Shallow Gas Program.pdf

IADC/SPE IADC/SPE 17256 A Shallow Gas Research Program by M. Grin rod,* 0. Haaland,* and B. Ellingsen, Statoil AJS * SPE Members

Copyright 1988, IADC/SPE Drilling Conference This paper was prepared for presentation at the 1988 IADC/SPE Drilling Conference held in Dallas, Texas, February 28-March 2, 1988. This paper was selected for presentation by an IADC/SPE Program
Introduction A SHALLOW GAS RESEARCH PROGRAM Up to the present (nov. 19&7) shallow gas has been reported in 155 of the 567 Abstract

exploration and appraisal wells drilled on

Operational problems associated with the

Seven blowouts and a number of kicks caused

occurence of shallow gas in offshore

by shallow gas have been recorded.

drilling led Statoil to initiate a massive

On the night of October 6th, 1985, a gas

research program. The program consisted of

blowout occured on the semi-submersible

the Norwegian Continental Shelf.

the following subprojects: Revised drilling

"West Vanguard", during exploration

procedures, shallow gas drainage,

drilling in block 6407/6, Haltenbanken,

multistage cementing, gas tight cement,

Norwegian sector.

diverter systems, rig ventilation systems

The well blew out from a shallow gas sand

and emergency anchor release.

while drilling ·the 311mm (12 1/4") pilot

Emphasis in this paper is placed on the

hole at 523m RKB, with the marine riser

experience from the West Vanguard blowout,

installed.

and results from the subprojects on

failed as a result of the extremely large

The diverter system and riser

diverter systems, drilling procedures,

flow of gas and solids.

quick anchor release and ventilation

The eroded flow diverter system and slip

systems.

joint packing leak allowed gas to flow into

Other papers (reference 1, 2) give

the drilling platform where the gas

information on development of gas tight

subsequently ignited.

cement, stage cementing and additional

The resulting explosion and fire caused

background on shallow gas drainage.

considerable damage. The platform was quickly evacuated. One person was lost. Damage to the rig amounted to several hundred million Norwegian kroner.

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A SiiALI!M GAS RESEARCH PROGRAM

2

The investigation after the West Vanguard blowout disclosed that significant

Erosive wear test of Diverter System

improvements were needed in diverter

Compo]1ents

systems. It was further evident that riser-less drilling would be the best

The West Vanguard blowout clearly

option under certain conditions. Air

demonstrated the weakness of diverter

ventilation systems had an influence on gas

systems. Holes were worn in the diverter

distribution, and the fire and explosion

bends in a period of less than 20 minutes.

pattern. The drilling procedures needed to

The largest hole was located in the first

be updated, and rig crews educated on the

bend where the diverter line was tied to

dangers of shallow gas.

the flowline. At this point the sand had cut its way through other piping and into the shale shaker room.

Shallow Gas

The second bend in the

diverter line was also eroded. Gas sands at shallow depths have frequently

Based on these facts an experimental

been penetrated.

investigation has been performed to

These sands have been

encountered as shallow as lOOm below the

determine the most suitable geometry for

seabed.

diverter components, and the relative

Predicting the exact presence of

shallow gas sands from seismic or sparker

erosion and abrasion resistance

investigations is still unreliable, and

characteristics of selected materials.

their existence is often exclusively

The test philosophy was based on comparing

disclosed by MWD and/or wire line logs.

the various coatings as well as piping

Shallow gas is primarily localized in sand

geometry.

lenses surrounded by shale in the upper

different coating were placed in a test

tertiary sediments.

loop to obtain comparable test conditions.

Identical components with

Common mild steel was used as a reference

The gas pressure in these pockets is

material.

normally at or slightly above the normal

In the appendix the mathematical

water gradient. Since the thickness of the

theory behind these tests is discussed.

sand pockets are normally limited to a

In summary the test results were:

maximum·of lOrn, the actual over-pressure in Particle velocity is the most

excess of a hydrostatic water gradient, is

significant factor on erosive wear.

small.

Particle size has very little influence

Because of the low pressure regime, spontaneous kicks with the riser installed

on the value of specific erosion.

seldom occur.

Increase of bending diameter reduces

The risks of losing primary

control lies mainly in swabbing or when

the value of specific erosion.

lost circulation occurs.

