect of the 50 second additional time to perform the soft shutin 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 choke pressure for the soft shut-in case exceeds that for the hard shut-in case by 60 psi. • A greater time difference in shutting-in the well will produce a larger influx, resulting in higher maximum choke pressure.

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BOP 10,000 psi

9 5/8 Hanging Casing

Nitrogen Injection in 1" Coiled Tubing String

5" Drill Pipe

2322' (MD) - 2304' (TVD)

P

T

(Measured from Rig Floor)

Pressure and temperature measurement

6 1/2" Collars (270')

P

T

4700' (MD) - 4640 (TVD)

Pressure and temperature measurement

8 1/4" Tri-Cone Bit Packer

• To investigate the water hammer pressure pulse experimentally, Sedco Forex in collaboration with other Schlumberger companies conducted a series of full scale experiments using a 1430m test well. • The well was instrumented with pressure transducers at surface, at 708 m and at the bit at 1430m. This enabled accurate measurement of the water-hammer pressure pulse at surface and in the wellbore. • Gas was introduced at the bit through coiled tubing. Care was taken to ensure that conditions were realistic. For example, a pressure injection profile was used to simulate drilling into a formation of 200 psi (1.38 MPa) overpressure and permeability of 100 mD (9.87e-14 m 2 ). • For both the hard and soft shut-in cases, 10 bbls were injected into the annulus at which stage the annular preventer was closed.

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The surface pressure measurements obtained during the hard and soft shut-in procedures are shown in this slide. The main points are : • Closure of the BOP produces a "water hammer" pulse after which pressures continue to rise until bottom hole pressure (BHP) stabilises. • In the case of the soft shut-in the pressure is lower until the choke is closed, after which the pressures build in a similar way to that for the hard shut-in except that the influx volume is now greater. • To examine the pressure pulse in more detail, the bottom part of the diagram 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 in the soft shut-in case. • Note that the water-hammer pressure pulse occurs BEFORE there is any significant increase in annular pressure. • In both cases (soft AND hard shutin) the pressure pulse amplitudes are small compared to the net pressure rise after bottom hole pressure has stabilised. [ • Note however that you cannot see the final shut-in pressure for the soft shut-in case as it does not start to build towards the final value until after the choke is closed at 230 seconds] 7

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• The downhole pressure measurements corresponding to the surface measurements shown in the previous slide are shown here at the casing "shoe" (708m) and at the bit (1430m). • The water hammer pulse amplitude is reduced to a slight shoulder on the pressure ramp at 130 seconds. This shoulder is about 36 psi in amplitude and is less than the surface pulse amplitude due to loss of energy of the wave as it travels down the annulus. • The effect of the water hammer pulse is even less significant compared 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 head as 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 velocity U1 from the annulus (see left-hand wellbore figure). • The BOP acts as a valve. If the valve were to close instantly (middle diagram) then a pressure pulse is produced which propagates down the wellbore. The pulse amplitude is given approximately by the first equation (this is described in detail in the enclosed SPE paper). "c" is the velocity of sound in the mud, "p" is the mud density and "u1" is the mud velocity at surface due to the influx from the formation (The mud pumps are not active at this stage). • In practice the BOP does not close instantly and there is an effective closure time, "Tc", during which there is an increasing constriction of the annulus. • For most practical situations, during the BOP close time "Tc", some of the surface pressure wave will have travelled down the well, reflected from 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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Technology Transfer HARD - SOFT shut-in ?

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

Sedco Forex Research & Engineering © Sedco Forex March '93

• This is a summary of experimental and theoretical pressure pulse amplitudes for the experimental tests and corresponding theoretical predictions. • Measured results for hard and soft cases are as shown. • There is good agreement with the theoretical predictions which are derived in detail in the paper. • At the casing shoe depth of 708m, the amplitude of the water hammer pulse is even lower than at surface. It is less than 36 psi, which represents 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 the amplitude of the water hammer pulse. • Note that the 60 psi is potentially more serious as it ADDS TO the final shut-in pressure, whereas the water-hammer pulse occurs BEFORE any significant annular pressure increase. • Remember that there was also a predicted 12.5% increase in pit gain due to the difference in shut-in method (assuming a difference in timing of only 50 seconds, which is a “best case” example).

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Technology Transfer HARD - SOFT shut-in ?

When is a Hard Shut-in Hard ? ● No reduction in∆P 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 still less than the final shut-in pressure.

Sedco Forex Research & Engineering © Sedco Forex March '93

• When does a hard shut-in produce the maximum pressure amplitude ? • A previous slide showed that the pressure pulse amplitude is reduced by 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 BOP closure 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 preventer was used. Taking a conservative estimate for effective BOP closure time as 10 seconds implies that there will be significant pressure pulse reduction for hole depths up to over 6700 m. This covers most operational situations 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 due to the upward travelling wave (eg extremely rapid ram operation), then the pressure pulse amplitude is still less than the typical shut-in pressure. For example, there was 120 psi predicted "water-hammer" in the case studied in the experiment compared to an annular shut-in pressure of over 200 psi.

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Technology Transfer HARD - SOFT shut-in ?

Conclusions

● Theory and experiment show small "water hammer" 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-in pressure rise • formation exposed tolower net pressure Sedco Forex Research & Engineering © Sedco Forex March '93

• Main conclusions are as outlined on the slide. The issues relating to SOFT and HARD are again separated following the scheme used 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 downhole pressure measurements. All the results are consistent. The effect of the pressure pulse produced during the hard shut-in was shown to be less than the higher pressures produced by the additional influx in the soft shut-in case.

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Technology Transfer HARD - SOFT shut-in ?

Field Implications

● Results favour HARD shut-in ● HARD shut-in results in : - minimum confusion - less influx volume - lower annular pressures ● Safety of personnel and equipment without risk to the well. Sedco Forex Research & Engineering © Sedco Forex March '93

• Final conclusions emphasising the field implications. • The aim is to maintain safety of personnel and equipment without risk to the well.

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It could be useful to finish with the default choke valve arrangement for drilling mode as shown in the Sedco Forex well control manual. After all, this is what it all boils down to in practice....... !

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Technology Transfer HARD - SOFT shut-in ?

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 small compared to shut-in pressure rise. ● Effect of closing choke in soft shut-in ? • pressure pulse produced • effect is a delayed water-hammer. Sedco Forex Research & Engineering © Sedco Forex March '93

Finally, some possible questions.............. Q - What happens if the operator and contractor disagree on shut-in procedure ? A - The shut-in procedure should always be agreed at the pre-spud meeting as it is very important that there are no conflicting requirements during the kick detection/shut-in operations. Sedco Forex will recommend the use of the hard shut-in procedure for the reasons described in the presentation. Q - What happens if the influx is larger and gas expansion produces a higher mud velocity than during the experiment ? A - Firstly, the large influx is most likely to be produced by a high flow rate from the producing formation so it is even more important that the well is shut-in as rapidly as possible without any further influx. We have shown that 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 slide 9 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/mud interface 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 can close more rapidly than the BOP. Note : It may be worth making this point anyway. 15

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