Hardfacing extends the lifetime of

The West Vanguard blowout is assumed to be a consequence of loss of hydrostatic head

bends. Efforts should be made to design

caused by gas expansion in the riser.

diverter systems as straight as possible, avoiding bends.

630

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M. GRINR(£?D

17256

A.(Z). HALAND

B. ELLINGSEN

3

Components and Bend Coating

significantly longer life than the short

The test programme included some 53

bending diameter of 0.83m had almost a 50%

radius bend.

The long radius bend with a

components. This comprised of 37 bends, 10

improvement in life span compared with the

hoses and 6 valves. In addition to the

short radius bend with a bending diameter of 0.26m.

reference steel bends tested, a total of five different bend surface coatings were

Bend Angle

examined for erosive wear resistance. Test Rig

It is difficult to isolate the influence of the bend angle since the tested bends had

The erosive wear test was carried out in a closed test loop

~here

different diameters. It would appear,

high abrasive silica

however, that the 60° bend tested with a

sand was recirculated.

One mobile diesel 3 compressor capable of delivering 0.7lm /s

bend, will last about 80 % longer than the

of free air at a pressure of 10 bar was

equivalent 90° bends.

diameter similar to the short radius 90°

used to accelerate the sand to a maximum velocity of lOOm/s.

Diverter Design

Comparison of surface coatings

During the erosive wear testing it became evident that the reinforcement of diverter

One of the hardfacing materials showed

components will not provide an acceptable

significantly better restistance to erosive

level of lifetime within economical or

wear than the other materials.

This

practical limits. The hardfacing material

hardfacing material contained 60 percent by

that performed best extended the life by

weight tungsten carbide bonding in an erosive-corosive resistant alloyd matrix.

about three and a half times compared with mild steel.

The amount of tungsten carbide is of great

Consequently every effort should be made in

importance.

designing a more satisfactory diverter

Tests also were performed with

65 percent by weight tungsten carbide and these showed even better results.

system. factor.

However

Geometry being the most important

cracking was more extensive than for the

Discontinuties in the diverter line must be

other materials.

thermal strain during the hardfacing

avoided; such as bends or changes in pipe diameter.

process, and can be improved by reducing

Where bends are unavoidable, these should

the hardness in the matrix.

be hardfaced.

This is due to the

The tests have proved the

importance of material selection. The other hardfacing materials tested were not significantly better than plain steel.

In addition the straight section following a discountinuity should be hardfaced at a

Bend Diameter

length of approx. 1,5 mas this section will be exposed to heavy turbulent flow and

Since two 90° bends with different

erosive wear.

diameters were tested, the analysis

Diverter valves should be located before,

provided a direct comparison of the

or at a sufficient distance from any

influence of bend diameter. The results

discontinuty.

showed that the long radius bend had a 631

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A SHALLOW GAS RESEARCH PROORAM

4

Where flanges are used, these should be

17256

The potential fire hazard from an

machined and matched to ensure that the

underwater release of shallow gas was

inside surface is perfectly smooth and free

investigated.

from steps and ridges. The testing proved that the most important

At the West Vanguard blowout the fire on

parameter effecting erosive wear is the

the ocean surface had a diameter of

particle velocity, and this parameter can

approximately 60 meters. The waterdepth was

be controlled by mounting a choke at the

220 meters giving a central gas zone of

ends of the diverter lines. On jackups and

some 35% of the water depth.

fixed platforms where the diverter system

Based on the experience from the actual

in most cases is the only protection if gas

blowout, theoretical simulations were

flow occurs, it is of great importance to

performed to investigate gas concentrations

reduce the flow rate.

on the ocean surface and above. Figure 1 is a projection of the gas cloud

Development of drilling procedures

resulting from the gas flow from the 762mm (30") wellhead after the marine riser was

The top hole drilling procedures for floaters were reviewed.

disconnected on the West Vanguard accident.

This changes can

This projection indicates the height of the

be summarised as follows:

flame up to 1 meter above sea level, which seems to be a fair assumption of what was

setting depth of the 762mm casing (30")

actually seen at the location.

based on

Figures 2

and 3 show two different projections with

expected leak off

wind velocity reduced to 3m/s.

20 inch setting depth

Here it

clearly shows how the shape, height and

site survey results

concentration of gas is highly dependent

increased mudweight in pilot hole to

upon the effect of wind, and that in this

min. 1.10 SG when drilling with marine

instance the concentration is within the

riser

limits of flameability up to a height of

MWD tool run on slick bottom hole

25 meters.

assembly

the fact that upgrading in design and

max. 30 m/hour rate of penetration

layout of the ventilation system is

However these figures support

important, since in certain instances Later the authorities' requirements led to

dangerous concentrations of gas can be

an amendment in the maritime certificates

present at 15-25 meters above sea level.

of all Norwegian flagged drilling vessels.

Reducing the amount of gas by 50% and

Stating that surface diverter systems

waterdepth to BOrn, dangerous concentrations

should only be used in the short period,

of gas up to 20m above sea level can arise,

until the riser is disconnected.

(ref. fig. 4).

This in

addition to the fact that several problems were experienced in the disconnection of the marine riser led to a philosophy where riserless drilling is accepted by Statoil as a safe and co9t effective top hole drilling procedure under certain conditions.

632

B. ELLINGSEN

M. GRINRQ)D

5

Special arrangements and modifications· in the use of a shallow gas stack:

The fact that dangerous concentrations of gas on a rig are not solved by drilling riserless, led to extra precautions when drilling through seismic anomalies without the marine riser installed. These can be summarised; by displacing the hole to sufficient mud weight, and combining the design of the hole size and bottom hole assembly so that dynamic kill is possible.

It will be necessary to be able to drift an existing specific profile on a 4 76rnrn · (18 3 I 4 ") wellhead housing through the 7621nrn (30") ID of the SGS. Flushing arrangement in the 762rnrn (30") wellhead connector will keep the wellhead area clear during cementing of the 508mrn (20") casing.

When shallow gas is encountered the pilot hole will be plugged back and 20" c~sing set immediately above. The conventional 476rnrn (18 3/4") BOP stack is then installed allowing the gas zone to be drilled with BOP control. In this case setting of a 406mrn (16") liner may be necessary. To avoid this time consuming operation Statoil is now in the process of incorporating a subsea shallow gas stack (SGS), which will allow setting of the 508rnrn (20") through shallow gas zones with BOP control.

Original minimum disconnecting requirements on 762mrn (30") connector in the upper section were: a)

69 bar pressure trapped under 6 shear ram equal to 3.1 x 10 N tension.

b)

2.7 x 10 5N tension caused by riser tensioners.

The SGS consists of an upper section and a lower section (fig. 5). The lower section has a 762mrn (30") wellhead connector with flushing ports, a spool with kill valve and a "dump" valve, 30" annular BOP, accumrnulator bottles, adaptor and framework. The lower section has a minimum inner diameter of 762mrn (30"). The upper section consists of a 762nun (30") connector capable of disconnection under extreme conditions, a shear ram, flex joint, choke/kill line, riser adaptor and framework. Minimum inner diameter of the upper section is 476mrn (18 3/4").

c)

3.4 x 10 5Nm bending caused by the flexible joint.

d)

0-10° angle on flexible joint.

The selected connector is capable of disconnecting under combined loads. The shear ram is designed to cut 127mrn (5") DP if emergency disconnection of the upper section is necessary. A control system and a flexible kill line are connected to the lower section of the SGS. This arrangement makes it possible to operate the lower section while the upper section and riser are

If gas cannot be controlled, or if formation breaks down, the upper section of the SGS will be released by means of the specially selected connector/disconnector system.

pulled.

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A SHALLOW GAS RESEAHCH PROGRAM

6

Anchor emergency guick release:

Rig Ventilation system

During the shallow gas blowout/explosion on

The air ventilation system on West Vanguard

board the "West Vanguard", the emergency

was divided into several separate systems

quick release on four anchor lines was

with air intake fans and air outlet fans.

activated.

Air ventilation had manual shut-down in

on·e of the winches failed to

case of a gas alarm.

release.

The air intake fans

were stopped manually when the well started to flow; however air outlet fans remained

Considering off-shore blowouts it has been

running.

obvious that:

This is considered to have contributed to Technical damages and weak points in

gas explosions in some rig areas.

the anchor system on several occasions

The explosion potential, when gas enters

have rendered. an emergency quick

the ventilation system, was proven during

release impossible.

the blowout, i.e the generator room

Emergency quick release has not been

exploded.

activated due to risk of ignition of

this risk were investigated.

Consequently measures to reduce

the blowout gases. The anchor system is vulnerable to

Following are the results which have been

explosions at deck level.

implemented:

Most existing anchor systems lack Gas detectors installed in all air

sufficient redundancy.

intakes. Automatic stop of all ventilation fans

Information concerning the following was

when gas alarms are activated.

collected and analyzed:

Remote control ~f valves on critical air intakes.

Anchor systems and emergency quick release systems on existing platforms

Shallow gas drainage

Procedures related to operations of these systems Accidents related to blowouts

Drainage of shallow gas could be an option

Alternative release systems

in the following circumstances:

Based on the results the following items

Before commencement of development

have been implemented on Statoil operated

drilling where the fixed installations

platforms during 1986/87: 1.

Manual release of the anchor winches.

2.

Quick release panel installed on the

cannot be located outside the shallow gas area. In exploration drilling where gas layers have a large areal extent and

bridge. 3.

several wells are to penetrate the gas

Improvement of electric/electronic

layers.

components. 4.

Increased sprinkler capacity.

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a. GRINR0D

17256

7

During exploration drilling in block 34/10

Increasing the lifetime of surface

- i.e. Gullfaks field, in ca. 60% of the

diverters can be achieved by reducing

wells shallow gas has been penetrated. In

velocity of particles.

1983 it was decided to drill the first

velocity can be obtained by end mounted

shallow gas investigation well near the

chokes.

Reduction of

Gullfaks "A" platform location. This well was drilled to 550m MSL and shallow gas was

-Increase of bend diameter and

revealed. A number of similar wells have

hardfacing of diverterpiping, and an

been drilled since to improve the mapping

increase in pipe diameter will increase

of shallow gas in general.

lifetime of diverter systems.

Well 34/10-30 was drilled and completed with the intent to attempt depletion of the

A theoretical model for gas

gas sand before the Gullfaks "A" platform

concentrations above sea level has been

was placed on its location.

developed taking into account rate,

The sand, 312-314,5m MSL, produced during

waterdepth and wind velocity.

35 days. The production rate was approximately 60.000

SCM/day~

the maximum

Riserless top hole drilling when

rate was 115.000 SCM/day (short period).

checking out anomalies should in all cases be done with hole size and

The reservoir pressure, in the shallow

mudweight designed to allow dynamic

sand, was 33.5 bar, i.e. 1 bar in

kill.

excess of normal hydrostatic pressure. This indicates a gas column height of

Shallow gas stacks will promote control

6m.

of top hole drilling and protect

The gas was 99% methane.

against gas flow to the rig.

Sand was produced into the well. A gravel pack was done.

Rig ventilation system should be

Permeability of the reservoir: between

designed for full automatic shut down

200 and 350mD.

in case of a gas alarm.

Due to circumstances and platform placement the test was abandoned and the well

References:

plugged. In total 2.800.000 SCM had been produced.

(Ref. 1.) The desired pressure

1.

and gas depletion was not obtained.

Grinr¢d. M, Tomren. P.H, Justad. T: "Development of the Gullfaks field" Paper IADC/SPE 17220, presented at IADC/SPE Drilling Conference, March

Conclusion

1988.

Changing drilling procedures seem to be

2.

Dings¢yr. E, Vass¢y. B and Grinr¢d. M:

the most important contribution to an

"Development and Use of a Gas Tight

increased level of safety for floating

Cement" Paper IADC/SPE 17258, presented

drilling. Drilling programs and

at IADC/SPE Drilling Conference, March

procedures for top hole drilling should

1988.

be designed to enhance safety at each location based on existing information.

635

8

3.

A SHALI.CM GAS RESEARCH PR:X;RAM

D. Mills and J.S. Mason: "Conveying Velocity Effects in Bend Erosion. Proc. Int Symp. on Freight Pipeline. Washington D.C. 1976". Subsequently published in the Jnl. of Pipelines, Vo. 1, No. 1, pp 69-81, 1981.

4.

D. Mills and J.S. Mason: "Particle Concentration Effects in bend Erosion Powder Techno!. Vol. 17, No. 1, pp 37-53, 1977".

5.

D. Mills and J.S. Mason: Evaluating the Conveying Capacity and Service Life of Pipe Bends in Pneumatic Conveying Systems. Proc. Powder Europa '78. PAC Conf. Wiesbaden. Jan.

1978"~

Jnl. of Powder and Bulk Solids Tech. Vol. 3, No. 2, pp 13-20 1979. 6.

D. Mills and J.S. Mason: "Analysis of Factors Influencing the Premature Failure of Pipe Bends in Pneumatic Conveying Systems. Proc. 2nd Conf. on Pneumatic Conveying, pp 93-103. ISBN 963 8091 64 7. HUngarian Sci. Soc. Mech. Engrs. Pees. Hungary, March 1978".

7.

D. Mills and J.S. Mason: "The Effect of Particle Size on the Erosion of Pipe Bends in Pneumatic Conveying Systems. Proc. POWTECH '70. pp H 50-96. !ChernE Conf. Birmingham. March 1979".

636

S'PE. 17256

SPE 17256 A P P E N D I X

BEND LIFE CALCULATIONS

Specific Erosion

Msf

Mbf

(ref. D. Mills and J.S. Mason)

mass eroded from component mass conveyed through component

g/tonne

tonne

E

where Msf

and

and

Mbf

E

Conveyed mass of sand before component fails

(g)

Eroded mass from component before failure

g

Specific erosion

The Time to Failure For a Component Tbf = Msf x 60 min ms Where Tbf and ms

time before failure flowrate of sand

tonne/h

In the test the sand was conveyed at a phase density (0) of 1.0 and the mass flow rate of the sand was the same as that of the air.

The Influence of Conveying Air Velocity Specific erosion increases with the increase in velocity according to E~ C 2.65

for sand conveyed through 90 degr. bends. Where C = Conveying air velocity m/s

The Influence of Phase Density Specific erosion decreases with increase in phase density according to EOc:' 0 -0.16

for sand conveyed through 90 degr. bends.

The Influence of Particle Size Particle size has little influence on the value of specific erosion, however, the influence o£ turbulence increases significantly with a decrease in the mean particle size. The influence of turbulence is additionally effected by phase density (0) and so for a 70 ~m sand, for example, the time quoted at 0=1 will vary by a factor of about +/- 2 compared with a 230 ~m sand.

637

SPE 1725b %BY VOLUME GAS IN AIR

% BY VOLUME GAS IN AIR

- · - · - · - · - 30.0 - - - - - - - - 15.0

===~

5.0 2.0 1.3

i

i

)(

)(

120 Z(M)

Z(M)

Y(M): O.OOOE + 00

Y(M): O.OOOE + 00

RATE : 120 KG/S UF WIND: 15M/S ' L: 15%, LFL: 5% WATER ' T-AIR: 10C DEPTH•• 2 20M, DIAMETER GAS ZONE· 60

RATE : 120 KG/S U WIND: 3M/S ' FL: 15%, LFL: 5% ' T-AIR• 10C ATER DEPTH• • ' T-GAS: 10C W • 220M, DIAMETER GAS Fig. 2-Weat V ZONE: &OM anguard gaa cloud with reduced wind velocity.

Fig. 1-Gaa oloud alter Weal Van guard accident. • M

% BY VOLUME GAS IN AIR

%BY VOLUME GAS IN AIR 40

---· ................

- · - · - · - · - 30 0 - - - - - - - - 15:0

5.0 2.0 1.3

.•... :::::::::

5.0 2.0 1.3

·····································

i

)(

10

40

70

Y(M)

z (M):

Z(M) Y(M): O.OOOE + 00

1.000E + 02

RATE : 60 KG/S WIND: 3M/S T ' UFL: 15%, LFL: 5% WATER DEPT'H· -AIR: 10C, T-GAS: 10C • 80M, DIAMETER GAS ZONE: 19M

RATE : 120 KG/S WIND: 3M/S T' UFL: 15%, LFL: 5% • -AIR· 10C WATER DEPTH: 220M. ' T-GAS: 10C . ' DIAMETER GAS ZONE· Fig. 3-Weat V • &OM anguard gaa cloud with reduced wind velocity.

Fig. 4-Gaa cloud with water depth and ' rate reduced.

638

SP£ 17256

SHALLOW GAS BOP

RISER ADAPTOR

FLEXIBLE KILL LINE CHOKE LINE

ANNULAR BOP DUMP

0

VALVE

FLUSHING LINE

W.H. CONNECTOR

Fig. 5-Shallow gas valve stack.

639